The Fishes 9780231893916

Table of contents :
Contents
Illustrations
Acknowledgments
1. The origin of fishes
2. Fishes without jaws
3. Sharks and their ancestors
4. Fishes with lungs
5. Structures concerned with swimming
6. Teeth and jaws
7. Breathing
8. Reproduction
9. The senses
10. More living fossils
11. Trouts and their relatives
12. Fresh-water specialists
13. Fishes of the sea
14. Summary
Index

Citation preview

The Fishes

.hew York Zoological Society

T H E COW-NOSED RAY

The Fishes by URL

COLUMBIA

UNIVERSITY NEW Y O R K

LANHAM

PRESS AND

LONDON

Copyright © 1962 Columbia University Press Second printing

and Columbia

Paperback

edition

Printed in the United States of America

1967

To H . G . R O D E C K

Contents

1

T h e origin of

fishes

3

2

Fishes without jaws

14

3

Sharks and their ancestors

21

4

Fishes with lungs

31

5

Structures concerned with swimming

39

6

Teeth and jaws

52

7

Breathing

56

8

Reproduction

59

9

T h e senses

64

10

More living fossils

72

11

Trouts and their relatives

76

12

Fresh-water specialists

90

13

Fishes of the sea

99

14

Summary

108

Index

113

Illustrations

PLATES

1-24

After page 116

FIGURES

1

Representatives of the three main groups of chordates

6

2

Family tree of the chordates

11

3

Fins of the brook trout

45

4

Fins and airfloat

49

5

Fins of the red snapper

6

Simplified family tree of

101 fishes

111

A cknowledgments

Several persons have given assistance in various phases of the preparation of this book, and I wish to thank them here. Claude W . Hibbard, Museum of Paleontology, University of Michigan, and Frank Mclnnis, in charge of the Detroit Aquarium, made material available for photography. Norman B. Wigutoff, Eloise Holzner, and Stewart Springer of the U. S. Fish and Wildlife Service made it possible for me to use to best advantage the resources of the photographic files of the service; D o r o t h y Reville and Gerald P. Cooper did the same with respect to the files of the New Y o r k Zoological Society and the Institute of Fisheries Research, Michigan Department of Conservation, respectively. Carl L. H u b b s read and made extensive comments on an early version of the book. T h e drawings and diagrams were prepared by Caroline Lanham.

The Fishes

1 The origin offishes Deep in human nature is an interest in the water. Give man or child freedom to move and he heads for the nearest river or lake or sea shore. T h e world of water has the fascination of the unfamiliar; we have always looked at it from the outside, and with net and line have drawn up the animals that live in it. It seems to hold riches to be had for nothing: a shell cast up on the shore, a silvery fish pulled out of dark water, a brilliant trout that materializes out of the chaos of a mountain stream. And the strangeness of the aquatic environment seems somehow brought to a focus in the fish. Observing the living fish is like watching no other animal. It is weightless as it hangs poised in the water, and it breathes by drawing in the drowning fluid. Fishes have lived in the water for a long time, for at least 400 million years. In their long history they have become adapted for life in all sectors of the aquatic world—the sunlit open ocean and its black depths, the washing surf, placid tropical rivers, and mountain tor-

4

The origin of fishes

rents. T h e end p r o d u c t s of this evolutionary history are some 20,000 species of fish of remarkably varied shapes, sizes, and habits. T h e kinds of fish now living o u t n u m b e r the kinds of all other vertebrate animals—mammals, birds, reptiles, amphibians—taken together. O n e of the aims of this book is to provide a historical background to the present world of fishes: from what beginnings and in what way their structure was fashioned. T h e brief sketch of the origin and history of fishes given here is in part based on a technical literature in which there is m u c h controversy. T h r o u g h conflicting opinions a single p a t h has been chosen which is intended to give an adequate introduction to the structure of modern fishes, although qualifications at various points indicate that the evolutionary history of fishes, a history that extends h u n d r e d s of millions of years into the past and is lighted by only the faintest gleams from the evidence of fossils and comparative anatomy, is a highly speculative subject. T h e oldest good fossils of reasonably complex animals are found in Cambrian rocks. In the famous Burgess shale locality in the Selkirk Mountains of British Columbia are well-preserved animals of many kinds—jellyfish, trilobites, crustaceans, and other invertebrate animals, but there are no fishes, nor have fishes been found anywhere else in rocks of Cambrian age, the period lying between 500 and 400 million years ago. It is likely that fishes were not yet in existence d u r i n g this phase of earth history, or at least not until near the end of the period. It is in rocks of the next geologic period, the Ordovician (400 to 3 6 0 million years ago), that we find the first trace of fishes. In sandstones of the foothills of the C o l o r a d o Rockies are fragments of bone

The origin of fishes 5 which in minute detail are like the bony plates of fishes known from good fossils of later geological periods. T h e fossil record thus tells us that fishes, probably fairly complicated ones, existed nearly 400 million years ago. It does not tell us how fishes came into existence. In rocks of later times there are many fossils that bear on the history of fishes after they were well launched on their career, and we will have occasion to refer to them, but to find evidence of how fishes originated we have to turn to animals now living. In the structure of living animals we find clues, some obvious, some not, to their evolutionary past. T h e evidence is indirect, so that interpretations of its meaning vary; however, it is worth while to consider some of the groups of animals other than fishes to see if they shed any light on the otherwise dark past of the group in which we are particularly interested. Fishes belong to a major group, or phylum, of animals termed the Chordata (chordates), a phylum that includes ourselves and as well some small, relatively simple animals that lack backbones. T h e chordates that do have backbones are called vertebrates. T h e vertebrates and their spineless relatives are linked together in the single phylum Chordata by three important structures that they have in common: (1) a notochord, a flexible supporting rod: (2) gill slits; and (3) a hollow tube of nerve tissue that forms the brain and spinal cord. This characterization has to be qualified, since the structures may be found in only one stage of the development of the individual. Thus, a man has notochord and gill slits only as an embryo; as the embryo develops, the notochord is replaced by backbone and the gill slits disappear or are modified.

6

The origin of fishes

tube, (b) the notochord, and (c) gill slits.

It is among the backboneless chordate animals that biologists look for clues as to how the fishes and—since fishes gave rise to the other backboned animals—how the vertebrates originated. T h e backboneless chordates are the acorn worms, sea squirts (tunicates), and lancelets, all animals that live only in the sea. Of the three, the lancelet, or Amphioxus, is most fish-like. In laboratory classes in vertebrate anatomy it is used to introduce the student to basic chordate structure, the structure on which the further specializations of the vertebrate animals are based. Amphioxus,

a slender,

vaguely minnow-shaped

animal

a few

inches long, lives in the sand of shallow coastal waters, where it sometimes is so abundant, as oflf the China coast, that it is profitably

The origin of fishes 7 taken for food. T h e animal is eyeless and without a well-differentiated head. It usually lies quiedy in the sand, with its mouth protruding, but can swim with fish-like body movements. It is the notochord that makes it possible for Amphioxus to swim vigorously, if somewhat aimlessly. This rod, which runs the length of the body below the spinal cord, has the consistency of firm jelly and has the merit of being incompressible. When the swimming muscles that lie alongside it contract, its incompressibility prevents the body from shortening, so that the whole power of the muscle contraction is spent in bending the body in the undulating swimming movements. Quite likely the evolution of the notochord was a crucial event in the sequence of changes that was finally to produce the vertebrates, since it laid the foundation for a vigorous swimming life. As we shall see, the complex of other adaptations for greater mobility which clustered about this adaptation defines the vertebrate animal. Leaving Amphioxus for the moment, we can inquire into the background of the notochord: where did it come from? Of the other two groups of spineless chordates, the acorn worms probably give us the smaller amount of information. They have a notochord, used to support the head end of the animal as it is pushed through mud or sand, and have gill slits, but they are at best only distantly related to Amphioxus. It is the sea squirts that give a useful clue as to how the notochord may have originated. T h e adult sea squirt is usually a rather shapeless lump of an animal that lives fastened to some underwater object. Its only reaction when picked up is to squirt toward the inquiring eye a jet of sea water. T h e young sea squirt, however, is quite different, being an active, tadpole-like animal. Seen under a

8

The origin of fishes

microscope, the young sea squirt distinguishes itself from other microscopic animals of sea water by its powerful swimming movements. In the tail of this miniature "tadpole" is a notochord. Now, a very small animal of the larval sea-squirt type could conceivably swim with a tail that lacked the notochord. Many of the microscopic protozoans swim by means of clusters of lashing cilia. One can imagine a swimming tail, furnished at first only with muscle fibrils,

gradually being improved by the addition of a few cells

packed with droplets of water (these render the cells incompressible) and the gradual organization of these cells into a notochord. Such a development would enable an animal of somewhat larger size, a millimeter or more in length, to swim powerfully. Thus, in the small sea squirts we may see a kind of animal in which the notochord could have arisen by gradual processes. Some sea squirts never change into the usual sedentary adult type, but become sexually mature in the mobile stage. Perhaps it was from such ancestors that animals resembling Amphioxus developed. Once the notochord appeared, the best way for the animal to get more speed was to increase its total size. It may be that the circumstance which favored the evolution of larger size, and hence more speed, was the desirability of getting away from enemies. Another suggestion is that the ancestial types invaded coastal streams, where only better swimming ability would enable them to cope with the river currents. In this perhaps oversimplified conception, then, Amphioxus is essentially an oversized sea-squirt larva. T h e small sea-squirt larva can get the oxygen it needs by diffusion from the water through the skin, directly to the deepest tissues (as the larva changes into the

The origin of fishes 9 sedentary and eventually larger adult, it acquires a circulatory system, which transports oxygen). However, some of the tissues of the larger Amphioxus are too far from the skin to get oxygen by diffusion, so that, correlated with its larger size, Amphioxus has evolved a circulatory system which keeps colorless blood, with oxygen dissolved in it, stirring through the body. T h e stage is now set for the evolution of the first fish, the first vertebrate, from an animal of the Amphioxus type, an animal a few inches rather than a few millimeters in length, having a good notochord and a circulatory system. T h e addition of a few more characteristics, all correlated with larger size, will produce the fish. One addition concerns the notochord. It is first strengthened, then entirely replaced by cartilage, and the cartilage in turn by bone. This adaptation gives better purchase for and resistance to the increasingly powerful swimming muscles. Another addition concerns the blood. For an animal the size of Amphioxus, the colorless blood, which carries about the same amount of oxygen as ordinary water, suffices as the vehicle for carrying oxygen. But with greater size and greater activity there must be some mechanism for concentrating oxygen in the blood. T h i s is accomplished by means of so-called respiratory pigments in certain of the cells that float in the blood. T h e oxygen-absorbing pigment used in this instance is hemoglobin, and is red. T h e evolution of this pigment was not a teleological or purposeful event, nor is the appearance of any other of the innumerable adaptations of a complex animal. Hemoglobin has appeared several times in the history of animals, probably by slight modification of nearly

10

The origin of fishes

colorless iron-bearing substances called cytochromes which occur in the living tissues of all animals. When circumstances favor variants with hemoglobin, the variants tend to persist. A premium on larger size made the acquisition of hemoglobin at this stage in the evolution of fish a near certainty. In some other animal groups, such as the squids, the pigment selected was the copper-bearing hemocyanin, which is not red. With these two additions, hemoglobin and a skeleton, the first vertebrate—a backboned, red-blooded chordate animal—had appeared. Another characteristic of fishes is, like hemoglobin, concerned with the oxygen problem. In the fish the gill slits—or, more specifically, the gill arches between them—are organs of breathing. The gill slits of the sea squirt are used, not for breathing, but for feeding. A current brings water laden with minute organisms into the mouth, and the water goes out through the slits minus the organisms. Amphioxus also feeds in this way, but in addition blood channeled through the gill arches picks up oxygen that has diffused through the skin surface of the arch. This probably was the situation that existed in the ancestor of the vertebrates. In a later development the surface through which diffusion occurred could be increased by plate- or thread-like projections of the arch, eventually forming complex gills of the kind found in modern fishes. At this stage the gill slits would be of only minor importance in straining out food, and would be used almost entirely for breathing. T h e transition between not-yet-fish and fish was gradual, and there was doubtless a time, perhaps in the late Cambrian or early Ordo-

The origin of fishes 11

2. Family tree of the chordates. The relative abundance of each group is indicated by the thickness of the branches. For past geologic times the estimates are highly conjectural; the widths at the top represent for each group the number of species now living. The problematic acorn worms are omitted. FIGURE

vician, when a distinction between the two was hardly possible. Transitional types are now extinct, and there is a clear-cut distinction between the fishes on the one hand and the sea squirts and Amphioxus on the other—with one exception to be noted in the next chapter. At this point a fish can be defined as a red-blooded, gill-breathing, backboned aquatic animal. Before we examine the major groups of fishes, we would do well to outline their over-all classification. T h e animals here termed fishes are grouped into four major divisions or classes:

12

The origin of fishes

1. Class Agnatha. Fishes without jaws. Includes lampreys and hagfishes or borers, and the extinct ostracoderms. 2. Class Placodermi. Extinct fishes with primitive jaws. 3. Class Chondrichthyes. Fishes with neither lungs nor airfloat. Skeleton typically of cartilage. The living species nearly all marine. Includes the sharks, rays, and chimaeras. 4. Class Osteichthyes. The bony fishes. Fishes with lungs (a primitive minority) or an airfloat (the advanced majority); some have secondarily modified or lost the airfloat. T h e class Osteichthyes is divided into three subgroups: A. Subclass Dipnoi—the true lungfishes, abundant in Devonian times, represented today only by a few species in the southern continents. B. Subclass Crossopterygii—the lobefins, also abundant in Devonian times, and with a single living representative, Latimeria

from the

seas near Madagascar. T h e ancestors of the Amphibia. C. Subclass Actinopterygii—the rayfins, which include all remaining fishes, nearly 20,000 species in all. A few primitive species (for example, the bichir of Africa) have lungs and apparently can breathe air, but in the vast majority the lungs have been transformed into an airfloat. Since this last category is immense, it is well to present the breakd o w n of the subclass, which divides it into three groups: a. Group Chondrostei. Bichirs, sturgeons, and their numerous extinct relatives. b. Group Holostei. Garpikes, the bowfin, and many extinct types. c. Group Teleostei. All remaining fishes, ranging from the herrings and their relatives at the more primitive end of the spectrum to the perches and related types at the other.

The origin of fishes 13 Finally, the great group of the teleosts is usually divided into many orders, with much disagreement among specialists as to the number of orders and the distribution of the many thousands of species a m o n g them. W e here concentrate on three major orders which include the great majority of living fishes. 1. Order Clupeiformes, including the herrings and trouts. 2. Order Cypriniformes, including minnows and catfishes. 3. Order Perciformes, including perches, bass, tunas, and many other types.

2 Fishes without jaws

W e can only guess as to the way in which the first fishes originated, but our speculations as to the origin of such essential characteristics as red blood, gills, and a backbone are designed to make sense in the light of what we know about animals now alive. W e would be pleased to be able to check our hypotheses against fossils of this crucial period in the history of life, but the fossils have not been found. T h i s is not surprising, since the animals concerned must have been small, at most only a few inches in length, and of soft and delicate structure. But once the evolution of fishes got well under way an event occurred which made possible a fossil record: the development of bony armor. Later the appearance of hard scales, teeth, and internal bones added to the likelihood that fish remains would be preserved in the rocks. Fossils of armored fishes first become common in rocks of the late Silurian, and by the Devonian they are abundant. Evidently there

Fishes without jaws

15

were by this time predators of a size and strength that made it imperative for these early fishes to develop armor. W h a t these predators were we do not know, but among the possible candidates are the eurypterids, aquatic relatives of the scorpions which are found in the same rocks as the armored fishes. Some of the eurypterids were as much as eight feet long, so that they dominated the fishes in point of size, and they were equipped with pincers that would seem to place a premium on protective armor. Another possibility is that there were primitive predatory fishes resembling modern lampreys which had sharp, boring teeth. Themselves unarmored, such predators would leave little trace in the rocks. The

early

armored

fishes

are

called

ostracoderms

("bone-

skinned"). T h e fact that they come first in the fossil record of fishes would cause us to think that they were primitive, and this is indeed borne out by their structure. T h e significantly primitive feature of the ostracoderms is their lack of jaws. T h e y have mouths, often armed with tooth-like plates and projections, but do not have the powerful rigid levers, the jaws, that are needed for shearing and grasping. T h i s primitive jawless stage in fish evolution is represented today by two small groups of unarmored, snake-like fishes, the lampreys and hagfishes. Both the extinct ostracoderms and their living jawless representatives are put together into a group termed the Agnatha ("without jaws"). T h e s e first known fishes had, then, a primitive feeding apparatus. T h e y also had primitive swimming equipment. Although equipped with relatively powerful muscles which drove the body forward by side-to-side lashing movements, they lacked the controlling fins

16

Fishes without jaws

necessary for precise and effective use of motive power which appear in later types. Many of the ostracoderms had only vertical fins, extending up or down from the midline of the body. These tend to prevent rolling from side to side as the animal moves forward. Apparently these early armored types were heavier than water, so that in swimming they had to be kept from sinking. T h e necessary lift was perhaps provided by the peculiar tail fin illustrated in the diagram of an ostracoderm in Figure 4. T h e longitudinal supporting bar, which was an extension of the backbone, was near the lower margin of the fin, leaving the thin upper membrane free to bend as the tail was lashed sideways through the water. Observations of working models of this kind of fin indicate that the membrane bent at an angle which deflected water vertically, driving the tail down. Possibly in normal swimming this tended to tilt the head end up, both compensating for the weight of the head and allowing the planing action of the under surface of the body to provide lift. T h e kind of tail fin just described is termed a reversed heterocercal fin, because it is the inverse of the heterocercal tail fin found in sharks (see Plate 5) and other primitive fishes. So unusual is this fin shape that the first pictures of these fossil ostracoderms were drawn upside down. T h e ostracoderm in Figure 4 has a helpless strait-jacketed look because it lacks the front pair of fins we expect to see on a fish. It is presumably because ostracoderms lack these miniature planes, which would hold up the front end of the body, that they have the anomalous tail fin. Some of the ostracoderms made up for this deficiency in part, at least, by having the underside of the body flattened or even

Fishes without jaws

17

drawn out into more or less sharp angles running the length of the body on either side, giving an effective planing surface underneath. Others had the front part of the body drawn out into flaps that corresponded, at least in function, to the front pair of fins possessed by later fishes. Such ostracoderms have a tail fin of the heterocercal or shark type; the reverse of the kind already described, it lifts rather than depresses the tail. Because of the planing action of the flaps at the front of the body, the front end does not drop. This arrangement, in which the tail fin provides lift for the rear and the pectoral fins or their equivalent for the front, is apparently an efficient one, and is found throughout the shark group. It is doubtful that any of the ostracoderms had really well-developed pectoral fins; this, together with the fact that many or most had no pectoral fins at all, makes it likely that as a group the ostracoderms were poor swimmers. Even though clumsy by the standards of other fishes, the ostracoderms must have been much better swimmers than their predecessors in the earlier stage when fishes were emerging from very small invertebrate ancestors. With relatively swift forward movement of a relatively powerful body, the evolution of sense organs which can give information on distant surroundings comes to be important. Most significant are the eyes, which in the jawless fishes appear as wellorganized structures for the first time in the history of the vertebrate animals. T h e armor of the head of the ostracoderms has apertures foi two eyes, and the structure of the brain, as well as the size of the nerve canals leading to the apertures, shows that the eyes were fairly good ones. A peculiar sense that is not covered satisfactorily by any term in

