The Anatomy and Physiology of Fishes

Fishes are cold-blooded vertebrates. This means that they remain at approximately the same temperature as the water surrounding them, in contrast to the whale or the water rat, which like ourselves maintain a much higher temperature. Aquarium fishes share with ourselves and other mammals, however, the possession of a backbone, or vertebral column, and are built on the same fundamental plan, having the same basic system of bones and organs as we do.

Fishes breathe oxygen, but it is usually absorbed only from solution in water by the gills, which are leaf-like organs, normally four on each side of the neck in a pouch covered by the operculum, or bony gill cover. The gills are richly supplied with blood vessels, and water is swallowed from the mouth and forced over the gills, leaving by a slit between the operculum and the body. The rate of fishes' respiratory movements is partly determined by the need for oxygen and its concentration in the surrounding water.

Fins

There are two paired and (in all but fancy goldfish and a few other fishes) three unpaired fins. The paired pectoral and pelvic (ventral) fins correspond, respectively, to the arms and legs of human beings and connect with bony girdles in the body which correspond to our own pectoral and pelvic girdles. The unpaired fins are the dorsal, the anal, and the tail or caudal fins, as shown in the accompanying figure. These fins are supported by rays, sometimes bony and sometimes made of cartilage. In some families the dorsal fin is split entirely into two parts, the forepart with spiny rays and the hindpart with soft rays. In the characins and some others, there is a small adipose fin, composed of fatty material with no fin rays.

Body

The fish body is composed mainly of a large lateral muscle on each side of the backbone, divided by sheets of connective tissue into segments corresponding to the vertebrae, which give rise to the typical flaking of the cooked fish. This is the main organ for swimming. The internal organs often occupy a very small volume, toward the front, so that much of the apparent trunk of the fish is really its tail (as distinct from the tail fin). This is indicated by the forward position of the beginning of the anal fin, which marks the end of the digestive tract. Fishes possess the usual organs familiar to students of human anatomy, with the exception of lungs and chest cavity; they have a stomach, intestines, a liver, a spleen, kidneys, and so forth.

Skin and Scales. The skin may be naked, or it may be covered by scales or by bony plates which in turn have an outer layer over them. The scales may be opaque or transparent; if they are transparent, the appearance and color of the fish may be due to skin pigments, not to scale color or formation, as in the calico goldfish. Bony plates may be seen in the Corydoras, or South American armored catfishes.

Peculiarities of Fish Anatomy

Air Bladder. In addition, fishes often possess a characteristic organ, the air bladder. This is a long bag filled with gas and lying in the body cavity. It may be entirely closed, or it may communicate with the alimentary tract by means of a duct, or tube. Sometimes it is divided into two rather distinct parts, which communicate with each other.

The air bladder controls the specific gravity of the fish, as the diving tanks of a submarine govern its buoyancy. In fishes with divided bladders, the center of gravity can be altered too. It actually corresponds to the lungs of higher vertebrates, and this fact is foreshadowed in the socalled lung fishes, which take air into their air bladders and breathe as we do.

Labyrinth. In the Anabantidae and other families, as mentioned in the preceding chapter, an entirely different organ, the labyrinth, is also used as an auxiliary air-breathing apparatus. This organ is situated near the gills, and air is passed through or into it and out through the mouth or gill slits.

Lateral Line. Careful examination of the majority of fishes will reveal a line running from the head along the side of the body. This is a series of tubes filled with a gummy secretion and with stiff bristles at the base. Its function is to detect vibrations of low frequency, and the ear is a specialized part of the lateral line system.

Nostrils. It may seem odd that fishes have nostrils, but they do—in fact, they often have four of them. They are organs of smell and do not perform any function in breathing, since they do not open into the mouth.

Swimming and Balancing

Most fishes swim by body movements, not fin movements. The fins are mainly balancers, with the exception of the tail fin, which often acts as a final thrusting member, propelling the fish through the water. In normal, medium-paced to fast swimming, the action is initiated at the head end of the fish, and waves pass down the body, culminating in a flick of the tail. The dorsal and anal fins prevent the fish from turning over in the water; the paired fins also perform braking and turning functions. In slow swimming, and in static balancing in the water, the pectoral fins are used. These fins are usually colorless, so that when the fish is still in the water, their gentle movement is unnoticed. Indeed, in a fish like the Siamese fighter, they have to be looked for quite carefully, in contrast to the bright colors of the rest of the finnage.

Some fishes, particularly some of the coral fishes and the stickleback, normally swim with the pectoral fins rather than the body, but this is an unusual habit.

The balance of fishes is controlled by three main factors:

The inner ear contains (as does ours) a system of sensitive sacs containing bones, called otoliths, which are balancing organs. The movement of the bones in the, sacs tell the brain of the fish about its orientation and movements.