18

Fishes without jaws

our system of naming the senses was also evolved by the ostracoderms. It may perhaps be best described as a kind of hearing that detects low-frequency vibrations in the water. T h e same sense is possessed by modern fishes and is based on the lateral-line system (discussed in a later chapter), which is partly enclosed in canals running through the bones of the head. Similar canals occur in the fossil skulls of the ostracoderms. T h e inner ear, which functions in balance, was present in the ostracoderms in simplified form, with only two semicircular canals. T h e living jawless fishes, the lampreys and hags, also have two canals rather than the three found in other fishes and the other vertebrates. It is believed that these canals were derived, in their evolution, from part of the lateral-line system. T h e solid bone of the ostracoderms was confined mostly to the skin, and was especially heavy in the head region. The internal skeleton, except for the skull, was of cartilage rather than bone. In restoration some of the fantastically armored ostracoderms look formidable, but only if one fails to take their size into consideration. Most were tiny fishes, only a few inches long; few reached as much as a foot in length. The armored, jawless ostracoderms became extinct at the end of the Devonian, but after a gap of 200 million years in the fossil record this primitive jawless stage comes to light again in the lampreys and hagfishes. That there should be no fossils connecting them with the ostracoderms is not surprising, since they have neither armor nor bony skeletons. Although the lampreys and hags are the most primitive living fishes, and therefore the most primitive vertebrate animals

Fishes without jaws

19

alive, they are not at all rare or obscure. T h e lampreys are abundant enough to make their weight felt on a massive scale in the affairs of animals and men. T h e sea lamprey normally lives in the ocean, where it feeds by fastening to large fish with its sucking, disk-shaped mouth, rasping through the skin with sharp teeth set on the tongue, and drinking the blood of its prey. T h i s lamprey can substitute a large lake for the sea, and has invaded Lake Ontario. After the Welland Canal was built, the lamprey was able to work its way around the barrier of the Niagara Falls and get into Lake Erie and the upper Lakes, where it slaughtered the populations of whitefish and lake trout that had been the mainstay of a great fishing industry. Whether living in the sea or a large lake, the lamprey must go into a river to spawn, for its young have a mode of life entirely different from that of the adult. T h e young lamprey lives a rather sedentary life in the mudbanks of streams, where it strains out small food particles from the ooze, using the gill slits as a sieve. It will be remembered that Amphioxus feeds in a similar way, and indeed the young lamprey, called an ammocoete, is something of a connecting link between Amphioxus and the fishes. Like Amphioxus, it has colorless blood. Upon reaching

a certain size, the young lamprey abruptly

casts primitive things aside: its blood turns red, and the delicate filaments around the mouth become transformed into the effective teeth of the adult lamprey. It is interesting to see that some of the lampreys have given up the lake- or sea-going phase of their existence, and with it the predatory blood-feeding stage. T h e brook lampreys, which are quite closely related to the sea lampreys, live as ammocoetes in streams, just as do

20

Fishes without jaws

their relatives. However, the transformation to the normal adult is incomplete: the teeth are blunt and useless, and the grown brook lamprey never feeds, living out the rest of its life span on fat stored during its youth. Although more advanced structurally than the ammocoetes, the adult lamprey is quite primitive, lacking jaws and paired fins. This same primitive pattern is found in the hags. Less is known of them because they live always in the sea. They are most often seen by fishermen, who bring up nets to find the eel-like hagfish, over a foot long, wriggling out of the hollowed-out bodies of the netted fish. Quite likely the hagfish is able to attack only a disabled fish, which it enters to feed on soft tissues. It is believed to feed usually on soft annelid worms, devouring them by using the movable plate-like teeth. An outstanding mystery in the fossil record concerns small, hard teeth called conodonts that have not been found associated with recognizable animal remains. One suggestion is that they belonged to animals resembling lampreys or hags and, like them, lacking other hard parts. Conodonts are abundant, and if this hypothesis is right, animals of the lamprey type have had a long and successful history. However, we have little knowledge of the evolutionary past of these primitive and gruesome fishes.

3 Sharks and their ancestors

Even before the A g e of F i s h e s — t h e D e v o n i a n — s o m e one or more g r o u p s of the j a w l e s s fishes had gradually worked out an invention that was to contribute m u c h to the future success of the vertebrates. T h i s invention, the biting j a w s , was based on raw materials already available in the j a w l e s s stage of fish evolution. S u p p o r t i n g the bars between the gill slits in the early fishes were cartilaginous or bony columns w h i c h were hinged in the middle, in the manner of a V with the opening facing forward. In some of the j a w l e s s fishes, one of these so-called gill arches lay close to the mouth opening, and its movement on the hinge j o i n t , normally functioning in the expansion and contraction of the throat region associated with breathing,

also

became associated

with opening and closing the

mouth. A s the connection with the mouth became more intimate, the function of

this arch shifted entirely from breathing to control of

p o w e r f u l m o u t h movements. T h e arch became larger than the foll o w i n g gill supports, its upper member was fused to the skull as the

22

Sharks and their ancestors

upper jaw, and its lower member protruded forward as the lower jaw. In more primitive fishes the similarity between these two jawbones and the two pieces of an ordinary gill arch seems obvious, and also is striking in fish embryos. With the development of powerful muscles to bring up the lower jaw, and of sharp teeth, the feeding apparatus became a mechanism that could pierce the defenses of prey formerly immune because of size or armor. Eventually fishes with jaws displaced all thejawless fishes except the few lampreys and hags that still exist. T h e inventors of jaws gave rise to a large and varied group ranked as a class and termed the placoderms. This group is of strategic importance because it included the ancestors of the living jawed fishes, the sharks and bony fishes. In the placoderms we find the first stage in the evolution of the jaw. Only a single pair of gill arches was involved, and behind the jaw was a complete gill slit. In later fishes the following gill arch too was pushed forward and combined with the primitive jaw to produce an improved mechanism; in the process the intervening gill slit was first compressed into a small tubular passage, the spiracle (found in sharks), and then, as in most fishes, disappeared. T h e placoderms also made early basic advances in swimming technique. They developed paired fins that look as if they would be of real use as planing surfaces. T h e oldest fishes whose appearance suggests that they were good swimmers were placoderms termed acanthodians, or "spiny sharks," trim little fishes a few inches long, with a history that extended from the Upper Silurian through the Permian. Not only did they have two pairs of fins in the position of the pec-

Sharks and their ancestors

23

toral and pelvic fins of later fishes, but some had five additional pairs lined up between (see the placoderm in Figure 4). Each of these fins was supported by a stout spine in front which served as a cutwater. Apparently there was no arrangement for adjusting the angle of attack of these fins in any precise way. If acanthodians like this were the ancestors of modern fishes, then the reduction in number of paired fins to four (the pectorals and pelvics) was brought about by the loss of the intermediate pairs. If the modern four-fin pattern had not been setded upon by the time fishes left the water—when fins were converted into legs—we would perhaps now have six- or eightlegged land vertebrates. This may be considered a minor evolutionary tragedy, for, as Mark Twain has pointed out, dachshunds could well use an extra pair of legs. Besides the lateral fins, acanthodians had well-developed dorsal fins and, on the underside of the body, an anal fin. Supported with spines on their leading edges, these median vertical fins would contribute much to the stability of the animal. T h e tail fin was of the heterocercal type found in sharks, which came later. With efficient fins and a flexible plating of scales, the trim acanthodians were the most modern-looking of the placoderms, and it has been suggested that they were a basic stock from which the living jawed fishes arose. Another group of placoderms that had good lateral fins were the stegoselachians. One of these looked something like a modern ray (a very flat-bodied kind of shark), with wide, wing-like pectorals. A group of placoderms which has left abundant fossil remains, the antiarchs, had slender, jointed flippers that may have helped the animal cling to the bottom against a current.

24

Sharks and their ancestors

E q u i p p e d with biting jaws and more advanced swimming equipment, the placoderms gained ascendancy over the ostracoderms. T h e greatest known fishes of the Devonian were placoderms found in the black shales near Cleveland, Ohio. Gigantic armored heads of these animals indicate that their total length may have been thirty feet. Between the first minnow-sized placoderms of the late Silurian and these monsters there was a span of perhaps 30 or 40 million years. T h e placoderms survived the ostracoderms, and had a successful career of another 100 million years before they too became extinct at the end of the Permian. Fishes of the two classes so far described, the Agnatha and the Placodermi, were not, so far as we can judge from their structure, swift or powerful swimmers capable of living free of the bottom for long periods of time. T h e two remaining classes, the Chondrichthyes (sharks) and Osteichthyes (bony fishes), include members that have developed techniques of swimming to a much higher level than anything that went before, and owe their preeminence in the seas and fresh waters to this fact. As the technical names of the two groups indicate, they can be distinguished by the skeleton, which is of cartilage in the sharks, bone in the other class. Unfortunately, this emphasis ori the skeleton, which dates from an early period in the study of fishes, tends to draw attention away from the most fundamental difference between the two groups. T h e so-called bony fishes have either lungs (a primitive minority) or a hydrostatic organ called the airfloat or air bladder which is a converted lung. By contrast, the sharks have neither lungs nor airfloat. T h e y are " h e a v y " fish, which means that they have had

Sharks and their ancestors

25

to solve the problem of advanced swimming in a way basically different from that hit upon by the bony fishes. T h e large and powerful sharks of the open oceans are seldom seen in aquaria, but in motion pictures their strength and grace are impressive. Most astonishing is the great spread of the shark's pectoral fins, which are held outstretched as it swims. They look like the stubby wings of a jet airplane, and they function like airfoils, providing lift as the shark is driven through the water by effortless sweeps of the tail. A typical bony fish, by contrast, holds its relatively small pectoral fins close to the body when swimming at high speed; it is a streamlined torpedo, with the necessary lift provided by its internal hydrostatic organ. It is quite possible that the cartilaginous skeleton of the shark, instead of being a primitive characteristic which has simply not been superseded by the supposedly more efficient bone, is an advanced specialization which saves weight. There may well be other such devices: large deposits of fats and oils, for example, which have a lower specific gravity than water, would serve to lighten the animal. T h e whales, among the mammals, are large animals with an exceedingly massive bony skeleton, but this is buoyed up by the lungs, by the layers of fat under the skin, and, in some, by the huge cask of oil in the skull. T h e tail fin of the shark is as distinctive as its pectoral fins. Of the heterocercal type, the fin has a firm backbone for support above and a flexible vane below, which in swimming movements gives lift for the tail end of the fish. Apparently this arrangement is nearly a necessity for heavy fishes of the shark or ostracoderm type. But, unlike the

26

Sharks and their ancestors

ancient ostracoderms, the sharks combine the heterocercal tail with efficient planing surfaces—the pectoral fins—and it is this combination which gives the sharks advanced swimming capabilities. Acanthodians, placoderm fishes which may have been near the ancestry of sharks, also combined the heterocercal tail with paired planing surfaces, but the latter were apparendy rather ineffectual structures compared to the pectoral fins of modern sharks. T h e swimming apparatus just described is characteristic of heavy fishes with a specific gravity appreciably higher than that of water. T h e few sharks that have been analyzed turn out to be about five percent heavier than an equal volume of water. By contrast, a fish equipped with an airfloat—one of the common fresh-water sunfishes, for example—weighs essentially the same as an equal volume of water. Experiments on these sharks demonstrate clearly that swimming movements produce lift as well as forward motion, since the shark is able to carry added weights amounting to as much as one fourth of its own weight. T h e airfloat fish is dragged down, unable to swim, by a trifling added weight. A heavy fish, of course, has to keep moving to stay aloft. T h e shark in an aquarium moves ceaselessly, sweeping back and forth in its tank like a leopard in a cage. T h e oldest known shark is a three-foot-long animal called Cladoselache, of the Upper Devonian. In reconstructions of this fish we recognize an advanced swimming type with a well-streamlined body and a pair of pectoral fins which give the fish the lines of an aircraft, but with wings suitably shortened for the denser medium of water. These fins are broadly based, so are not movable except by bending.

Sharks and their ancestors

27

On either side of the trunk near the tail fin are two lateral keels which may be adaptations concerned with speed, for they reappear in such modern fishes as the swift mackerel sharks and the powerful tunas and albacores. In modern sharks the pelvic fins of the male are modified so that when held together they function as an organ for introducing spermatozoa into the female; fertilization is therefore internal, and the young shark can develop inside the female. Most modern sharks give birth to living young, an adaptation with advantages for life in the open seas. T h e ancient Cladoselacfu did not have pelvic fins of this type, although other sharks of Paleozoic times did have them. Cladoselacfu had abandoned (if its ancestors ever had it) the bony plating of scales characteristic of most primtive fishes, and had instead a tough hide studded with minute teeth. These were teeth in a rather literal sense, since on the rim of the mouth the same structures became enlarged to form the several rows of teeth. This kind of skin is present in living sharks as well, and has been sold as a kind of sandpaper called shagreen. T h e bases of the teeth are embedded in the skin; the protruding tips are bent backward, presumably to reduce friction. By Carboniferous times there were sharks with quite modern pectoral fins, fitted with a notch at the base behind, which allowed for a much greater range of movement than did the broadly joined fin of Cladoselacfu. T h e angle of attack of the hydrofoil could now be altered at will to give better control of swimming. Sharks differ from all other living fishes except the lampreys and hags in that their gill slits are not covered with a plate or operculum.

28 Sharks and their

ancestors

The operculum is a device, to be described in more detail in a later chapter, which helps pump water through the gill slits. Clearly the sharks get along without it, and movement of the individual gill arches is used for breathing. Perhaps the incessant swimming of many sharks also helps keep water flowing through the gills. Sharks are essentially fishes of the ocean, although they may go up large rivers and have been landlocked in tropical lakes. In the sea they are exceeded in size only by the whales, and their numbers are sometimes overwhelming. Small sharks may swarm in fishing grounds until they drive fishermen away by monopolizing the baited hooks and tearing the nets as they are gathered in by the thousands. Living sharks are commonly divided into two main groups: the sharks in the strict sense, usually of rather normal fish shape, and the rays, which are flattened, some to a degree that allows them to lie on the sea floor like a rug. There are in addition the grotesque chimaeras, so different from the other sharks that they sometimes are put in a separate class, the Holocephali. The largest fishes in existence are sharks. A captured specimen of the whale shark (not related to whales except in point of size) measured forty-five feet. This great fish is a harmless creature with minute teeth only one eighth of an inch long. Like some of the true whales, it feeds on small animals strained out of the water. The very large basking shark, hunted by fishermen for its oil, also feeds in this way. Others of the sharks are formidable predators equipped with rows of large, razor-sharp teeth. The great white shark ( C a r c h a r o d o n ) reaches a length of fifteen or twenty feet and has heavy, triangular, saw-edged teeth. Teeth of an extinct Carcharodon. so abundant in

Sharks and their ancestors

29

marine deposits of Florida that they have been mined for phosphate fertilizer, are larger than one's hand, and, judging from the teeth of the living related shark, may have come from fish thirty or forty feet long. Particularly in the second group of sharks, the rays, the teeth may be heavy rounded molars fit for crushing prey—clams or snails, for example—protected by thick shells. T h u s , few groups of marine animals, whether they be fleet or stolid with heavy armor, escape the attention of the voracious sharks. Rays have developed the pectoral fins into large wings that merge into the sides of the body along much of its length, giving the animal the appearance of an aircraft of the "flying wing" type. In swimming, however, the fin is not held rigid, like an airfoil, but moves with a rippling motion or strokes like the wing of a bird. It is the pectoral fin, not the tail, that provides such fishes with the power for swimming. Many rays live rather sedentary lives on the sea bottom, but others, such as the manta, live in the upper waters and are superb swimmers. T h e manta ray, with a wingspread of twenty feet, strokes its wings in massively graceful slow-motion flight as it cruises along to feed on small animals channeled into its mouth by a pair of flattened horns on either side of the head. Sawfishes are rays that have developed a long snout, armed on either side with teeth. Since some of the sawfishes are twenty feet long, this snout, wielded by thrashing the powerful body, is a ponderously heavy weapon capable of slashing and disemboweling large fish. A sawfish in an aquarium used the saw to impale its prey with a

30

Sharks and their ancestors

single sidewise stroke; it then had to descend to the bottom to rub the fish off and eat it. Some of the sedentary rays defend themselves with a stabbing spine wielded by lashing the tail. T h e spine of these sting rays is fitted with grooves lined with tissue that drips poison; the sting inflicts agonizing pain and may even kill human beings. A more spectacular defense is that of the electric rays or torpedos. These have large masses of muscle that exaggerate the ability of all living tissues to generate minute amounts of current: instead of being used for movement, these modified muscles go all out to produce electricity, and a large hundred-pound ray can produce enough electricity to stun a man who steps on it. It is said that the ancient Greeks used the rays for a kind of shock therapy by draping one over the head of a patient. In nature, these fish doubtless use the charge to stun prey as well as for defense. T h e few living chimaeras or spook fishes probably are survivors of an ancient and once more abundant group. Their teeth are peculiar crushing plates. Some of the odd ornamental projections on the head of the male are used to clasp the female. Most of the species live in deep water, and fishermen often bring up these strange fish, two or three feet long, in trawls.

Fishes with lungs W e have seen that the sharks, although ancient and in many respects primitive in structure, are still important in the sea. But even here they have been overshadowed by the bony fishes, and the rivers, once a stronghold and perhaps even the original home of the sharklike fishes, have been swept nearly clean of these pioneers. Although there are sharks in Lake Nicaragua, for example, and in large southern rivers, sharks are so uncommon in fresh water as to merit special notice. The streams and lakes now belong to the bony fishes, or class Osteichthyes, whose over-all dominance is shown by the fact that about nine tenths of the living kinds of fish belong to this group. In an estimate of the relative values of the structural features peculiar to the bony fishes, probably first place would go to the airfloat rather than to the bony skeleton. Our discussion of the evolutionary origin of these fishes, then, will center on the origin of the airfloat Tropical and semitropical conditions seem to have been widespread during most of geological history, the present harsh frigidity

32

Fishes with lungs

of the so-called temperate regions being exceptional. T h e great tropical river systems flow vigorously in the rainy season, but in the dry season dwindle to a chain of stagnant water holes. In the daytime the sun-drenched green plants in these warm waters may provide enough oxygen for fish life, but at night the respiration of the plants, together with that of the abundant microorganisms of decay, takes up so much of the oxygen that fish may suffocate. T h e ancient fishes of the fresh waters, then, had before them an extensive environment that they could invade if they could evolve the ability to live in oxygen-deficient water—if they could learn to breathe air. Among the fishes that solved the problem were the placoderms, perhaps some that resembled the acanthodians if not actually members of that group. Probably this evolutionary advance took place in Silurian times, before the dawn of the Age of Fishes. It may be that several groups of fishes solved the problem almost simultaneously, and perhaps in different ways, but the solution of particular interest to us is the evolution of lungs. Fishes that lived in suffocating waters probably breathed at first by coming to the surface to use the oxygen-rich layer of water there. In a further development they may have actually gulped air above the surface to absorb oxygen through the moist skin in the mouth and throat. T h e feathery gills could not be used for this because their filaments,

when out of the water, collapse into a compact non-

absorbent mass. With the habit of gulping air established, the earliest air-breathers could advance by improving the blood supply around the gullet, which would enable them to take up oxygen more rapidly.

Fishes with lungs

33

Finally, the oxygen-absorbing surface could be enlarged by developing a p o u c h on the underside of the gullet. T h i s became long and forked, extending back in the body on either side of the other internal organs. These oxygen-absorbing pouches, well supplied with blood vessels, are lungs. Embryos of modern land vertebrates sketch out this history as they develop, for the lungs develop as out-pocketings of the digestive tract. Even if we could have been on the scene when the fishes developed lungs, we could scarcely have predicted the ultimate significance of

the invention. O n e of its consequences, perhaps easily

foreseen, was to make possible the vertebrates' invasion of the land, one of the most important events in the history of life. Some of the Devonian fishes with lungs gave rise to the amphibians, which in turn, by way of the reptiles, produced the birds and mammals. In another and more unexpected development the invention of lungs influenced profoundly the history of fishes themselves, for these sacs of air became transformed into the airfloat, and with the acquisition of this hydrostatic organ, fishes were able to improve their swimming equipment radically. Fishes thus equipped inherited the waters of the earth. T o d a y an overwhelming majority—more than ninety-nine percent —of the fishes in the class Osteichthyes either have the airfloat or are descended from ancestors that did have it. Only a small minority have lungs. It is this primitive minority that will concern us here. Presumably at one time all the bony fishes had lungs (or, to p u t it the other way around, probably

it was the evolution of lungs that

founded the group of bony fishes). But by p r o d u c i n g land-going

34

Fishes with lungs

amphibians as well as fishes with good airfloats, these primitive types assured themselves of almost unendurable competition, and survive today only as a dozen or so species in the outlying continents of the Southern Hemisphere or in the deeps of the ocean. Referring to the classification on page 12, it will be seen that the lung-breathers belong to the class Osteichthyes. The first subclass, Dipnoi, contains all the "true" lungfishes. These live in rivers of South America, Africa, and Australia. T h e second subclass, Crossopterygii (lobefins), contains a single recently discovered living species (there are many fossils), the famous Latimeria. This deep-water animal has lost its lungs, but, judging from a chain of evidence involving fossils, it came from ancestors that had them and is the nearest living relative, among the fishes, of the amphibians. In the third subclass, Actinopterygii (rayfins), only the most primitive living members, the bichir (Polypterus) and its relatives in Africa, have lungs. It is in the advanced rayfins that we find the typical airfloat fishes. T h e true lungfishes are more closely related to four-footed land animals than are the rayfins, although apparently not so closely as the lobefins. T h e most primitive of the lungfishes is, appropriately, found in Australia, the land of living fossils. This fish, called Neoceratodus, has paired fins on the ends of short stumps, somewhat as in the lobefins; in the other lungfishes, of Australia and Africa, these fins are much degenerate, and in some cases have changed into long, wispy sense organs. Also, the Australian fish is stout-bodied, of more normal fish shape than the slender, eel-like lungfishes of other continents.

Fishes with lungs

35

The Australian lungfish was known to early settlers long before scientists became aware of it. When it was discovered, its teeth were seen to resemble fossil teeth long known from Triassic rocks of Germany. The fish is a big one—five feet long—and the settlers took it for food by dynamiting the deep permanent water holes where it lives with the famous and primitive egg-laying mammal, the duckbilled platypus. Although it has a good set of lungs, their function is not yet understood. Some say that the fish surfaces at night to expel air with a hiss like that of a seal breathing. One observer who entered the controversy as to whether or not lungfish use their lungs noted cautiously that a specimen in an aquarium at the Queensland Museum did not surface for air between the hours of nine A.M. and five P.M. Perhaps it will be necessary to observe the behavior of the fish in water of different oxygen concentrations to get answers to the problem. Best known of the lungfishes probably are the African species, which belong to the genus Protopterus.

Some of these hibernate in

hard balls of clay, which can be shipped long distances. Put in water, the clay cocoon melts away and the fish comes to life. African lungfishes live in rivers and swamps from the White Nile to the Niger and the Congo. In the water the fish breathes with its gills, but in the dry season it can burrow into the mud and survive even when the mud is baked to dryness. T h e fish lines its retreat with a waterproof membrane of dried mucus. Its mouth is applied to a tube of this material which opens into an airshaft leading from the cocoon to ground surface. Drawing breath once every hour or so, the fish can survive in this state of dormancy for as long as three years.

36

Fishes with

lungs

One species uses the dry season as a time of reproduction rather than dormancy. It lives in swamps that do not completely dry out. Although the surface becomes dry enough to walk on, the ground water remains near the surface, and it is in this subterranean water that the fish lives. The male stays in a tunnel that opens to the surface through an airshaft. So stale is the water that the fish has to stay near the shaft and rely almost entirely on air-breathing. While the swamp is still flooded, the female lungfish lays her eggs in the tunnel, then escapes to the open waters of the river when the dry season comes. The male remains behind to care for the eggs and young. At first the young breathe by means of tadpole-like external gills in the aerated water near the airshaft, but soon take to air-breathing, like the adults. Unlike the lungfishes of Africa and Australia, that of South America—the loalach, or Lepidosiren—is

absolutely dependent on air,

and will drown if held under water. Before the great shallow swamps of the Amazon basin bake dry in June, the fish burrows deep into the mud, where it wraps its tail around its face and sleeps until the rains come again. This four-foot long, eel-shaped fish forces its way into mud and through dense vegetation with agility, feeding on snails and plants, and storing quantities of orange fat against the dry season. The female lays her eggs in a long tunnel dug in the bottom of the swamp. The male guards them, and is able to stay under water because he sprouts a set of gills—not the usual fish gills, but red filaments on the pelvic fins. The young loalach also has a set of external gills, in this instance on the head. People of the region hunt the loalach with spears, and the swamp alligator also preys on it.

Fishes with lungs

37

Some writers have thought that the present distribution of lungfishes—Africa,

Australia, South America—proves that these conti-

nents once formed a single land mass. T h i s argument fails to take into account the fossil lungfishes, which are found in the United States, Europe, and India. T h e three surviving lungfishes are merely outliers of a once world-wide group. T h e lobefins or crossopterygians were, in the Devonian, the dominant bony fishes (see Plate 13). T h e y range through rocks of the next 200 million years, to disappear from the record at the end of the Cretaceous, about 70 million years ago. It was thought that this group of heavy, ungainly fishes, equipped with stumpy paired fins prophetic of the legs of land vertebrates, had been for tens of millions of years extinct. But in 1938 a living lobefin, the Latimeria,

was

taken from deep water off Madagascar. T h i s living lobefin does not have lungs, but there is indirect evidence that the extinct lobefins were air-breathers and had lungs. T h e most primitive of the fossil amphibians are called labyrinthodonts from their peculiar teeth, whose enamel is crinkled into the dentine in

a complicated pattern. Extinct lobefins of a group called the

Rhipidistia also had such teeth, and this fact, together with certain characteristics of the skeleton, indicates that they probably were the ancestors of the Amphibia, which are, of course, equipped with lungs. Latimeria is in another subdivision of the lobefins, the coelacanths, a group that reentered the sea, where the lungs, if any, degenerated. But the coelacanths are much like the rhipidistians, so that biologists are intensely interested in Latimeria.

Its brain and heart in particular

38 Fishes with lungs closely match what biologists think the brain and heart of the extinct ancestors of the first land vertebrates must have been like. Since its discovery (the first known specimen had long been dead and was in poor condition) Latimeria

has been observed alive—a

magnificent steel-blue fish over five feet long, covered with large, circular, overlapping scales. Its eyes are very large, corresponding to the fact that it lives in dim light hundreds of feet down. It has escaped discovery for so long because it apparently does not come to the surface, and because the craggy rocks of the sea floor where it lives cannot be swept with nets. It is an axiom of biology that the Southern Hemisphere is a good place to look for primitive animals, and we are not disappointed when we search through southern rayfin fishes for a type resembling the ancestors of this group. T h e fish is the bichir, or

Polypterns

(a group name including about ten similar species), of Africa. The heavy scales, which fit together in mosaic fashion, resemble those of some ancient placoderms and of the Paleozoic bony fishes; there arc also primitive peculiarities of the skeleton. Most important, instead of the airfloat of the typical modern rayfin, the bichir has a pair of lungs. These are smooth inside, not folded as in the true lungfishes, so probably are not very efficient, lacking the large surface for absorbing oxygen that folding gives. As yet the function of these lungs in the life of the animal is not well understood.

5

Structures

concerned with swimming

W e have thus far reviewed the historical background of the structure of modern fishes by taking up the main groups of primitive fishes, both living and extinct. Before discussing the main groups of the modern fishes we will consider the basic structure of modern fishes and its functional meaning. One of the most important of the structures concerned with swimming is relatively inert and is hidden within the body of the fish. T h i s is the airfloat (also called the air or swim bladder). It transforms a heavy mass of flesh and bone into a weightless craft that soars in the water, one that is instantly responsive to the slightest movement of fin and swimming muscle. Anyone who has cleaned a fish knows the airfloat. Children like to pop this silvery bag tensely filled with air. It is situated in the top of the body cavity, above most of the other internal organs, a logical location for a buoyant organ. It is possible to calculate the volume of a float which would ex-

40

Structures concerned with swimming

actly balance the weight of the fish in the water—that is, of a float which would increase the total volume of the fish to the point where its specific gravity would be unity. A fish with such an airfloat would hang suspended in the water, neither rising nor sinking. Considering only fat-free tissues, the airfloat would have to occupy about seven percent of the volume of the fish. It is interesting but not totally surprising to find by actual measurement that the volume of the float in a variety of fresh-water fishes is about seven percent of the total. In the heavier salt water of the sea, we would expect that the float would be somewhat smaller; and it actually is, averaging about five percent of the volume of the fish. Large quantities of oils and fats also can appreciably lighten the fish, making for even smaller airfloats. When the fish moves to different depths, the changing water pressure tends to alter the size of the float: in deeper water the increased pressure squeezes the float into a smaller volume; nearer the surface the decreased pressure allows the float to expand. These changes in the volume of the float of course change the specific gravity of the fish, so that as it moves deeper it becomes heavier, or if it goes nearer the surface it becomes lighter. Obviously, if the fish is to keep its specific gravity or submerged weight constant, it must have some mechanism for adjusting the volume of the float. This is done by adding or removing gas (usually a mixture of nitrogen and oxygen, as in air, but sometimes mostly oxygen). Among living fishes there are three ways of regulating the volume of the airfloat. One can arrange these logically in a progression from crude to advanced, and this logical arrangement seems also to be their chronological sequence. T h e fishes of the kind now having the

Structures concerned with swimming

41

crudest method of regulation appear first in the fossil record, and those with the most advanced type appear last. T h e most primitive float is one which retains the windpipe that once served for air-breathing. T h e fish with this open float increases its volume by swallowing air, and decreases it by ejecting air through the duct leading to the gullet. T h i s kind of float has one or possibly two disadvantages: the fish presumably cannot regulate the volume precisely, and it has to come to the surface to get air for filling the float. T h e primitive open float occurs mostly in fishes that live in fresh water, particularly in streams, where accurate adjustment to depth is not overwhelmingly important because the water is relatively shallow. Also, many of these fish rest on the bottom and thus find it useful to be somewhat heavier than water. T h e trouts are a group of fishes that have the unmodified open airfloat, the chubs another. W h e n a fisherman grasps one of these fishes to take out a hook, it is likely to make a prolonged belching noise as air is forced out of the float through the " w i n d p i p e . " A good many of the fishes with the o p e n float have an improved model in which oxygen can be added while the fish is underwater. T h e oxygen comes from the water; it is picked u p by the gills, and is carried to the float by the circulatory system. Here a layer of blood capillaries secretes oxygen into the float and maintains the gas pressure at as high a level as necessary. Examples of fishes with this modified open float are the pikes and eels. In the pike the capillaries are poorly developed and fill the float slowly, but the eel's capillaries are well developed and are organized into a definite structure called the red gland. Presumably the fish with the modified open float can

42

Structures concerned with

swimming

regulate its specific gravity with some accuracy as it descends, and it is independent of the surface. Excess gas is ejected through the open tube leading to the digestive tract. Many—perhaps most—fishes have floats of the third type, the closed or "perfected" float. Here, in addition to the red gland that forces oxygen and sometimes nitrogen into the float, there is another group of blood capillaries called the oval that take gas out when the situation requires it. A ring of muscles around the oval can contract to close it off from the float or expand to expose it. T h e duct or relict windpipe, no longer needed, is discarded. A fish with this advanced float can adjust its specific gravity precisely at any depth: the sunfish, for example, hangs poised in the water without effort, with only a slight fanning movement of the pectoral fins to counteract the current flowing through the gills. The float in some of the deep-sea fishes is remarkable in that it contains gas at a pressure as high as 200 or 300 atmospheres. A steel tank of great strength would be required to contain gas at this pressure out of water, and obviously the only thing that keeps the deepsea fish from disappearing in an impressive explosion is the fact that it is surrounded by the crushing weight of water. It is only recently that the remarkable mechanism that maintains this enormous pressure, a device of interest to engineers, has become understood. T h e weakly p u m p i n g heart and the fragile blood vessels maintain the enormous differential in oxygen pressure—about one atmosphere in the water, a few hundred in the float—by applying the countercurrent exchange principle. When a tiny vessel bringing blood to the

Structures concerned, with swimming

43

float comes near the high concentration of oxygen in the float, it immediately becomes flooded with oxygen. This minute vein would take excess oxygen away from the float, causing it to collapse, were it not for the fact that the vein lies in close contact with an incoming artery. The vein loses its excess oxygen to the artery, which returns the oxygen to the float. The complex network of such veins and arteries in the wall of the float thus acts as a barrier to prevent the escape of the gas. Most technical advances have disadvantages, and this is true of the closed float. The red gland and oval add or remove oxygen slowly, so that if the fish comes up from deep water too quickly, the oval cannot take up enough oxygen; the float swells, and the fish becomes too light and maintains balance only with violent muscular effort. If the fish rises still higher, it turns belly up and is carried helplessly to the surface by the swollen float. The more primitive open float allows the fish to discharge air quickly through the duct, so that it can come rapidly from the depths to the surface without danger. The perfected airfloat thus represents a gain in sensitivity with a sacrifice of flexibility. Once adapted to a given depth, a fish with this modern equipment cannot swim quickly for more than a few yards toward the surface. Probably this is the reason why these fish, seen in deep clear water, often follow a lure upward for a distance, then turn back. The airfloat of some fishes is a hearing aid. Sound waves in the water cause the walls of the float to vibrate, and either an extension of the float itself or a chain of bones conducts the vibrations from the float to the ear.

44

Structures concerned with swimming

T h e airfloat also is used to make sounds. A good many species of fish make noises for courtship or to keep together in schools. Special muscles vibrate the float, or the fish grinds its teeth, with the float acting as a resonator. S o m e fish are noisy enough to be heard above water, but the extent of auditory communication by fishes was not realized until underwater listening devices were developed during the war in connection with the operation of submarines. T h e r e is a long-standing belief that fishes respond to changes in barometric pressure; the Germans call certain loaches

Wetterjische

because they leave their usual station at the bottom of a pond and c o m e to the surface at the approach of bad weather. T h e airfloat is a logical instrument for detecting changes in barometric pressure, at least for fishes that lie on the bottom without moving from one depth to another. T h e biological significance of a response to changing barometer is not known, but there is some evidence that low-pressure fronts are connected with insect activity, a matter of considerable interest to fish. T h e shape, structure, and arrangement of the

fins—the

external

indication of a fish's mode of s w i m m i n g — a r e in a general way correlated with the presence of and the type of airfloat. W e have seen that, in the sharks, the shape of the caudal fin and of the pectoral fin is an expression o f the fact that they are heavy fish, kept aloft by planing devices rather than hydrostatic devices. W i t h the gradual evolution of the airfloat, the ancient shark-like tail fin was redesigned. T h e b a c k b o n e which extended to the tip of the upper lobe was withdrawn and the upper lobe of the fin became equal in size to the lower, so that the fin became symmetrical. T h i s

Structures concerned with swimming

45

FIGURE 3 . Brook trout. T h e fins are (a) dorsal, (b) pectoral, (c) pelvic, and (d) anal. Behind the dorsal fin is the small, fleshy adipose fin characteristic of the trout group and some others but not present in most fishes. At the end o f the body is the caudal or tail fin.

evolutionary process was gradual, and many fossil types, as well as a few living ones, have tail fins more or less intermediate between the shark type and the modern fish type (see the tail fin of the alligator gar in Plate 9). When the trend toward symmetry was completed, producing the homocercal tail fin (see Plate 9 for an early record of this invention), the upper and lower lobes of the fin were equally flexible. As a result the fin no longer functions in giving lift, but only in contributing to forward speed, in smoothing out the swimming movements (a minnow with a badly injured tail fin has a ragged swimming motion, one imitated by certain types of artificial lures), and in reducing turbulence. T h e r e are several degenerate kinds of tail fin in relatively sluggish, bottom-dwelling fishes, but the active swimmers reveal two main types: the rounded, square, or slightly forked fin that occurs in most

46

Structures concerned with swimming

fishes (trout and pike, for example), and the crescentric fin of the tuna and its relatives. These are extremes that are connected by intermediate forms. T h e "high aspect ratio" (a wide span, compared to the area) of the crescentric fin means that the turbulence behind it will be low, that there will be little drag; but the narrowness of the fin means some sacrifice of power. Such a fin is well suited for fishes like the tuna and mackerel that cruise constantly at high speeds. The broad, nearly square fin of the trout delivers more power, although at the price of generating turbulence and therefore drag or resistance, and is good for sudden bursts of speed needed to catch other fish or to plow through swift currents. Pelvic fins were not much affected until comparatively late in the evolution of bony fishes. When the airfloat became highly perfected, so that the specific gravity of the fish could be adjusted with extreme precision, the pelvic fins were moved forward to near the "shoulder." This matter is taken up in more detail in connection with the spinyfinned fishes. T h e thickened, relatively rigid pectoral fins of such ancient and heavy fishes as the sharks act primarily as airfoils, thus compensating for the absence of the airfloat, but the thin, highly mobile pectoral fins of the bony fishes are used for braking and steering. They come into play when the fish suddenly turns or comes to a stop; when not in use, as when the fish is swimming forward, they can in some fishes be held flat against the sides, offering no resistance to the water. Sometimes the pectoral fins take on the main responsibility for swimming. In the mazes of coral reefs, where there is little clear track for the sweeping movements of the body-tail swimming motion of

Structures concerned

with swimming

47

most fishes, many of the inhabitants have large wing-like pectoral (ins that with sudden strokes drive the fish through the water in short dashes or propel it smoothly with rowing or flapping motions. The sculpins of fresh water and tide pools offer another example; these fish lie passively on the bottom, but when goaded into action, they dash ahead with a burst of speed, leaving behind a cloud of mud stirred up by the stroke of the large pectoral fins. A few fishes use the pectoral fins, not for swimming, but to keep the fish out of the water as long as possible. The flying fishes (family Exocoetidae) have pectoral fins comparatively as large as the wings of a bird, and on these outstretched flight planes they can glide for a quarter of a mile. It is a cheerful sight on an ocean voyage to see these sparkling azure animals scattering before the ship, like grasshoppers on the prairie. Once in the air, the flying fish can only glide. The high flight velocity, probably about thirty miles per hour, is built up as the fish swims near the surface. The peculiar tail fin is adapted to launch the fish at top speed: the lower half of the fin is enlarged, and as the body lifts clear, this part of the fin is left in the water to give powerful thrusts, like the propeller that drives a hydroplane. The ability to fly is obviously of great value to these fishes in escaping predators, but the swift dolphin fish ( C o r y p h a e n a ) that specializes in hunting them is said to track its prey by following the shadow. There is in the Amazon basin a flying fish, a three-inch-long characin, that really flies—not glides—a short distance by rapidly buzzing its wing-like pectoral fins. The dorsal and anal fins, on opposite sides of the body, confer

48

Structures concerned

with

swimming

stability, like the feathers of a dart. In some fishes—the American bowfin, for example—the dorsal fin is long and ribbon-shaped, and drives the fish slowly through the water by its rippling, undulating movement. In the remoras (see Plate 19) the dorsal fin has been greatly altered to form an adhesive sucker used to cling to large fish, ships, and sea turdes, thus making it possible for the remora to travel far without effort. Each spine of the fin has been split lengthwise and fused with its neighbor to make the suction cup. Natives in several parts of the world have learned to use remoras to catch sea turtles. Leashed with a line tied around its tail, the remora fastens on so firmly that the heavy, struggling turde can be hauled in successfully. T h e thin, overlapping scales of the modern fish (see, for example, the scales in the carp photo, Plate 3) are well adapted for swift, powerful, and supple swimming movements. It will be remembered that the ancient acanthodians were armored with a heavy mosaic of ganoid scales, which must have rendered their movements relatively ungainly. These heavy scales persisted into the class Osteichthyes, and all of the bony fishes of Paleozoic times had them. In the subsequent evolution of the bony (or airfloat) fishes, scales of the ganoid type were gradually replaced—most fishes of the early Mesozoic still had them, but by the end of the Mesozoic most had the modern type. One fish of Mesozoic times had ganoid scales on the main part of the body and thin overlapping scales on the flexible tail region, which was used to a greater extent in swimming. Although the scales of the modern fish are thin and appear delicate, they are surprisingly tough, being constructed of thin layers of

FINS Ostracoderm

AND

AIRFLOAT

stage

Reversed hetero c e r c a l tail f in. No p a i r e d f ins. N o a i r f loat.

Placoderm stage Heterocercal tail fin Several paired fins' N o air f I o a t . S hark stage

E a r l y r a y f i n stage ( t r o u t )

Advanced r a y f i n stage (bass)

Heterocercal tail fin . Two pairs of f ins. N o airf loat .

Homocercal tail fin Two pa irs of fins . Open a i r f l o a t .

Homocercal tail fin. Two pai rs of fins, both forward. Closed a i r f l o a t .

F l G l ' R F . 4. In these diagrams showing a sequence of major inventions concerned with the swimming apparatus, the median (vertical or unpaired) fins are shown with stippling, the paired (horizontal) fins with black, and the airfloat with diagonal lines.

50 Structures concerned, with

swimming

bone and crisscrossing fibers of horny material. The scales, which are produced in the deeper layers of the skin, grow throughout life, although unevenly at different seasons, so that expert examination of a scale can often give the age of the fish (see Plate 2). All fishes have slimy skins. The slime decreases friction between the body surface and the water, and bars infective microorganisms, especially fungi. The slimy skin and scales together are nearly waterproof and help to keep the body fluids from being fatally diluted by the surrounding water. Fishes rival birds in brilliance, but whereas the color of birds is lifeless, staining the dead feathers, in fishes it is living and changing. The living cells of the skin contain pigment granules—red, yellow, and black—which can flow out into an irregular set of channels in the cell or can withdraw into a single minute spot. Nerves that branch finely among the pigment-bearing cells control these movements. The fish then changes color "when necessary." This is usually done for concealment, but the Siamese fighting fish blazes into color as part of a good fight. Certain dark fishes, when put into a white aquarium, blanch in a few seconds or minutes. If such a fish is killed and a bit of its skin is fixed for microscopic examination, the black pigment granules are seen to be contracted. Some fish match color as well as shades of gray; a few even match the pattern of the background. Besides these quick chameleon changes, there are slower changes, extending over weeks, in which the absolute amounts of pigment in the skin are altered.

Structures concerned

with swimming

51

A white skin pigment called guanine produces the silvery sheen of the fish. The microscopic structure of the guanine layer may be such that it reflects blue light. In combination with the pigment cells, the blue light yields purples and greens.

6 Teeth and jaws

Fishes as a group eat nearly anything edible found in or even near the water, but most are carnivores, feeding on smaller animals that in turn live on the abundant plant life of inland and marine waters. T h e mobility, strength, speed, and highly developed sense organs of the fish give it an eminent place in the hierarchy of aquatic life. It is in the teeth and jaws that we find the most evident structural expression of the feeding habits of the fish. Both upper and lower jaws of such primitive types as the sharks are fashioned from the first gill arches (the bones that support the first pair of gills), but in modern types the original upper j a w has been pushed toward the midline to form the palate, and a new j a w is formed from toothed bones that developed in the skin of the upper lip. T h e palate usually keeps its teeth, and other parts of the gill arches that touch on the mouth or t h r o a t — t h e bones (hyoid) that s u p p o r t the tongue, and the pharyngeals—also may bear teeth. Compared with the other vertebrate animals, the fishes thus have a remarkable array of teeth.

Teeth and jaws

53

A fish that preys on animals of appreciable size has sharp, slender teeth that enable it to grip the victim before swallowing it whole. The pike, for example, has a formidable set of jaw teeth—sharp, bladelike structures set rigidly in the jawbone and capable of inflicting painful injury on a human being. It has, besides, smaller teeth set flexibly in the palate; these easily bend backward, allowing the prey to go in, but will not bend forward. T h e predatory appearance of this equipment is matched by the diet—not only other fishes, but ducks and muskrats which the pike attacks at the surface. Even the needle-like, inconspicuous teeth of the brook trout, which feeds mostly on insects and other small invertebrate animals, are effective: the fisherman learns of their existence from the crisscross of cuts he gets on his finger when removing lures from the mouths of his catch. T h e razor-sharp teeth of the piranha or caribe of tropical American streams are used to slice off bits of flesh, so that these fishes are able to prey on animals larger than themselves. Many aquatic animals have defensive stony armor—the molluscs and corals, for example—but the fishes have evolved dental equipment to cope with them. T h e triggerfishes, of tropical seas, have powerful chisel-like teeth that can pierce the shells of clams and oysters. Some of the cichlids, of tropical streams and lakes, crush molluscs with molars situated in the throat. Parrotfishes, of coral reefs, have powerful beaks, made by the fusion of the jaw teeth into a continuous cutting block that can snip off pieces of the solid reef. Other parrotfishes feed on seaweed, clipping off pieces with the jaws; then, using molar teeth in the throat, chew it with the thorough deliberation of a cow chewing her cud.

54

Teeth and jaws

A large and varied group, collectively called the tube mouths and including such fishes as the sea horses and pipefishes, have the mouth modified into a kind of pipette or medicine dropper, with which they inhale the prey by suddenly drawing in water. Besides making for an efficient suction apparatus, the shape of the snout enables them to poke the mouth into out-of-the-way places in search of food. If the prey is small and sedentary, the fish may not have teeth. T h e familiar carp and related minnows, so-called "leather-jawed" fishes, are toothless. At least there are no j a w teeth, but in the bottom of the throat are a set of crushing molars which work against the base of the skull. Herrings, which feed on microscopic plants and animals, are quite toothless, straining such food out of the water with fine, comblike structures (gill rakers) set in front of the gills. If the prey is large for the size of the fish, the jaws as well as the teeth may be intensively specialized. In the blackness of the sea floor, prey is scarce and not easy to find. Some of the predatory fishes living there have luminous lures to attract prey, or even have "searchlights," equipped with focusing lenses, to help find it; but, at best, finding a meal must be a rare event, and such a fish has j a w s and stomach capable of making the most of it. T h e gulpers, for example —fishes related to eels—have a skull only about the size

of one's

thumbnail, but this is perched on the tip of a relatively enormous pair of jaws that can gape open as wide as a human mouth. T h e very long, recurved, needle-sharp teeth hold on to the prey relentlessly, and it is eventually engulfed in the distensible stomach. Such an animal is little more than a bag adapted for getting itself around another fish.

Teeth and jaws

55

T h e f o o d s u p p l y of the fish that lives in small streams and brooks is supplemented b y land-dwelling insects that fly near the surface or fall from o v e r h a n g i n g shrubbery into the water. In feeding on these, the dash and alertness of the trout comes into play when it leaps to capture a l o w - f l y i n g insect or darts after floating or drowned insects that swirl by in the current. A remarkable little fish of the Orient captures streamside insects in a quite different manner. T h e archer fish (Toxotes) knocks its prey off l o w branches b y ejecting a drop of water from its mouth with a velocity and accuracy sufficient to bring d o w n an insect three feet away. S o m e archer fish kept in a garden p o n d were said to quench lighted cigarettes held by persons sitting nearby. O n c e the fish gulps d o w n its p r e y — u s u a l l y i n t a c t — t h e powerful action of enzymes and acid soon liquefies the food. Fishes have the usual vertebrate plan for the digestive tract: a muscular, churning stomach w h i c h secretes hydrochloric acid as well as enzymes, followed by an intestine (longer in fishes that eat plant material) that secretes additional digestive enzymes and absorbs the nutrients. T h e most primitive of the rav-finned b o w f i n — h a v e in

fishes—sturgeons,

garpikes, and the

the intestine a structure called the spiral valve

w h i c h lies in the short, straight intestine like a spiral staircase. A similar arrangement exists in the sharks and lungfishes. For some reason the spiral valve has been abolished in the more modern fishes. M a n y of these last have a cluster of blind p o u c h e s or caeca attached to the front end of the intestine w h i c h may serve to accumulate digestive secretions.

7 Breathing

Hardly any part of the land environment is made uninhabitable for vertebrates by lack of oxygen, since this gas is invariably present in the air by the amount of twenty percent, and since it quickly diffuses into places where it is being used up. In the water, by contrast, dissolved oxygen is often so scarce that water-breathing fish find it difficult or impossible to live. Dissolved oxygen diffuses through the water very slowly, so that the fish must rely on or create strong currents of water to bring in fresh supplies of oxygen. A cold, foaming mountain stream may have seven or eight cubic centimeters of dissolved oxygen per liter of water, and a warm, quiet stream only three or four cubic centimeters. Fish are often adapted to specific conditions of oxygen concentration. Trouts, for example, will barely survive at oxygen concentrations of three cubic centimeters per liter, but carp will thrive in such water. Streams filled with organic wastes, as from sewage disposal systems, are likely to be deficient in oxygen because the bacteria decom-

Breathing

57

posing the wastes use up the oxygen; at night, in particular, photosynthesis by green plants does not make good the oxygen deficiency, and fishes may suffocate in such water. In the winter a thick blanket of snow over the ice may black out the green plants in the water so that oxygen is not replaced as fast as it is used, and the fishes then suffer winter kill. T h e typical water-breathing fish breathes by drawing water over the feathery gills, which provide a large surface through which oxygen can diffuse into the blood. A flatfish two feet long, for example, has a gill area of about ten square feet. There are usually four gills on each side of the throat, hidden from view by the gill covers. T h e working parts of the gills, the parts through which oxygen enters the blood, are the

filaments—delicate,

blood-red structures, usually in

two rows on each gill. Each filament has its surface increased by minute leaf-like projections above and below. T h e filaments stream out behind the gill in the current of water that flows backward through the gills. T h e same countercurrent exchange principle that is used in the airfloat expedites the transfer of oxygen from water to blood. The blood that is pumped to the gills has been depleted of oxygen in the body tissues. It flows forward through the gill filaments, in the direction opposite to that of the stream of oxygen-laden water. When water is experimentally forced through the gills in the same direction as the blood flow, the gills can pick up less than ten percent of the oxygen in the water, instead of the usual fifty percent. The fish pumps water through the gills by means of muscular movements of the gill covers, which are constructed of flat bones that slide over one another, allowing the covers alternately to bulge

58

Breathing

outward and flatten. As they bulge, water is sucked in through the mouth and into the gill

chamber, the chamber meantime being

closed off behind by a flap of skin so that the water will not come in the wrong way. T h i s flap (the gill-cover membrane) slides easily over the body and keeps a tight seal. When the covers flatten, the water, kept from running forward by the arrangement of the gill filaments or by flaps at the front of the mouth, forces open the gill-cover membranes and escapes to the outside (Plate 8 illustrates the breathing movements of the Northern pike). Beyond the gills and on into the body tissues the blood flows sluggishly, since it has been held up by the elaborate system of blood capillaries in the gills. T h i s situation has been remedied in the more advanced vertebrate animals, whose oxygen-absorbing capillaries are in a separate circuit, not interposed between heart and body tissues. Upon its return to the fish heart the blood, spent of oxygen, is received into two thin-walled, lightly muscled chambers arranged in sequence. T h e s e step up the pressure of the blood so that it can be forced into the heavily muscled ventricle, which is the effective pump of the circulatory system. It drives the blood into the gills. In the deep waters of the Antarctic seas are the only known vertebrates that have colorless rather than red blood. These fishes, of the family Chaenichthyidae, a group related to the perches, lack red blood cells. Apparently the chaenichthyids are sluggish, not requiring much oxygen, and the surrounding water is saturated with oxygen, which makes their respiratory problem relatively simple.

8

Reproduction

Modes of reproduction vary widely. Some of the common oceanic food fishes produce enormous numbers of eggs and large quantities of sperm or milt, then simply gather in shoals during the spawning season and discharge eggs and sperm simultaneously. T h e effort here is mainlv physiological: there has been great investment of nutrients in producing the sex cells, but little courtship activity has occurred, and no care is given the young. T h e eggs—25 or 30 millions for a ling (a relative of the cod), 6 or 7 millions for the cod—are naked cells, almost invisibly small, made buoyant by the one or more microscopic droplets of oil they contain. These eggs float in the ocean currents at the mercy of a host of minute carnivores; there is high mortality, and the odds are millions to one against a fertilized egg developing into a mature fish. Other fishes have elaborate courtship procedures, and in one way or another care for their young so that the few offspring have a good chance of survival.

60

Reproduction

Rather highly evolved methods of getting the young off to a good start in life are found even in such supposedly primitive fishes as the sharks. Of the sharks that lay eggs, nearly all produce large ones— a few inches to a foot long—that usually are protected by a tough, horny shell. Most sharks use a quite certain method of producing young that are well developed before facing the world on their own: the female retains the eggs in her body until they hatch, and gives birth to the young alive. If the procedure is as simple as that just described, the shark is said to be ovoviviparous. But most sharks have some arrangement for giving the young more food than that supplied by the yolk of the egg; such a shark is viviparous. In some viviparous sharks the walls of the uterus secrete a kind of milk. T h e young shark either absorbs this nutrient fluid through blood vessels or drinks it. The long secretory filaments growing out from the walls of the uterus may actually grow into the throat of the young. Other livebearing sharks have a parent-embryo relationship not unlike that of the mammals. The yolk sac of the embryo (which at first absorbs the egg yolk) grows into the uterine walls to form a placenta that extracts nutrients from the blood of the mother. With such advanced methods of prenatal care, the number of young can be small, and some sharks produce only seven or eight young at a time, in contrast to the millions of eggs laid by some of the otherwise more advanced fish types. As was pointed out in a previous chapter, sharks fertilize the eggs internally by means of a penis-like structure formed from the pelvic fins. Details of the courtship behavior of sharks are little known. Sharp spines on the large pectoral fins of the males of some rays indi-

Reproduction

61

cate that there may be combat between the males; it is possible that the sexual behavior of some sharks is complex. For the most part unknown except to the specialized observer of aquatic life, the fishes of our streams and lakes each year engage in nest-building activities that rival those of birds. Most of the fishes of inland waters—many thousands of species—are nest-builders. Usually the nest is quite simple—analogous, say, to that of such ground-nesting birds as the sandpipers or plovers—and is merely a hollow cleared out of the stream bed. If the eggs are left in an exposed nest of this kind, one of the parents, usually the male, guards them. Often the eggs are not attended, but have been covered with coarse gravel or stones which protect them from enemies and at the same time allow water to circulate past the eggs and bring oxygen to the developing embryos. Chubs and shiners often make nests of this type. The familiar sunfishes (Centrarchidae) nest in shallow sandy shores, where the open nests, each guarded by a brighdy colored male, dot the underwater landscape like craters on the moon. The tiny sticklebacks of northern streams do as well as any bird. The male works for days gluing bits of vegetation together (the glue is a special secretory product of the kidneys) to form a small roofed nest. He then strenuously courts a female, finally urging her into the nest, where she deposits a few eggs, which he fertilizes. He hunts up another female, then another, until the nest is filled. The eggs, and even the

young fish for some time after hatching, are carefully

guarded by the male. The several hundred species of cichlids (a group that replaces the sunfishes in tropical fresh waters) all care for their young. In one

62

Reproduction

species both male and female work to hollow out a basin in the sand. Apparently this nest serves only as a stimulus for the stages of mating and fertilization, for as soon as the nest is filled with eggs, the male takes them out to store them in his mouth until they hatch. He cannot eat during the incubation period. The female of this species shows the mouth-brooding behavior in a rudimentary way, since she picks up a few of the eggs; but she is inexpert, and forgetfully swallows them. In some other species of cichlid the ritual is the same, except that it is the female who picks up the eggs and broods them in her mouth. Fishes that live in the sea usually do not build nests, a fact which might reasonably be related to the circumstance that there are few places to rest such a structure in the open ocean. Many of the inshore species do construct nests or use crevices. Some have, however, evolved interesting devices for protecting the eggs. In the pipefishes (relatives of the sea horses), for example, the female, during the mating embrace, forces her eggs into a pouch on the underside of the abdomen of the male, who carries them about until they hatch. The young may return to seek shelter in this pouch. The male sea horse also cares for the eggs: his pouch is even a sort of uterus, since its lining furnishes nutrients for the developing embryo. Here the male truly bears the young; the only contribution of the female is to insert the eggs. Motion pictures of the birth of the young sea horses give the impression that the male is put to great inconvenience by the process. As we have seen, most of the sharks are live-bearers. One would expect that the highly evolved bony fishes also would have perfected

Reproduction

63

methods of bearing the young alive, but the overwhelming majority have not. A few marine species, scattered through several unrelated groups, are viviparous, as is one entire fresh-water family, the Poeciliidae or live bearers. T h e y are f o u n d in fresh waters as far north as the upper Mississippi valley and as far south as Argentina, but are especially numerous and varied in the tropics. They furnish a large number of aquarium fishes—the guppy is one—whose bright colors and courtship antics make them favorites. Since they are viviparous, fertilization is necessarily internal. T h e anterior edge of the male's anal fin is more or less rigid and lengthened; this structure is used to place packets of s p e r m , w h i c h h o l d together in the water, in the reproductive opening of the female. T h e females give the impression of being shy and evasive, but there are different degrees of the expression of this trait in different species. Someone has made the generalization that the modified fin of the male is longest in those species with relatively unresponsive females and shortest in those with cooperative females. In the oviparous majority the eggs are naked, and are fertilized after deposition. T h e male sheds sperm over the eggs immediately after they are laid, and the spermatozoa swim the short distance through the water to the egg. T h i s method of fertilization is effective; it was once erroneously thought that

external fertilization under

natural conditions yielded only a small percentage of fertile eggs. Even when fertilization is external, courtship and mating activities may be as elaborate and intense as in those fishes with internal fertilization.

9 The senses

T h e fish dominates most of the aquatic animal life about it, not only on account of its superior size and strength but also because of its large central nervous s y s t e m — b r a i n and spinal c o r d — a n d

its re-

markable array of sense organs w h i c h give a detailed and far-reaching awareness of its surroundings. M o s t noticeable of the sense organs are the eyes, large structures w h o s e constant movements impress the observer with the alertness of the living fish. In its basic structure and d e v e l o p m e n t the eye of the fish is like that of the land vertebrates: a transparent lens, derived from the skin, focuses on a curved sheet of nerve cells, derived from the brain, that are sensitive to light. T h e main difference concerns the manner of focusing. T h e human eye changes the curvature of the lens; the fish eye changes the distance between lens and retina, like a camera. In the relaxed state the fish eye is in focus for near objects. W h e n distant objects are brought into focus, the lens is pulled closer to the retina.

The senses

65

Although the visible world of the fish of deep water is close around it, because of the limited transparency of water, the sense of vision is usually important and in many fishes is keen. Fishermen maintain that even slight differences in the patterns of artificial flies are detected by trout. For many years some ichthyologists ridiculed the fisherman's concern for color in flies, thinking that fishes were color blind. In recent years, however, the fact of color vision in fishes has been well established. There is indirect evidence in the brilliant colors that males display in courtship—the females must be able to see them. Some fishes are able to change the color of their skin to match that of the background. Also, fishes can be trained to come to certain colors for food. By using colors of various intensities, the experimenter rules out the possibility that the fish is only discriminating among shades of gray. There is rather direct evidence from the microscopic anatomy of the eye. In the retina of fishes living in brighdy lit waters are structures called cones, and the cones of the human eye are known to be responsible for color vision. So far as known, the sharks lack cones in the retina, so presumably lack color vision. One of the most characteristic sense organs of the fish—it occurs nowhere else in the animal kingdom except in some of the amphibians—is the lateral-line system. T h i s system is composed of two tubes that run the length of the body, one on each side, and branch in a more or less complicated way over the head. T h e tubes lie just under the skin, and open to the exterior through a number of short canals which pierce the overlying scales. Each tube is filled with mucus and, according to some authorities, contains clusters of sen-

66

The senses

sory cells so arranged as to be stimulated by slight movements of the mucus. T h e r e has been some controversy about the microscopic structure of the system; perhaps the above interpretation will turn out to be incorrect. In some experiments designed to determine the function of the lateral-line system,

pikes were blinded to eliminate the sense of

vision. If a dead minnow was carefully placed near the head of the blinded pike, the pike remained indifferent, since these fish do not feed by smell; but if the m i n n o w was moved, the pike swung its head sideways and unerringly snapped u p its prey. An operation designed to abolish the lateral line, one in which the nerve that parallels it was cut, destroyed the ability of the pike to respond to objects moving in the water. Another blinded fish, a species of minnow, turned and snapped at a slender glass rod (presumably odorless) which was moved a fraction of an inch near its side, and followed accuratelv when the rod was drawn through the water. Blinded fish of at least some species learn to swim easily in the aquarium, avoiding obstacles, but cannot do so if the lateral-line nerve is cut. Apparently pressure waves created by the swimming movements are reflected off surrounding objects and detected by the lateral line. Experiments like those described indicate that the lateral line is able, by its sensitivity to pressure waves, to inform the fish of events that take place in the water some distance away. Even more alien than the lateral-line system is a sensory system recently discovered in some of the fishes that are able to generate

The senses

67

electricity. T h e electric eel, certain African catfishes, and some oddshaped African fishes known as mormyrids can deliver noticeable or even powerful electric shocks. T h e shock obviously serves to defend the fish, and sometimes it is potent enough to stun prey. It now turns out that there is a third use for the electrical generators in these animals. W h e n an electric eel, for example, lies still, the generating tissues are inactive, but when the animal moves, it generates pulses of low-energy electricity. In this and some of the other electric fishes it has been shown that these pulses can serve as a navigating device, notifying the fish of nearby objects in the water. W h e n a conductor, such as a piece of copper wire, is dipped into the water, the behavior of the fish alters. Evidently there is some sensory system which is able to detect changes in the electrical resistance of the surrounding water. W e know the source of the electric current involved, but the sensory system which detects the currents is still unknown. It is possible that the pulses serve for communication between individuals as well as for navigation. T h e s e electrical detection systems are of some theoretical interest because they may answer a question which opponents and even sympathetic partisans of evolution theory have asked about the origin of the electric shocking mechanisms of such fishes as the electric eel. How could this defense mechanism have arisen by gradual accumulation of small variations, they ask, since no one can imagine any effective defense being provided by the very weak currents which are all that evolution could, have had to work with in the beginning? However, just such weak currents are used in detection systems, and it may well be that the ability to generate electricity originated as part

68

The senses

of the sensory apparatus and, only after it had been brought to appreciable strength in relation to this function, was raised by natural selection to the astonishing power shown in the electric eel. T h e sense of taste, in both fish and man, distinguishes only a few qualities, while that of smell distinguishes a seemingly endless number of qualities. A person with a cold finds that the world of food, for example, is not a very complex one when his sense of smell is eliminated. Organs of taste and smell are alike in that both contain thin-walled sensory cells that are stimulated when soluble molecules come in contact with them. One obvious difference between the two sense organs is that those for smell are in the nose, those for taste are elsewhere; in fishes, oddly enough, the taste organs may not even be in the mouth. Another difference is that the nerve cells sensitive to odors connect directly to the brain by way of a slender fiber, while the cells concerned with taste require another cell, a nerve cell, to conduct the information to the brain. Some fishes have a poorly developed taste sense, with taste buds only well back in the mouth, where they make it possible to test food before it gets into the digestive tract. A pike snapped up worms soaked in quinine, but quickly spat them out. Other fishes have taste organs on the lips and on soft barbels that project from the head around the mouth, and the carp has taste organs scattered over the general body surface. Such organs assist in locating and identifying food that is grubbed out of the bottom mud, for example. T h e "nose" of the fish consists of two separate pouches, one on each side of the head, that have no connection with the mouth or

The senses

69

throat. Usually each p o u c h has two nostrils (shown in the photograph of the carp, Plate 3), which allow a current of water to flow past the sensory cells lining the cavity. A fish such as the pike seems to rely little on its sense of smell: it will pass by a cloth sack containing food; but an eel, with a keen sense of smell, is attracted to the sack and tries to get at the food inside. So keen is the sense of smell in the salmon that it is able to recognize by the odor of the water which of the innumerable tributaries it passes on its way upstream is the one where it was born. This homing ability, as shown by experiments that approximate the natural situation, is abolished when the nostrils are plugged. This sort of sensory ability must concern odor, not taste, because it resides in the nose, it detects exceedingly small amounts of dissolved substances, and it discriminates among many kinds of substances. T h e ear of the fish, like that of man, has two functions: balance and hearing. These two senses, which would seem to be quite unrelated, are both based on the same kind of apparatus: fine sensory hairs projecting into a cavity filled with liquid. T h e hairs, when moved by currents in the liquid, stimulate nerve cells that lead to the brain. In the organs of balance, changes in the position of the head swish currents of liquid against the sensitive hairs; in those of hearing, compression waves in the air or in the water are converted into compression waves in the liquid inside the sense organ, which cause minute movements of the hairs. T h e sense organs in the ear, then, may be basically like the lateralline system, since both consist of liquid-filled channels and movable sense organs. And it is likely that both sets of sense organs in the

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ear—the semicircular canals, which function in balance, and the sacculus, which functions in hearing—came originally from part of the lateral-line system that branches over the surface of the head. Some of these canals presumably sank deeper into the skull where, isolated from the influence of currents washing against the head of the fish, they could take on their new roles. Each of the three semicircular canals is placed at right angles to the other two, so that if the head moves in any direction, the liquid in one or more of the canals is set in motion and stimulates the sensitive hairs situated in swellings at the ends of the canals. Since the fish moves in three dimensions, the sense of balance is important, and the cerebellum, the part of the brain which coordinates the information coming from the organs of balance, is well developed. T h e sacculus, the organ of hearing, is divided into three chambers. Suspended inside these cavities are curiously ornamented enameled bones, the otoliths or " l u c k y " bones. Their shape differs from one group of fish to another, so that the expert can often tell what kind of fish an otolith came from. As the fish grows, the otolith increases in size by adding new layers of bone, and in cross-section shows rings like those of hailstones. Sometimes one ring corresponds to a year's growth. T h e lowermost of the three chambers is the one that in mammals is drawn out into the coiled, snail-shaped cochlea. Fish do not have this elaborate structure, and lack the mammals' fantastically keen sense of hearing. But this cavity is sometimes an important part of the hearing apparatus. Laboratory experiments show that fishes have a rather good sense of hearing; even noises made in the air are trans-

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mitted through the water sufficiently to be heard by some fishes. Fish can be trained to begin searching for food when a whistle blows some distance away from the aquarium, and can even be trained to distinguish pitch, to tell one note from another. It is likely, however, that in nature the fish uses its ears mostly to detect vibrations that originate in the water or in the earth of the riverbank or shoreline. The fishes with the best sense of hearing appear to be the carp, catfishes, shiners, and their relatives, whose specialized hearing apparatus will be discussed in a later section.

10 More living

fossils

Before we reach the main body of the advanced, modern types, we encounter a few outliers in the rayfin group that remind us of the extinct and primitive fishes that have been described already. After the float was invented, the ancient upturned tail fin and the cumbersome ganoid scales were discarded, but only gradually; at least, it was a long time before the fishes without these primitive traits became the majority. T h e float probably appeared in the late Devonian, and not until early Tertiary times, about 150 million years later, had the majority of fishes, for example, discarded the ganoid or mosaic scales. T h e few survivors of the ancient types, fishes that have the float but also the anachronistic upturned tail fin and ganoid scales, live oply in the Northern Hemisphere. This is in odd contrast to the distribution of the living relics of the older " l u n g " stage of the evolution of fishes, which are all confined to the Southern Hemisphere. The living representatives of the more recent stage in evolution are the garpikes, the bowfin, and the sturgeons and their relatives.

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Sturgeons are great shark-like animals, the largest fishes of inland waters. Like the sharks, they have a naked, fine-grained hide (except for widely separated rows of heavy guard scales), an upturned (heterocercal) tail fin, and a mouth placed well back on the underside of the head (see Plate 10). In keeping with the shark-like tail fin, they swim like a heavy fish, with continuous sweeping movements, and do not rest suspended in the water. Although they have an airfloat—it is the source of commercial isinglass—the sturgeons are heavy fish, with a specific gravity higher than that of water. There are also shark-like features of the internal anatomy—the spiral valve of the intestine, and the cartilaginous skeleton. The several species of sturgeon are like salmon in that they spend part of their lives in the ocean or in large lakes, and part in rivers, where they spawn. It is in the great rivers that drain into the Caspian Sea and in the Columbia River that the largest specimens have been caught, twelve or thirteen feet long and weighing over half a ton. Once they were so abundant in the Great Lakes that caviar—sturgeon eggs—was given free at beer counters to make people thirsty. Now the fish has been nearly exterminated in the Lakes, and smoked sturgeon is a luxury sold by the quarter-pound. In spite of its size, the sturgeon is not a formidable predator. The small, toothless mouth, resembling that of a sucker, is used to inhale small animals from the bottom. Four sensitive barbels or "feelers" hanging below the head help to locate food. The sturgeon in an aquarium often swims a short distance above the bottom, dragging the tips of the feelers as it goes. The paddlefish ( P o l y o d o n ) of the Mississippi is named for the large

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flattened snout, which is furnished with sense organs that assist in finding concentrations of its prey—microscopic animals suspended in the water that are strained out by the fine gill rakers. Several fishes of the ocean have solved the problem of extracting the abundant microscopic animal life from the water, but the paddlefish is the only river fish of great size that has done so. The Oriental equivalent of the paddlefish is Psephurus,

of the

great rivers of China, said to reach twenty feet, although the largest measured specimens appear not to exceed twelve feet. Garpikes are slender, cylindrical fishes of the fresh waters of eastern North America and of Central America. They range in size from the four-foot gar of the north to the great alligator gar of the southern United States, which grows to nine feet. When the dinosaurs flourished on the land, relatives of the garpikes dominated the seas and inland waters, and had the diverse forms and habits that have been taken over by more modern fishes. The tail fin of the garpikes is intermediate between the upturned type and the modern symmetrical fin (see Plate 9), with the backbone continuing some distance along its upper margin. So heavy is the mosaic of ganoid scales that the hide of the alligator gar is said to turn an ax blade, and the diamond-shaped scales were used by Indians for arrow points. Only in the garpikes are ganoid scales to be found among living North American fishes, although such scales are common enough as fossils. T h e very

long, slender jaws of the garpike are set with large

needle-shaped teeth. T h e fish, camouflaged by its resemblance to a floating log or stick, sidles up alongside its prey, then strikes with a sudden sidewise slash of the head. Its slender jaws allow it to steal

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bait easily from a hook, and anglers catch garpikes by threading the bait on a wire noose which tightens on the jaw of the fish. T h e backbone of the garpike is in one way more like that of reptiles than of fish. T h e usual fish vertebra—as seen in a canned sardine, for example—is hollowed out on each end like a candy mint, but the garpike vertebra is convex in front and concave behind, so that each moves on the next by means of a strong ball-and-socket joint. A survivor of a group that once occurred also in Europe, the bowfin now lives only in the Mississippi basin and north and east to the Great Lakes and Vermont. It is more advanced than the garpikes in that the ancestral ganoid scales have been transformed into a flexible, light covering of rounded and overlapping scales. It shares with the garpikes the primitive spiral valve in the intestine. Although the tail fin is almost symmetrical, the upturned remnant of the backbone is still evident. Extending more than half the length of the body is the low, ribbon-shaped dorsal fin, which with rippling movements drives the fish slowly through the water. When the water is too foul for gill breathing, the bowfin comes to the surface for air, using the spongy, lung-like airfloat for respiration. In the spring the bowfin clears out a large nest in reeds standing in shallow water. T h e few thousand creamy yellow, sticky eggs are guarded by the male. After the young hatch, the male continues to care for them, swimming about in a circle to keep them from scattering and fending off other fishes. When frightened the male dashes away with a sudden sweep of the tail which stirs up a concealing cloud of mud and sends the young into hiding at the bottom.

11 Trouts and their relatives

All the groups of fishes so far dealt with, although extremely varied and of interest because they are relics of a long early period of fish evolution, together make up only a small fraction of the species now living. About ninety percent of the 2 0 , 0 0 0 kinds of fish in existence belong to a group technically called the Teleostei. T h e teleosts belong to a larger group, the Actinopterygii (see page 12), or ray-finned fishes, which may be divided into three groups: Group 1. Chondrostei: bichirs, sturgeons, and their numerous extinct relatives. Group 2. Holostei: garpikes, the bowfin, and many extinct types. Group 3. Teleostei: all remaining fishes, ranging from the herrings and their relatives at the more primitive end of the spectrum to the perches and related types at the other. O f the fishes listed above, the bichirs are the most distinctive, and sometimes are put in a separate group of their own; they were taken up here in the early chapter on ancient air-breathing fishes. T h e pre-

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ceding chapter included the sturgeons and all the Holostei. It is to the teleost fishes that we now turn our attention. Although our formal classification files them away under a single low-ranking division at the tail end of the series, the teleosts not only far outnumber the rest of the fishes, but are the most diverse of all groups and have taken over ways of life that were the domain of more primitive types. T h e catfish grovel on the bottom like the ancient ostracoderms; the tuna disputes the high seas with the great pelagic sharks; the mudskippers may well be better air-breathers than the ancient lungfishes. T h e teleosts have invaded highly varied environments: Arctic ponds that thaw for only a few weeks of the year; hot springs of the desert; the deep sea floor. Some have even learned to glide in the air to escape their enemies. T h e teleost fishes began to dominate the aquatic scene in the Cretaceous, a time when on land the dinosaurs had reached the climax of their evolutionary history. With the appearance of the teleosts their more primitive relatives, which were related to the garpikes and bowfins, gradually dwindled in number and were pushed permanently into the background. In the span of some 60 million years the teleosts burst into the many thousands of varied types that now swarm in the waters of the world. T h e classification of this extensive and varied group of animals is, as might be expected, a difficult matter. Most authorities divide the teleosts into a large number, twenty or more, of major groups or orders; hardly any two authorities agree on the number of orders, their names, or their contents. T h e method used here to give some idea of the main structural

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types within this remarkably diverse group of animals is to center the discussion on the three largest orders: 1. Order Clupeiformes, including the herrings and trouts. 2. Order Cypriniformes, including minnows and catfishes. 3. Order Perciformes, including perches, bass, and many marine types. These three orders include most of the fishes in existence; the first and last also illustrate two widely different and important modes of structural organization. T h e first order, especially in structure of fins and airfloat, is relatively primitive, and the early teleosts were mostly members of this order. One would think that the trout, vibrant with energy and able to cope with a foaming mountain stream, would be of a most advanced type, but this is not so, and as a good example of the first and most primitive order we may select this fish. T w o of the primitive characteristics of the trout concern the fins. First, they are soft. One can pick up a trout without fear of being stabbed by sharp fin rays; a sunfish (one of the perciforms) has rays that can cut to the bone. T h e significance of the spiny fins, which involves more than simple defense, will be taken up later. Secondly, the hind pair of fins of the trout, the pelvics, are placed where "hip"' fins belong, well back on the body, in the position of the hind legs of a lizard or salamander. In advanced types, again taking the sunfish as an example, the pelvic fins have moved forward to a position near the shoulder fins. Another primitive feature of the trout is internal. It has the open airfloat, with a duct leading to the throat, and lacks a well-developed

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red gland. Presumably the adjustment of its specific gravity is only approximate, so that the fish is probably slightly heavier than the water and lies on the bottom when at rest (see Plate 22). Although the trout reminds us of rushing mountain streams and forests of conifers, it actually is tied closely to the ocean, both by habit—most species of trout, if the way is clear, go to the sea for part of their life—and by the fact that some of its nearest relatives, the herrings, are marine. In the ancient Cretaceous sea that flooded the midland of North America lived the giant herring Portheus. Skeletons of this fish, up to fifteen feet long, have been quarried from the gray shales that flank the Smoky Hill River in western Kansas. One specimen has under its ribs the skeletons of some good-sized fish it had eaten. T h e modern herrings (the generic name of one of them, Clupea, gives the name to the order Clupeiformes) are small

fishes—besides

the true herring, the anchovies and sardines belong to this g r o u p but are very abundant, and are taken from northern seas by the billions of pounds each year. T h e menhaden, a foot-long herring of the North American Atlantic coast, is mined at the rate of one billion pounds a year to be converted into oil and fish meal. This fish is an unusually efficient mechanism for converting otherwise inaccessible resources of the ocean into a form that people can use. T h e energy of the sunlight that falls on the sea is first trapped by microscopic plants, mostly diatoms, that store droplets of oil. T h e usual route from diatom to man is by way of small animals that graze on the diatoms and are in turn eaten by fish large enough to be taken with nets. Since the small intermediate animals use a good deal of energy

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Trouts and their relatives

in their own affairs, man comes out short in the transaction. T h e menhaden is one of the few fish that strain out diatoms directly, thus bypassing the intermediates. T h i s incredibly abundant animal has a long history of usefulness, for it was the fish generally used by the Indians to fertilize their cornfields. Some of the herrings have small buoyant eggs that drift in the currents of the ocean; some migrate to shallow water to lay their eggs in seaweed. Yet others leave the sea to deposit their eggs in rivers. T h e best known of these is the shad of the eastern United States, a giant of the tribe that reaches a weight of six pounds. Another herring of the so-called anadromous type—one which leaves the sea to spawn in fresh water—is the alewife. A large population is found in Lake Ontario, where it may have been introduced by man. Making its way through the canal around Niagara Falls, it reached the upper Great Lakes. Along with the sea lamprey, which invaded by the same route, the alewife—still with no good use found for it—clogs the nets of fishermen with its numbers. T h e aristocrats of the herring group are marked by the adipose fin, a short, thick structure on the back between the dorsal and tail fins which lacks supporting rays. T h e y live in cold, oxygen-rich waters; usually 6 0 ° F. is near the upper limit of their tolerance. T h e trout fisherman who explores the streams of a doubtful region recognizes the frigid bite of the trout stream and, as he wades it, sees the trout darting to shelter with their familiar swiftness. T h e r e are three royal families: the whitefishes (Coregonidae), the graylings (Thymallidae), and the trouts and salmon (Salmonidae). T h e s e three are so similar that some authorities put them in the single family Salmonidae.

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T h e whitefish group is especially well represented in the Great Lakes region, where there are many species. Part of their diversity here results from the fact that populations were left isolated in lakes as the northern ice sheet retreated. Left to themselves, the populations went their own evolutionary way, becoming distinct from their neighbors, much as small isolated groups of primitive peoples develop their own dialects or languages. Thus, the Ives Lake cisco (a small whitefish) occurs only in the deep waters of a single lake in the Huron Mountains of Michigan, another species only in Siskiwit Lake of Isle Royale. These small coregonids of the Lake region, all belonging to the genus Coregoyius, go by a variety of common names—herring, chub, cisco, tullibee, kiyi. One trait common to all these fishes is that they have a delicious flavor when smoked. T h e true whitefishes are more massive, averaging two feet long and becoming as heavy as twelve pounds. They live in deep water, where they are caught in deep-set gill nets. A combination of over-fishing, the sea lamprey, and perhaps additional unknown factors has almost eliminated the once important Great Lakes industry based on the whitefish. T h e most brilliant of the royal group are the graylings

(Thymal-

lus), of which there are about a half-dozen species in North America and Eurasia. T h e grayling has an uncommon reputation for delicacy. Fresh-caught specimens smell like thyme (this is the origin of the scientific name), and the French call it the "fish that feeds on gold." T h e great sail-like dorsal fin and the body are varicolored: silver, gold, violet, blue, olive brown. In North America the grayling population is now centered in western Canada and Alaska. T h e retreat of the Pleistocene ice sheet

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left two colonies isolated farther south, one in Michigan and o n e in M o n t a n a . Anglers came even from Europe to fish for the Michigan grayling, which swarmed in rivers o f the northern half of the L o w e r Peninsula. Writers tell of parties that caught two or three hundred grayling in a day or two. Northrup, who visited the Au Sable River in 1 8 8 0 , says of the grayling: He is a simple, unsophisticated fish, not wily, but shy and timorous. He is a "free biter," and is bound to disappear before the multitude of rods waved over his devoted head. T h e sport he affords in his capture, the taste he gratifies in the frying-pan, and the allurements of the charming stream he inhabits, all conspire with his simplicity to destroy him. Could he but learn wisdom from his crimson-spotted cousin, and would the sportsman have pity on this beautiful and gentle creature of the smoothly gliding rivers, he would long live to wave the banner of beauty and glory in the cold, clear streams of the North. But that cannot be. W h e n the forests of Michigan were stripped away, the grayling c o u l d not survive in the silted streams. A few hung on until the 1 9 3 0 s , but the Michigan race of the grayling is believed to be now extinct. T h e M o n t a n a grayling—which differs slightly, if at all, from the M i c h i g a n r a c e — i s native to such tributaries of the Missouri above the Great Falls as the Madison and Gallatin rivers. It is now maintained by rearing and stocking. Apparently some wild or native fish still live in streams and lakes against the crest of the mountains west o f A n a c o n d a , near the site of o n e of the last important battles of the Indian wars. In the trout group are the salmons, trouts, and charrs. It is a

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source of continuing confusion that these common names do not correspond to the natural cleavages in the group, which delimit three genera: Salmo, Oncorhynchus, and Salvelinus. The "true" or blackspotted trouts are all Salmo, but, as one would guess from the scientific name, one of the species is called a salmon and is in fact the original salmon, long known from Europe. The same species (the Atlantic salmon) also runs up the rivers of northeastern America. Early Russian explorers discovered the salmons of the Pacific. These fishes differ slighdy in structure from the Atlantic salmon, and are put in the genus Oncorhynchus, which includes only the Pacific salmons. The speckled or brook trout of the eastern United States is not, properly speaking, a trout at all; in Europe similar fishes are called charrs, and the term is coming into use for our brook trout. Our brook trout and the Old World charrs belong to the single genus Salvelinus. The American lake trout also is closely related to the charrs and sometimes is put in the same genus with them. The lake and speckled "trouts" have even been crossed, to give a hybrid game fish, the splake. The American species of Salmo were widely split, east and west, by the basin of the Mississippi River. To the east, in rivers from the Hudson northward and west through Lake Ontario, was the Atlantic salmon, Salmo salar. On the west, from the Pacific to the western fringes of the Rio Grande, Arkansas, and Missouri river systems, were two trouts, the rainbow and cutthroat. Man has altered this original pattern by planting trout, especially the rainbow, in new waters, and by exterminating the salmon and cutthroats over parts of

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their range. Also, he has brought the brown trout of Europe into American waters. Since it finds polluted and warm water more congenial than American trouts do, the brown trout (Salmo tmtta)

is a mainstay of fish-

management programs. It is warier than the natives and can survive large numbers of average fishermen, maintaining itself even in heavily fished streams. One of its weaknesses is its habit of feeding at night, when anglers sometimes catch it in complete darkness, noting the strike by feel and sound. On their way west, members of the Lewis and Clark expedition found their first trout at the falls of the Missouri, where they caught several fine specimens two feet long. This trout, the cutthroat, is now known by the scientific name Salmo clarkii. T h e other western trout is Salmo gairdnerii.

the rainbow, named for a doctor with the Hud-

son Bay Company who sent the first specimens to Europe. Of the two, the cutthroat originally had the wider distribution, living in cold lakes and streams from Alaska to California, and from the mountains of New Mexico north through the main range of the Rockies. A fisherman exploring the western trout waters in the middle of the nineteenth century would have found a fascinating mosaic of different races of cutthroat trout. In the headwaters of the South Platte and the Arkansas he would have found the green-back trout; in Twin Lakes, overlooking the valley of the Arkansas, a yellowfinned race; over the Divide to the west, the Colorado River trout; to the south, the Rio Grande trout; and farther west, the Utah trout. And there were others, all differing in color and such structural details as the size of the scales.

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T h e original mosaic of races of cutthroat trout has been disrupted and defaced by overfishing, by the pollution of streams, and by the introduction of new fish, especially the rainbow trout. More aggressive than the cutthroat, the rainbow drives it into the colder and more inaccessible headwater streams. Also, when the rainbow from the Northwest is introduced into mountain cutthroat streams, the two fishes hybridize and the native blood is gradually submerged. Of the races of cutthroat trout, the yellowfin trout of T w i n Lakes, the Crab Creek cutthroat of Washington, and the San Gorgonio trout of southern California are now extinct. T h e green-back of the Colorado Rockies was presumed to be extinct, but was recendy rediscovered. Civilization wiped out the Utah trout everywhere except in a single stream (Trout Creek), but it is now restocked elsewhere. T h e Piute cutthroat of the eastern Sierras survives, isolated by falls in a stretch of Silver King Creek. As recently as 1950 a new species of trout (the Gila trout), which has characteristics of both cutthroat and rainbow, was described from the remote headwaters of the Gila River in New Mexico. One theory to account for the distribution pattern of the western trouts assumes that the original American species was the cutthroat, which invaded the continent from Asia. There is in Kamchatka a native trout much like the American species. This trout may have worked its way down the American coastal rivers, up the Snake River system, and across the Continental Divide, possibly near the present boundary between Montana and Wyoming, where there are lakes and streams that drain in both directions. From here it could proceed southward, following the mountain chain into New Mexico, then

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westward through the Colorado River system and the old drainages of the Great Basin that now are desiccated and have no outlets, and eventually back to the Pacific Northwest by way of the Sierras and Cascades. Along the route, different colonies could have prospered in more or less isolated river systems and lakes, where they would gradually develop characteristic racial features, in the same way that the isolated coregonids of the Great Lakes region acquired local peculiarities. According to this theory, the rainbow trout gradually emerged from its cutthroat ancestor toward the end of this great circling of races, probably in the southwestern United States. In this area it would have been difficult to decide whether a given local trout belonged to the cutthroat or rainbow series. Farther north and west the migrants would have been definitely rainbow. Here they entered territory occupied by their ultimate ancestors, the original invaders of the continent. In their long, circuitous march through western North America the migrants would likely have diverged so much from the original stock that they would be adapted for a somewhat different mode of life and might be more or less infertile with the ancestral stock; at any rate, any initial disinclination to cross would be accentuated by natural selection. This kind of process is believed to be important in producing new species of animals. A hybrid between two species adapted to somewhat different modes of life will probably not be adapted for either; hence it will be less viable than non-hybrids. Usually it is only under artificial, sheltered conditions, or certain new conditions, that a hybrid is superior to the native types. Parents with behavior patterns

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that prevent crossing will therefore leave more descendants that themselves reach sexual maturity; and this sterility barrier will be improved on by natural selection in succeeding generations. According to this theory, then (there are others), the rainbow emerged as a full-fledged species in the Northwest by evolution of mutual avoidance reactions in both the immigrant rainbow and the old cutthroat populations. It is a fact that these two species now live there side by side without crossing. In the Rocky Mountain area no such evolutionary divergence took place, and imported rainbows are more or less fertile with native cutthroats. The basic pattern of life within the trout group seems to be this: within the first year or first two years of life the fish goes downstream to the sea. Here, with an abundant food supply, it grows to good size—two feet or more in length—and then at sexual maturity leaves the sea to spawn in a river. This pattern is clear-cut in the Pacific salmon, and bounds the life history of the individual, for the fish dies after spawning once. The trouts (Salmo) may spawn more than once. The lake trout (Salvelinus), which lives in large lakes, does not usually go to the sea, although some of the Arctic populations do so. The closely related brook trout does migrate to the sea if there is a cold-water connection, but it, like the true trouts, will live and reproduce if landlocked. If restricted to a stream, however, it may never grow very large, and the landlocked brook trout of a small mountain stream becomes sexually mature when no more than six inches long. Differences in size and color between trout that have been in the sea and those that have not cause a good deal of confusion as to names. At one time ichthyologists thought the steelhead, a large,

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silvery fish, and the rainbow trout to be different species, but now most believe that the steelhead is merely a rainbow that has lived in the ocean (rainbows that migrate from rivers into the Great Lakes also are larger and more silvery than stream rainbows, without the bright side markings, and are also called steelheads). Other facts complicate the situation: all individuals of some races of rainbow trout migrate to the sea; in other races, some individuals do, some do not; and in others, all stay in the stream even though there is access to the sea. T h e sea-run cutthroat trout of the Northwest are called coasters. T h e great steelheads are seen in rivers only when they come in to spawn. T h e nesting site or redd of the steelhead is a massive affair— twelve feet long, half as wide—consisting of an aggregate of half a dozen or more basins, each a foot or two in diameter, dug in the stream gravel. At spawning time the female steelhead selects a site at the head of a riffle, where water circulates strongly through the gravel of the stream bed. A convoy of males attends her: a dominant male, usually the largest, who successfully mates with her, and several smaller males, both steelheads and stream rainbows, who are kept at a distance by vicious charges of the leading male. T h e female turns on her side and clears out gravel with powerful sweeps of her tail. The thrashing of the female in the shallow water and the running battle among the males make a commotion that can be heard a hundred yards away. When a basin is completed, male and female range themselves side by side over it and simultaneously extrude milt and eggs. T h e heavy eggs, 500 to 1,000 of them, fall to the bottom in a

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cloud of sperm and are immediately covered with gravel by the female. From a day to a week is required to construct the entire redd and stock it with eggs. In about a month the eggs hatch, and for another month or so the tiny fish remain hidden in the gravel, absorbing food from the yolk sac that remains attached to them. T h e n they emerge into the fiercely competitive life of the stream, which decimates their numbers within a few days or weeks.

12 Fresh-water specialists

T h e powerful and elegant salmonids are, in point of numbers, diversity, and amount of territory occupied, completely overshadowed in the streams and lakes by fishes that seem to have little to recommend them, a group represented by the familiar minnows, suckers, and catfishes and known technically as the Cypriniformes. T h e cyprinoforms dominate the warmer waters of the northern continents and swarm in the great river systems of South America and Africa. T h e y account for more than half of all living fresh-water fishes. T h e s e fresh-water specialists—only a minority live in the s e a — have a structure that distinguishes them from all other fishes: a set of bones that extend from the airfloat to the inner ear. T h e bones are fashioned from parts of the first few vertebrae, and move on one another in such a way that they transmit vibrations of the airfloat to the organ of hearing, somewhat in the way that the hammer, anvil, and stirrups—minute bones of the human ear—transmit movement from the eardrum. T h i s would imply that these fishes have good ears, and,

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judging from the little comparative work that has been done on hearing in fish, they do indeed have an acute sense of hearing. It is well known to anglers that carp, which are large minnows, are among the most wary fish. Perhaps they are unusually sensitive to the vibration of footfalls on the bank. Judging from the fossil record, the ancestral cypriniform fish invented the hearing apparatus at least 60 million years ago. In retrospect, it seems that the structure must have contributed much to the ultimate success of the group. It is interesting to speculate on the special significance of the hearing aid for these fish. One thing that all or nearly all have in common is the fact that they live in fresh water. A peculiarity of this environment is that it is exposed to many highly organized and efficient land predators such as mink, otter, and herons, as well as other less specialized predators that get fish only incidentally. T h e notable wariness of most stream fishes—they dash into hiding at the least footfall on the bank or a glimpse of movement—is well justified. So the unusually exposed nature of the stream environment, placing a premium on a highly sensitive auditory apparatus, may have been the central fact behind the presence of the chain of auditory bones (technically, the Weberian ossicles) that characterize the vast and diverse group of typical fresh-water fishes, the Cypriniformes. T h e "hearing-aid" fishes are closely related to the Clupeiformes (herrings and trouts) of the preceding chapter and, in fact, are little more than such fishes with the peculiar chain of bones added. They therefore display the primitive fin arrangement (pelvic fins far back), their fins are not spiny (except for the defensive stabbing spines of

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the catfish), and they have the open airfloat. Many also have the adipose fin of the trout group. T h e Cypriniformes did not arrive on the scene early enough to get to Australia, which has been isolated from the rest of the land masses since Eocene times or earlier. Only a few catfishes reached this continent, and these are descendants of marine catfishes. Among the familiar representatives of the group are the catfishes (Siluridae), carps and minnows (Cyprinidae), and suckers (Catostomidae). Some less familiar (because mostly tropical) but verv important representatives are the characins and gymnotids. T h e paradise for fishes of fresh water is the Amazon basin, which Louis Agassiz called " a fresh water ocean with an archipelago of islands." Agassiz, in 1865, took a vacation from his duties at Harvard to collect fish in the Amazon, during which he and his assistants took some 8 0 , 0 0 0 specimens. He estimated that there were about 1 , 8 0 0 species in his collection, and that the maze of Amazonian waterways contained ten times the number of species found in all the rivers of Europe, from the Tagus of Iberia to the Volga, and more than lived in the Atlantic from the North Pole to Antarctica. Most of the Amazonian fishes are hearing-aid fishes, and the most important are a primitive group called the characins. Although the more advanced types—minnows, carp, catfish—are nearly or altogether toothless and inclined to be sluggish, many of the characins are powerful, active fish, some of them formidable predators. Such a one is the famous piranha or caribe of jungle adventure stories, a characin the size of a man's hand or larger, with powerful jaws and triangular razor-sharp teeth. T h i s fish is said to have killed men and

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cattle. Aquarists use special nets for handling piranhas, which can cut ordinary cloth nets to pieces, and sport fishermen use wire leaders to angle for them. South America was cut off by sea (although more recently and less continuously than Australia) from the other continents during most of the time spanned by the evolution of the modern fish groups, and the fishes there evolved with little interference from those of other land masses. Somewhat in the way that the marsupials filled the Australian continent with mammals of diverse types, the characins of South America radiated out into types that are elsewhere represented by other groups. For example, there are in the Amazon "imitation" trout, pike, and carp which are really characins; and there are also many unique types among the 700 or so South American species of characin. There are between twenty and thirty species of completely blind cave fish in the world, belonging to widely different groups. When fish invade these dark subterranean waters, selection in favor of good vision is relaxed; the complex genetic mechanism behind the perfect eye then deteriorates without penalty. How quickly the eyes can be lost in this way is not known. One of the small stream characins of Mexico (see Plate 23) is especially interesting in this connection because it seems to show stages in the process. Individuals far back in the caves are completely eyeless; those near the entrance have imperfect eyes; and those in the lighted stream outside have perfect eyes. These small fish live well in aquaria, and the completely blind ones will cross with normal fish, giving offspring with eyes that range from degenerate to nearly perfect.

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A m o n g the hearing-aid fishes of South America are the knife-fishes or gymnotids, worthy of note because among them is the famous electric eel. Fish biologists have only recently developed the electricshock technique for stunning and collecting fish, but the electric eel has in all likelihood been using this method for quieting its victims for many millions of years. W i t h a sudden discharge of electricity the eel can immobilize small fishes and other animals some distance away in the water. A fine specimen eight feet long was seen to stun a netful of small carp held in the water twenty feet away. Electric shocks seem to be held in special dread by both human beings and other animals, so that the eel is well defended. Humboldt, studying the eel in South America at the end of the eighteenth century, found the Indians extraordinarily afraid of these yellowish, snakelike animals, and he himself soon learned the reason: " I do not remember having ever received from the discharge of a large Leyden jar a more dreadful shock than that which I experienced by imprudently placing both my feet on a gymnotus just taken out of the water. I was affected during the rest of the day with a violent pain in the knees, and in almost every j o i n t . " He observed that a single shock taught even a frog to fear the sight of an eel. Natives caught some of these fish for him by driving a herd of horses into a pond, where the eels, driven up from the mud by the trampling hoofs, discharged shock after shock against the bellies of the horses, often stunning the animals, so that two were drowned. As the melee quieted, the Indians were able to capture the exhausted eels in safety. A major group of the Cypriniformes, much larger and more varied than the gymnotids, is that of the catfishes. In the north these range

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in size from the giant wels of eastern Europe (said to swallow children, and at least it is big enough: 400 pounds) and the great channel cats of the Mississippi down to the tiny mad-toms of streams in the eastern United States. They are without scales, have sharp defensive spines hidden in the shoulder and dorsal fins, and usually lack bright colors. The brightly patterned mad-toms are exceptional; to match their warning colors, they inflict fiery stings with their poisonous spines. The northern catfishes give little idea of the diversity of the group. It is in South America that the catfishes have achieved a remarkable variety of forms. Some are small, delicate armored types (armored catfishes) that look as if sculptured from stone (see Plate 15). Some very small kinds live as parasites in the gills of another fish, where they drink blood from these soft tissues. In the torrential streams that rise on the east wall of the Andes are catfishes with pelvic fins modified into a sucker used to cling to rocks. Africa, too, has a varied group of catfishes, including an electric species and the strange Nile catfish that swims upside down to feed at the surface. The usual color pattern, white on the belly and dark on the back, is completely reversed in this fish, so that it has concealing coloration even though it swims on its back. In North America the characins are almost absent (one lives in the Rio Grande) and the catfish are a poor lot compared with the great South American assemblage. Here it is the minnow family (Cyprinidae) and the sucker family (Catostomidae) that represent the hearingaid fishes. Many of the thousands of species of cyprinids—shiners, chubs,

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specialists

dace—are much alike, and the specialist often uses a microscope to tell them apart. Although some of our minnows are brilliant—the golden shiner is in breeding season as bright as a new coin, and other species are marked with green, red, or yellow—most are rather dull in color and uniform in structure. Most of the cyprinids bury their eggs in gravel heaps, and the male guards this nest. One of the extreme variants with respect to egglaying behavior is the European bitterling. T h e male finds a live clam of the right species, then drives his mate to it. T h e female is equipped with a long egg-laying tube, which she inserts into the siphon of the clam, depositing the eggs in the clam. T h e male then deposits milt in the siphon. Others of the minnow group depart from the norm in point of size. T h e squawfishes of such large western rivers as the Columbia and Colorado are giant minnows of as much as thirty pounds. T h e powerful mahseer, a gamefish of India, is another large minnow, and the carp reaches thirty pounds or more. Europeans grow the carp, originally an Asiatic fish, in ponds, where it efficiently converts grain and potatoes into meat. T h e three great groups—Clupeiformes, Cypriniformes, and Perciformes—include most species of fish now alive, but there are many interesting groups that do not fit into any of the three. Among these is a group of small fishes, the cyprinodonts, which differ from the first two groups in that they have a perfected or closed airfloat, but lack the perch-like fin structure which usually goes along with such an airfloat. Unlike the true minnows (Cyprinidae), they have toothed jaws, so are called the toothed minnows. These fishes are well represented in the aquarium shop. Professional explorers are still bringing

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in new species of these brightly colored fishes, found in tiny streams and puddles of tropical jungles. T h e most popular of the toothed minnows belong to a group called the live bearers. Their habit of giving birth to living young make some of the species very easy to rear. T h e dependable guppy, for example, can give a bad time to the kindhearted aquarist who cannot bear to pour fish down the drain. Most North American cyprinodonts are not live bearers. These oviparous types include the top minnows and killifishes. Some of our species are brightly colored enough to make good aquarium fishes, but they are not much used. Some of the top minnows of the western United States interest the biologist because they live in water hot enough to kill ordinary fish. At one time streams flowed through the Great Basin country into the Columbia and Colorado rivers, but during the past few thousand years a long-term cycle of aridity has blotted them up, leaving dry canyons and a few springs, with now and then short creeks that flow for a few hundred yards or a few miles before disappearing into the sagebrush. Most of the larger fishes in the rivers did not survive in the area, but smaller kinds persisted in brooks and springs, where, in isolation, thev evolved into distinct species or races. Ichthyologists have in recent years made some remarkable discoveries of new fishes in the scattered waters of the Great Basin. T h e fishes that were driven into warm springs as the drainages dried up had to adapt or die. Adaptation to water temperatures over 100° F. is, apparently, beyond the capabilities of any fish, but some of the Great Basin cyprinodonts live in water just at this high temperature. One small warm spring south of Ely, Nevada, which emerges in a chest-deep basin and flows a short distance to end in a marsh, swarms with a

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race of top minnow that occurs nowhere else. T o a human being the water (98° F.) feels only lukewarm, but if individuals of the same species of minnow (although of a different race) from a cool stream some miles away are put into the warm spring, they die in convulsions in a few minutes. In the warm spring the native minnow metabolizes at a much faster rate (as measured by oxygen consumption) than does its cool-water relative in its own cool stream. T h e warmwater fish lives quite well if put in the cool stream; there its metabolic rate drops to about that of its cool-water relative. T h e true eels are another group that differ enough from the herring group and the Cypriniformes to be put in a separate category. T h e y evolved a snake-like form and discarded the hind pair of fins to become adapted for wriggling through mud and the crevices of reefs and rocky shores. Most are marine. T h e morays, of sharp teeth and evil tempers, are the largest and most diverse group of eels. Fresh-water eels presumably had oceanic ancestry. T h e European eel, when full grown, leaves its river and strikes out across the Atlantic on a journey that lasts several months and ends in the deep ocean southwest of Bermuda, three or four thousand miles from the starting point. T h i s long migration to the spawning ground is necessary, considering the peculiar development of the young. T h e lanal eel is a transparent, ribbon-shaped animal that drifts slowly north, then east in currents for about three years before it is transformed into a miniature eel. By the time this occurs, the ocean currents have brought the larva to the coasts of Europe, where it finds the streams needed for the next phase of its life. T h e American fresh-water eel also spawns in the sea near Bermuda. Its larva completes development more quickly and drifts to the American coast in a year.

13

Fishes of the sea The third of the great orders is that of the Perciformes, the spinyfinned fishes. These highly specialized types, probably the last of the main orders to appear in evolutionary history, have made their domain the ocean, where they are dominant among the brilliant fishes of coral reefs, the fleet inhabitants of the open sea, and the sluggish bottom-dwellers. A collection of fishes from a locality in tropical seas will show the dominance of the spiny-fins. In 1899 the U.S. Fisheries vessel Fish Hawk explored the waters around Puerto Rico, gathering in all about 260 species. Another forty which they did not collect were already known, bringing the total fish fauna in the region to 300 species. A dozen or so species lived in the short rivers of the island, the rest in the reefs and bays of the coastal waters. Of the total of 300 species, about 200 belonged to the order Perciformes. The strong, needle-sharp spines in the fins that give the order its name are the most obvious characteristic of these animals, but it is

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likely that the spines are related, in a roundabout way, to a hidden internal characteristic, the closed or perfected airfloat. Equipped with both red gland for admitting air and the oval for letting it out in precisely regulated amounts, the airfloat can adjust the specific gravity of the fish with great exactness. T h e fish can thus live at any depth, completely free of any necessity to rest on the bottom, and remaining suspended in mid-water without effort. Fishes that evolved the first crude airfloats were, as has already been described, able to redesign the fins, particularly to change the structure of the pectoral and tail fins. The perch group, with the perfecting of the airfloat and the achievement of complete rather than near weightlessness, has been able to push the remodeling to its logical conclusion. Formerly the pelvic fins were needed to support the rear end of the body. Now they can be used, not for lift, but for steering only. Since the best location for steering is near the front of the body, the pelvic fins of the perciforms have been moved forward, so far that they lie underneath the pectoral or shoulder fins. Here they serve efficiently for braking and for making quick turns. When one of these fishes is stationary in the water, the pectoral fins, which lie well above the pelvics and behind the gill opening, beat continuously in a gentle fanning motion. This movement of the pectorals is necessary to keep the fish stationary. As the fish breathes, it draws water in through the mouth and jets it backward through the gill openings, which tends to drive the fish forward. T h e backwater motion of the pectorals counteracts this tendency. T h u s we see that the change in fin position which characterizes the perciform fishes—the forward migration of the pelvic fins—is con-

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5. Red snapper. The fins are (a) dorsal, (b) pectoral, (c) pelvic (sometimes termed ventral), and (d) anal. In this spiny-fin fish the front parts of the dorsal and anal fins are supported by firm, sharp rays. The caudal or tail fin, which terminates the body, has flexible supporting rays only. T h e gill openings are concealed by the operculum or gill cover (e); compare with Plate 19, in which the shark's gill slits are visible. FIGURE

cerned primarily with increased maneuverability, with quick stops and precision turns. This capability is enhanced in many of these fishes by a deep, thin body form, which gives an extensive controlling surface in making turns. T h e familiar sunfishes and many of the reef fishes have this deep-bodied form; it is obviously ill-adapted for the hurly-burly of swift streams, and is found typically in fishes of still waters. T h e stiff spines on the leading edges of the median fins add the final touch to an efficient steering apparatus. At first sight, it might seem that the most important function of these spines would be to make things unpleasant for predators. Nearly every boy remembers

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Fishes of the sea

being bloodied by a sunfish that he has carelessly picked up. But it may be that this function is only an adjunct, although a happy one, to an original and primary function of improving maneuverability. Most important of the spine-supported fins is the large dorsal. Usually, approximately the first half of this fin has spines for supporting rays, while the back half has soft, flexible rays. The spines of the front half have strong muscles attached at the base which can raise and lower the spines, and this part of the fin can be folded down neatly into a groove. A sunfish or black bass when at rest will lower and raise this fin indiflerendy, but when swimming forward at good speed it folds the fin down. When the fish turns, the fin flashes up, and the strong spines give an unyielding controlling surface. T h e diversity of the perciforms puts many of them outside the description given above. Hundreds of species have thrown away just those structural advantages which would seem to account for much of the success of the group. These deviants are sluggish, sedentary types that have become adapted for living on the bottom. Often they have discarded the airfloat, and their spines, highly perfected, with sharp points and equipped with poison glands, contribute little to the control of swimming but are valuable for defense. T h e ugly stonefish of coral reefs has poisonous dorsal spines that can seriously and painfully wound a person who steps on them. Another perciform fish with dangerously poisonous spines is the weever, whose name is said to be related to the word "viper." T w o of the poison spines of this fish are on the gill covers. Although the perciforms dominate the oceans and may rightfully be called the "fishes of the sea," they are common in lakes and

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streams, where they sometimes seem to overshadow the numerous but obscure minnows and catfishes. Among the best-known types are the sunfishes, the delight of the boy angler because they obligingly take the lure, sometimes even a bare shining hook, at his feet, in plain sight. The sunfishes (bluegills, pumpkin-seeds, etc.) belong to the important family Centrarchidae, which is native to North America. The black bass (large- and small-mouth bass) are merely gigantic sunfishes. The family of perches, in the strict sense, includes the yellow perch, a standby of the still-water fisherman, and the powerful walleye of rivers and lakes. In general, the fresh-water relatives of the perch are adapted for life in quiet waters, but some, such as the sculpins and darters, are able to live in rushing streams. They hug the bottom, and their huge pectoral fins, shaped like butterfly wings, propel them with short bursts of speed from the shelter of one stone to another. It is in the sea that the perciforms reach their greatest variety and abundance. Packs of huge predators range the open sunlit waters; feeble scavengers lie on the dark sea floor; a brilliant display of varied types lives in the tropical reefs. In the Gulf of California commercial fishermen capture the basslike totuava, a fish three to four feet long that feeds in the shallow waters near the head of the Gulf. From his small boat—probably powered by an old automobile engine—the fisherman watches the horizon for diving birds. At the feeding site he anchors in the murky green water, perhaps twenty or thirty feet deep, and prepares to chum for the totuava. He cuts off a few inches of a stick of dynamite, slits it open with a knife, and crams in a dynamite cap with a

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Fishes of the sea

short fuse of carefully calculated length attached. He lights the fuse and tosses the explosive out into the mass of crying sea birds that are diving to feed on the school of small fish. It sinks. In a few seconds there is a sharp thud, water boils up over the spot, and at a wide radius a perfect ring of small silvery fish erupts from the water. Inside the ring of startled fish all are stunned, and the totuava swarm in to feed, joined by even more sea birds that come winging in from all directions at the sound of the explosion. With baited lines, the fishermen haul in the big fish hand over hand, each about as large as a man can manage. Probably the best swimmers of all fishes are the perciforms of the mackerel group: the mackerels, tunas, albacores, and swordfishes. T h e small mackerels are so well adapted to a life of swift and constant movement that they have come to rely on the rush of water through their gills for oxygen, and suffocate if kept motionless. It is said that an aquarium the size of a swimming pool is needed to hold them for study. Tunas are gigantic mackerels that grow to three quarters of a ton. These superb swimmers are thoroughly streamlined—even the eyes are planed off flush with the head and look as if they were painted on. T h e pectoral fins, sometimes very long and saber-like, fold back into shallow grooves on the sides of the body for high-speed swimming, and are put out for drags in sudden turns. T h e bone-hard tail fin is shaped like a slender crescent, and drives the fish through the water with an extremely rapid and powerful side-to-side vibratory motion, the main part of the body remaining relatively rigid. The body is narrowed to a slender stalk in front of the tail fin; above and

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below, this stalk is furnished with rows of small finlets. Their function is not understood, but they seem to be correlated with high speed. Also there are horizontal keels on either side of this body region. Perhaps they serve to strengthen this stalk, and may help streamline it for the side-to-side movements. T h e same kind of keel is present in some of the swift pelagic sharks. In short bursts of speed the tunas and their large relatives can probably reach forty to fortyfive miles an hour, and are probably the swiftest of fishes. One realizes how fast this is when he remembers that a trout, which seems to flash away with almost invisible speed, can do little more than ten miles an hour. T h e constant operation of the huge banks of powerful muscles keeps the body temperature of the tuna considerably above that of the surrounding water; it is one of the few warm-blooded fish. If the temperature of the water is as high as 24° C., which is sometimes the case, the body temperature may be well above that of man. T h e fishing industry packs these muscles into some millions of cans of tuna each year. T h e tunas are hunted by fast, wide-ranging boats out of Japan and the West Coast. Sometimes the fleets scout for the huge fish, which can be seen a long way off in the clear water, with airplanes or helicopters, but usually the captain relies on such signs as diving sea birds, which prey on schools of the same fish as do the tuna. Once at the fishing grounds, the boat may dump overboard tanks of live small fish to draw the tuna nearby and start them feeding. It takes too long to bait a hook each time a fish is caught, so the fishermen use a durable feathered lure on a barbless hook. Depending on the size of the tuna sighted, one, two, or three men,

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each with a pole attached to the single lure, jerk the big fish out of the water and flip it back over their heads into the boat. T h e tunas and their relatives are not dependent on a precisely regulated airfloat. In one species of tuna the airfloat varies in size in different individuals; apparently, like the human wisdom tooth, it is degenerating and may be on the way out. These fish seem to rely on movement to keep from sinking. T h e related mackerels also are variable with respect to the airfloat; it has completely disappeared in the Atlantic mackerel. A m o n g the relatives of the perch that have become adapted for life on the bottom is a group that has carried structural modifications for this mode of life to such an extreme that it sometimes is placed in a separate order. T h i s group is that of the flatfishes, some six hundred kinds of marine fish that include such important commercial species as the flounders, soles, and halibuts. Other bottom-dwelling fish usually become flattened from top to bottom—the skates and rays are examples of this kind of specialization—but the flatfishes have become flattened from side to side, in the manner of a sunfish. T h e y lie on one side, close to the sea floor, with the eyes directed upward. Some of the changes that took place during the evolution of the flatfish group are gone through, in a general way, during the life of each individual. T h e very young flatfish swims upright, like a normal fish, has eyes on the sides of its head, and has an airfloat. Sooner or later it begins to stagger, listing more and more to one side, either right or left, depending on the species. O n e of the eyes begins to migrate across the forehead or through the skull, coming to rest beside the other. T h e fish by now spends most of its time lying on one

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side. T h e airfloat disappears. T h e side that lies against the bottom becomes pallid, the side that is uppermost acquires pigment that makes the fish resemble its surroundings. It is now committed to lying on one side when at rest, but it is still able to swim, and some of these flying carpets are rather effective predators, catching other fish even near the surface. Angler fishes are bottom-dwelling types that rest on their stomachs. These sedentary predators lure their prey with a fleshy, sometimes worm-like appendage of the long first spine of the dorsal fin, which dangles over the mouth. T h e huge mouth opens automatically when the lure is touched. Many of the deep-sea anglers have luminous lures. One species even has a luminous lure attached to a pole, a line, and a bony hook. Other spiny-finned types have become adept at living out of the water. T h e crinkled walls of moist cavities above the gills are used to absorb oxygen from the air. Mud skippers are terrestrial fishes of tropical mangrove swamps. They hitch themselves along with the pectoral fins, but when hard pressed they flip themselves a yard or more at a time with the tail and make good speed. O n e traveler who visited the island of Truk says that when he chased some of these fish across an open field, they escaped by jumping into a ditch, swimming across, climbing the bank, and dashing away to safety on the other side.

14 Summary

In imagining the events of the conquest of the world of water by the fishes

we have been compounding two near unknowns. W e have

only bits of direct knowledge concerning the history of the fishes t h e m s e l v e s — a few fossils that are as tantalizing as they are informat i v e — a n d the indirect evidence from live fishes. A l s o , we know little about the nature of the aquatic w o r l d — w h a t are the problems presented to the animals that live in this strange environment. It is in the study of the world of water, particularly the vast environment that is the sea, that natural science faces its greatest frontier. Perhaps the recent invention of the self-contained underwater-breathing apparatus ( S C U B A ) , which makes it possible for human beings to enter the aquatic environment on something like equal terms with fishes, will turn out to be one of the important events in the history of our growing awareness of nature. T h e preceding brief outline of the historv of fishes should be regarded as a tentative framework with which to organize our observa-

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tions, from which to ask questions. Textbooks on the evolution of fishes are out of date nearly as soon as written, which means that, as in all developing sciences, the theoretical framework is being constantly remodeled as new information comes to light. What has been taken here as a useful theory concerning the main outline of fish evolution considers the fishes to have originated from some small marine invertebrate animal resembling the young of certain modern tunicates. In invading rivers that flowed into the sea, these invertebrates gradually acquired a complex of characters centered on more powerful swimming which transformed them into animals which we can call the first fishes. T h e crucial and, in a sense, primary characteristic of this complex was the large size of an active animal, which brought in its train the development of oxygen-carrying red blood, a large notochord or backbone, and the eyes that mark the fish as well as all other vertebrate animals. By late Silurian times this level of organization permitted the development of many diverse fishes, perhaps at first mainly in rivers, but also invading at least the fringes of the oceans. Survivors of this first, or jawless, stage of fish evolution are the lampreys and hagfishes. Jaws were invented by one or more stocks of the jawless fish by the late Silurian, and placed the fishes at a level which assured their dominance over the invertebrate animals. T h e most primitive group of jawed fishes, the class Placodermi, was a varied group with members that reached great size, in contrast to the jawless fishes, which never exceeded a foot or two in length. Before dying out, the placoderms gave rise to the two classes that now dominate the waters of

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the earth—the sharks (Chondrichthyes) and bony fishes (Osteichthyes). Sharks are well adapted for life in the sea; if they were ever important in fresh waters, they have been displaced there by the bony fishes. Differing mainly from bony fishes by lacking an airfloat, these "heavy" fishes either live on the bottom or compensate for the absence of the hydrostatic organ by the design of the tail and pectoral fins, and perhaps in some cases by emphasis on accumulations of oil in the tissues. Some sharks are eminently suited for life in the open seas, being powerful and tireless swimmers, and bearing their young alive, as do the successful marine mammals, the whales and porpoises. So far as number of species is concerned, however, the sharks are a small minority, and the bony fishes are in this sense the dominant group. This great class, living in virtually all waters of the world, is so varied as to defy brief characterization of its diversity. Of interest because they represent the earliest stage in the evolution of the class are the lungfishes and lobefins, now with only a few species holding out in the southern continents. T h e lungs, first evolved as air-breathing devices, lost this function in one or more stocks of lungfish as they became transformed into the airfloat. Descendants of the lungfishes that had this hydrostatic device founded the group of fishes that contains the great majority of living species. T h e lobefins were an immensely important founding group in another respect also, for some of those that retained the lungs went on to give rise to the amphibians, and through them to reptiles, birds, and mammals.

FIGURE 6. Simplified family tree of fishes, showing major adaptive steps in their evolution. It is possible that the sharks and other modern fishes had separate origins from the p l a c o d e r m s , in w h i c h case the true paired fins w o u l d have been invented twice. T h e lungfishes, w h i c h are o m i t t e d , w o u l d o c c u p y the same position as the closely related lobefins.

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Summary

T h e airfloat fishes are in a subgroup (subclass) termed the Actinopterygii, or rayfins. T h e majority of the rayfins, and hence most living fishes, belong to only three orders: the Clupeiformes (trouts and herrings), Cvpriniformes (carps and catfishes), and Perciformes (perches, tunas, etc.). T h e first of these has a rather primitive airfloat, a fact

reflected in the relatively primitive arrangement and

structure of the fins. T h e last has the most highly advanced airfloat, and the arrangement of the fins (pelvic fins brought forward) and their structure (spiny supporting rays) takes full advantage of the efficient hydrostatic organ. T h e Cypriniformes are predominantly fishes of streams, where their highly developed hearing apparatus, u n i q u e in fishes, is presumably of considerable use to them. T h i s outline of the history of fishes does not attempt even to summarize all of the often controversial literature on the subject. And the literature, voluminous as it is, can do little justice to the actual history of this group of animals as it has unfolded during half a billion years. T e n s of millions of years of the older periods, when the fish life was rich and varied, are crowded into a few pages about a few fossils. T h e r e has been not one steady march of progress, as a simplified account may suggest, but many different lines, each leading to remarkable and interesting types that have become extinct or live today as so-called minor groups, managing to survive, and hence basically no less successful than the few well-known groups which concerned us here.

Index

Acanthodians, 22, 23, 26, 32 Actinopterygii, see Rayfins Adipose fin, 45, 80, 92 Agassiz, Louis, 92 Agnatha, see Jawless fishes Acorn worms, 7 Air breathing, 32-33, 75 Air bladder, see Airfloat Airfloat, 24, 26, 31, 33, 39-44, 49. 78-79, 100, 106, 110 Albacores, 104 Alewife, 80 Amazon, 92 Ammocoete, 19, 20 Amphibians, 34, 65, 110 Amphioxus, 6 - 7 , 8, 9, 19 Anal fin, 45, 63, 101 Anchovies, 79 Angler fishes, 107 Archer fish, 55 Armored catfishes, 95 Backbone, 75, 109 Bass, 103 Barbels, 68, 73 Bichir, 34, 38, 76 Birds, 4, 110 Bitterling, 96

Blind fishes, 93 Blood, 9, 109; absence of red blood in fishes, 58; circulation, 58; flow through gills, 57 Bluegill, 103 Body temperature of tuna, 105 Bone, 9 Bony fishes, 12, 24, 31, 3 3 - 3 4 , 110 Bowfin, 55, 72, 75 Breathing, 5 6 - 5 8 Brook trout, 45, 53, 83, 87 Brown trout, 84 Caeca, 55 Carcharodon, 28 Caribe, see Piranha Carp, 54, 68, 71, 91, 96, 112 Cartilage, 9, 73 Catfishes, 67, 71, 92, 94-95, 112 Catostomidae, 92, 95 Caudal fin, see Tail fin Cave fishes, 93 Caviar, 73 Centrarchidae, see Sunfishes Cerebellum, 70 Chaenichthyids, 58 Channel cats, 95 Characins, 9 2 - 9 3

114

Index

Charrs, 83 Chick embryo, 6 Chimaeras, 28, 30 Chondrichthyes, see Sharks Chondrostei, 12, 76 Chordates, 5, 6 Chubs, 61, 81, 95 Cichlids, 6 1 - 6 2 Cisco, 81 Clupea, 79 Clupeiformes, 13, 78, 91, 112 Cladoselache, 26, 27 C o d , 59 Coelacanths, 37 Color, 5 0 - 5 1 Color vision, 65 Colorado River trout, 84 Conodonts, 20 Coregonidae, see Whitefishes Coregonus, see Whitefishes Coryphaena, 47 Countercurrent exchange, 42, 57 Crab Creek trout, 85 Crossopterygii, see Lobefins Cutthroat trout, 8 4 - 8 8 Cyprinidae, 92, 9 5 - 9 6 Cypriniformes, 13, 78, 90-98, 112 Cyprinodonts, see Toothed minnows Cytochromes, 10 Dace, 96 Darters, 103 Diatoms, 7 9 - 8 0 Digestive tract, 55 Dipnoi, see Lungfishes Dorsal fin, 45, 49, 75, 101-102 Dolphin fish, 47 Ear, 43, 69-70, 90-91; balance, 6 9 - 7 0 Eel, 69, 98 Eggs, 5 9 - 6 0 Electric catfish, 95 Electric eel, 67, 94 Electric rays, 30 Electricity: production by fishes, 30, 6 7 -

68, 94; use in navigation and detection, 67-68 Enzymes, digestive, 55 Eurypterids, 15 Exocetidae, see Flying fishes Eye, 6 4 - 6 5 Family tree: of chordates, 11; of fishes, 111 Fertilization, 63 Fins, 4 4 - 4 9 , 78, 9 9 - 1 0 2 , 111 Fish, definition, 11 Flat fishes, 57, 106-107 Flying fishes, 47 Ganoid scales, 48, 72, 74 Garpikes, 55, 72, 7 4 - 7 5 Geological periods, 11 Gila trout, 85 Gills, 5 7 - 5 8 Gill arches, 10, 21-22, 52 Gill covers, 57-58, 101 Gill rakers, 54, 74 Gill slits, 10, 27-28 Golden shiner, 96 Graylings, 80, 81-82 Great Basin, 9 7 - 9 8 Green-back trout, 84, 85 Guanine, 51 Gulf of California, 103 Gulpers, 54 G u p p y , 63, 97 Gymnotids, 92. 94 Hags, see Hagfishes Hagfishes, 15, 18, 20, 109 Hearing, 44, 69-71, 91, 112 Hearing-aid fishes, 9 0 - 9 8 Heart, 58 Hemocvanin, 10 Hemoglobin, 9, 10 Herrings, 79-80, 81, 112 Heterocercal tail fin, 17, 25, 26, 72 Holocephali, see Chimaeras Holostei, 12, 76 Hot springs fishes, 9 7 - 9 8 Hyoid, 52

Index Intestine, 55 Isinglass, 73 Jawless fishes, 12, 15, 109, 111 Jaws, 52, 109 Killifishes, 97 Kiyi, 81 Labyrinthodonts, 37 Late trout, 83, 87 Lampreys, 15, 18-20, 109 Lateral line system, 18, 65-66, 6 9 - 7 0 Latimeria, 3 7 - 3 8 Ling, 59 Live bearers, 63, 97 Loaches, 44 Loalach, 36 Lepidosiren, see Lungfish, South American Lobefins, 12, 37, 111 Luminous fishes, 54 Lungfishes, 12, 34, 110; African, 34; Australian, 35; digestive tract, 55; fossil, 37; South American, 36 Lungs, 110; origin of, 33 Mackerels, 104, 106 Mad-toms, 95 Mahseer, 96 Mammals, 4, 110 Manta, 29 Menhaden, 7 9 - 8 0 Milt. 59 Minnows, 54, 66, 9 5 - 9 6 Morays, 98 Mormyrids, 67 Mouth-brooding, 62 Mud skippers, 107 Nests, 61, 75 Nile catfish, 95 Nose, 6 8 - 6 9 Nostrils, 69 Notochord, 6, 7, 8, 109 Oncorhynchus, see Salmons, Pacific Osteichthyes, see Bony fishes Ostracoderms, 15, 16-18, 24 Otoliths, 70

115

Oval, 42 Ovoviviparity, 60 Oxygen, 32, 56 Paddle fish, 7 3 - 7 4 Paired fins, 22, 111 Palate, 52 Parrotfishes, 53 Pectoral fins, 26, 27, 29, 45, 4 6 - 4 7 , 100, 101, 103 Pelvic fins, 27, 45, 46, 100, 101 Perches, 103, 112 Perciformes, see Spiny-fin fishes Pharyngeal bones, 52 Pigments, 5 0 - 5 1 Pike, 53, 58, 66, 68, 69 Pipefishes, 62 Piranha, 53, 9 2 - 9 3 Piute trout, 85 Placoderms, 12, 2 2 - 2 3 , 24, 32, 109, 111 Poeciliidae, 63 Polyodon, see Paddle fish Polypterus, see Bichir Portheus, 79 Protopterus, see Lungfish, African Pstphurus, 74 Puerto Rico, 99 Pumpkin seed, 103 Rainbow, trout, 83-88 Rayfins, 12, 76, 111, 112 Rays, 28, 29, 30 Red gland, 41, 79 Red snapper, 101 Redd, 88 Remora, 48 Reproduction, 59-62 Reptiles, 110 Respiratory pigments, 9 Retina, 65 Reversed heterocercal fin, 16 Rhipidistia, 37 Rio Grande trout, 84 Sacculus, 70

116

Index

Salmo, 83, 87; salar, 83; trutta, 84; clarkii, 84; gairdnerii, 84 Salmons, Pacific, 69, 83, 87 Salmon, Atlantic, 83 Salvtlinus, see Charrs San Gorgonio trout, 85 Sardines, 79 Sawfishes, 2 9 - 3 0 Scales, 48-50; ganoid, 48 SCUBA, 108 Sculpins, 103 Sea horses, 54, 62 Sea squirts, 6, 7, 109 Semicircular canals, 70 Senses, 6 4 - 7 1 Shad, 80 Shagreen, 27 Sharks, 12, 2 4 - 3 0 , 31, 55, 60, 65, 73, 105, 110, 111 Shiners, 61, 71, 95 Shoulder fins, see Pectoral fins Siamese fighting fish, 50 Siluridae, see Catfishes Skeleton, 24, 31 Skin, 27, 50 Smell, 68 Species formation, 86 Speckled trout, see Brook trout Sperm, 59 Spiny-fin fishes, 13, 99-107, 112 Spiny sharks, 22 Spiral valve, 55, 73, 75 Squawfishes, 96 Steelhead, 88 Stegoselachians, 23 Sticklebacks, 61 Stomach, 55 Stonefish, 102 Sturgeons, 55, 72, 73 Spiny-finned fishes, 9 9 - 1 0 7 Suckers, 92, 95 Sunfishes, 61, 78, 103

Swim bladder, see Airfloat Swordfishes, 104 Tail fin, 44-46, 49, 104; heterocercal, 16, 17, 25, 26, 49, 72; homocercal, 45, 49; reversed heterocercal, 16, 49 Taste, 6 8 - 6 9 Teeth, 27, 28, 29, 5 2 - 5 4 Teleostei, 12, 76-77 Thymallus, see Graylings Tongue, 52 Toothed minnows, 96-97 T o p minnows, 9 7 - 9 8 Totuava, 103-104 Toxotes, see Archer fish Triggerfishes, 53 Trouts, 45, 55, 56, 7 8 - 7 9 , 8 3 - 8 9 , 112; brook, 45, 83, 87; brown, 84; Colorado River, 84; Crab Creek, 85; Gila, 85; green-back, 84, 85; Lake, 83, 87; Piute, 8 5 ; rainbow, 8 3 - 8 8 ; Rio Grande, 84; San Gorgonio, 85; speckled, 83; Utah, 84, 85; yellowfin, 84, 85 Tube mouths, 54 Tullibee, 81 Tunas, 104-106, 112 Tunicates, see Sea squirts Utah trout, 84, 85 Uterus, 60 Vertebra, 75 Vertebrates, 5 Viviparity, 60 Walleye, 103 Weberian ossicles, 90-91 Weever, 102 Wels, 95 Wetterfische, 44 Whale shark, 28 Whales, 25 Whitefishes, 80, 81 Winter kill, 57 Yellowfin trout, 84, 85

Plates

1

Sea lampreys and brook lampreys

2

Scale of a bluegill, showing growth rings Head of a sea lamprey

3

T h e carp

4

Spiny dogfish sharks Mouth of a shellfish-eating shark

5

Sand tiger sharks

6

Mouth of a mackerel shark

7

Backbone of a red snapper

8

Northern pike exhaling and inhaling

9

T a i l fin of an alligator gar Early example of the homocercal tail fin Pectoral fin of a red snapper

10

Large-mouth black bass Sturgeon

11

Bichir, most primitive of living rayfins Scales of an ancient rayfin, Paeleoniscus

12

Lungfish from South America and Africa

13

Fossil jaw of a lobefin fish

14

Pacific sardines Quillback carpsucker

15

Tiger catfish and armoured catfish

16

African mormyrids

17

Four-eyed fish

18

Salmon jumping falls

19

Remoras hanging from a sand shark Pacific ocean perch

20

Sunfish

21

Bluefin tuna

22

Oceanic skipjack Brook trout resting on bottom

23

Albacore Blind cave fish

24

Angler fish luring prey Steelhead trout over spawning nest

Photographs not otherwise credited were taken by the author

WiOii

PLATE

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1. ABOVE: Sea lampreys feeding on river chub (left) and common sucker.

Brook lampreys preparing a spawning bed in a stream in southern Michigan. These lampreys do not prey on fish. BELOW:

PLATE

2.

ABOVE:

Scale

of

a

bluegill (sunfish) from a Michigan l a k e . A n n u a l c h a n g e s in growth

rate

varying

thickness

cause

rings

of

to be laid

down in the scale. Distances between numbered points repr e s e n t a year's growth, LEFT: Head of a mature sea lamprey, showing

the

sucking

armed with teeth.

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mouth

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photc,

3. Carp. The paired nostrils for one of the two nasal pouches are visible in front of the eye.

PLATE

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(Brigkam

4. ABOVE: A haul of spiny dogfish sharks from the Atlantic illustrates the abundance of these fish, BELOW: Mouth of a small shellfish-eating shark, with flattened teeth adapted for crushing rather than shearing.

PLATE

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5. The heterocercal tail fin and the wide planing surfaces of the pectoral fins are displayed by this pair of sand tiger sharks.

PLATE

PLATE 6. Melville speaks of the "teeth-tiered sharks." This view into the mouth of a mackerel shark exhibits several rows of powerful teeth.

PLATE 7 . B a c k b o n e of a bony fish, the red snapper. Its c o n s t r u c t i o n is both light and strong.

8.

PLATE

Northern

pike.

LEFT: T h e fish is " e x h a l i n g , " pushing water out underneath the gill cover, whose edge is marked by a crescent extending upward

from behind the

pectoral fin. A pair of membranes,

one

from

the upper

j a w and one from the lower, meet to nearly seal the front of the

mouth,

thus

preventing

water from escaping forward. ABOVE: T h e fish is "inhaling"; the

mouth

withdrawn

membranes and

not

are

visible,

and a flexible membrane at the edge of the gill cover is tightly pressed against the side of the body,

keeping

water

from

coming in under the gill cover.

PLATE 9. RIGHT: Tail fin of a n alligator g a r . This primitive fish has a tail fin intermediate between the ancient heterocercal type and the symmetrical or homocercal type characteristic of most rayfin fishes. The fin is almost symmetrical in oudine, but the backbone extends well into the upper part of the fin; compare with the tail fin of the black bass (Plate 10) and with the fossil below.

One of the earliest known examples of the homocercal tail fin, which was invented by the rayfin fishes. This specimen of Leptolepis, now in the University of Michigan Museum of Paleontology, is from the Jurassic of Bavaria. In the same rocks was found the famous reptile-like bird Archeopteryx.

Pectoral fin of a rayfin fish, the red snapper.

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IS. Fah and WOdlift Srrvici (HaUm

10. A B O V E : Large-mouth black bass. The spiny portion of the dorsal fin is lowered. The external indication of the lateral-line system is a lighter line running horizontally through the black band in the tail region, B E L O W : The sturgeon, although equipped with an airfloat, has a specific gravity well above that of water. Like the similarly heavy sharks, it has a heterocercal tail fin and wide, thick, pectoral fins. PLATE

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Society

photo)

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Society

11. A B O V E : The bichir or Polypterus of Africa is the most primitive of living ravfin fishes. Besides the primitive lungs, it retains the ancient platy covering of ganoid scales, placed side by side in rows rather than overlapping. PLATE

Scales of an ancient rayfin fish, Paeleoniscus, from the Permian rocks of Germany. These fossils resemble the scales of the living bichir. T h i s specimen is in the University of Michigan Museum of Paleontology. RIGHT:

Xrw York Zedogicel Soci/fy PLATE 1 2 .

ABOVE: A S o u t h

American

lungfish reaching above the water for air;

it will

drown

if held under.

LEFT: An African lungfish breathing air.

fttw M

Z-UfUml Strirt-I

PLATE 13. This fossil jaw of a lobefin fish from the Devonian of Ontario is among the earliest known powerful, strong-toothed fish jaws. Scattered in the matrix are fragments of crinoids, small marine animals related to starfish. Specimen in the University of Michigan Museum of Paleontology.

( S fuk a,td WMlift Sm iti (MalUm

14. ABOVE: A crowd of Pacific sardines in an aquarium. RIGHT: A quillback carpsucker feeding by "vacuuming" with the extensible mouth. PLATE

photo)

I

Mew York ZaeUfiemt Society

PLATE 15. ABOVE: T h e tiger catfish, BELOW: An armored catfish. These two species from South America indicate some of the diversity of catfishes on that continent.

Vm Yvri Zeotofwal

Wirf»

.Vru York Zocbgit*!

Society

16. Two fishes of the African mormyrid group. In addition to the odd drawn-out head, this group is noteworthy for the relatively large size of the brain in proportion to body weight. Some species are known to be protected by electric shock equipment and use electrical pulses for navigation. PLATE

Mtw York Zsebgusl

Secuiy

Ktm Ytri Z—Uficml I « *

17. The four-eyed fish, a toothed minnow of South America, has eyes adapted for both aerial and underwater vision. PLATE

Mtw r«rt ZmUpril

( S FM ami WiUlift Strvin (KtU, pkrtc)

PLATE

18. Salmon jumping falls at Brooks Falls, Alaska.

Jftw Tcri Zaetcpeml 5enefp

Remoras hang like huge lice from a sand shark, which provides them with a resting place and transportation in the open sea. BELOW: A trawl catch of Pacific ocean perch (Sebastodes), drawn up from deep water off the Washington coast, illustrates the abundance of some of the marine spiny-fin fishes.

PLATE 1 9 . ABOVE:

I S. Fai —i WiUlift Srrvia

V S. «,k