The muscles themselves convey messages of position and movement, and it is possible that the lateral line also does so. In a fish, it is likely that only active movements bring forth the inner ear and muscular perceptions. It has also recently been discovered that many fishes are equipped with a kind of radar device, the muscles acting as broadcasters of electrical impulses which are reflected from surrounding objects.

The eyes are very important in most fishes, not merely for normal visual perception, but because the fish so adjusts itself, if possible, that the two eyes receive equal amounts of light. This is the same reaction that causes insects to fly into a light. In the aquarium, its effects are seen if the light entering the tank is not from overhead, when the fishes may be observed swimming along at an angle, sometimes a very odd sight. Continued slanting illumination is said to cause disorders in the fishes subjected to it.

Metabolic Rate and Oxygen Need

The rate at which an animal uses up energy, produces heat and waste products, and consumes oxygen is called the metabolic rate. An understanding of the factors which modify it is of primary importance to the aquarist. Since fishes are cold-blooded, they differ fundamentally from ourselves in that they have an increased metabolic rate as the temperature rises and are hungriest when warm. We consume a great deal of energy, which we get from our food, in maintaining our body temperature constant and normally well above that of our surroundings. A fish doesn't do this but merely obeys a basic chemical law which causes body processes to go faster, the higher the body temperature. Thus a fish turns energy over at a much greater rate in warm water than in cold water.

Another factor influencing the metabolic rate is activity. A resting fish consumes less energy than an active fish. The higher the temperature, the more energetic a fish tends to be, so that an elevated temperature acts doubly in causing greater energy consumption in most species—the fish is using more energy not only because it is warmer but also because it has to swim more to catch and to consume and digest more food. This action has an upper limit, however, probably determined by the lowered solubility of oxygen in warmer waters. Thus, at about 80°F., the average fish reaches its maximum oxygen consumption and maximum appetite.

A further factor influencing metabolism is age. Young fishes are growing relatively faster than older fishes and also they use up oxygen and foodstuffs faster per unit of body weight. There are no exact recorded measurements for fishes, but if they are anything like birds and mammals in this respect the difference is one of several hundred per cent—i.e., an ounce of adult barbs needs only a fraction of the oxygen per minute that an ounce of young barbs needs.

A final important factor, especially in livebearers, is sex and pregnancy. Gravid female livebearers need a good deal more oxygen than even younger fishes or the males and will suffocate first in an overcrowded tank containing adults and young. This is because they are breathing for their young as well as for themselves.

Growth, Age, and Size

In common with most living things, fishes grow most rapidly when they are young and gradually slacken off as they become older. Although there is an approximate maximum or fully grown size in most species, this maximum is really only the top of the growth curve, which does not flatten out entirely under favorable conditions, with plenty of food, oxygen, and room.

A starved or semi-starved growing mammal is likely to die and will certainly be stunted in growth. However, if it survives at all, it will not be much smaller than normal, although it will be thinner and wretched looking. A semi-starved fish may die, but if it lives it will grow very little or even not at all, and it may reach breeding age at a tenth of the normal weight. There are often tremendous differences among members of the same batch of fry growing under apparently identical conditions in the same tank. Some become so much larger than others that they can eat them, and so get larger still. This difference is usually seen when the baby fishes are underfed, so that the lucky ones happen to swallow most of the available food in the early stages or are hatched before the others and are then progressively better able to grab whatever comes, at the expense of their smaller brethren. When there is plenty of suitable food, much smaller differences are normally seen, and an even batch may frequently result in wildtype fishes such as barbs or characins. Fancy fishes, such as some of the goldfishes, usually throw a fair proportion of runts even under good conditions, which would indicate a genetic rather than a nutritional cause.

Thus fishes of the same species may differ enormously in size at the same age, depending on the food and possibly the room available to them in their earlier stages. The exact extent to which swimming space really matters has not been determined, but it is suspected that, as long as adequate food and air are available, swimming space makes little difference and the apparent effects of crowding are usually due to insufficient food and air.

Starved fishes may catch up to the normal if given the chance, and it is not clear whether they can be permanently stunted within, say, a year or so of hatching. If kept on short rations for a longer period, the average aquarium fish is said to be permanently affected, but there seem to be no records substantiating this claim. The usual effect of aquarium life in itself is to stunt in comparison with the size found in the wild, with one or two very curious exceptions, such as Copeina guttata, which grows to 4 inches in the aquarium but has never been caught in Nature longer than 3 inches. It has been found, however, that in very large tanks with everything as nearly perfect as possible, including much live food, fishes grow as large as in the wild. The limiting factor is probably oxygen and carbon dioxide exchange and adequate food, as noted above.

Salt Tolerance and Excretion

The kidneys and the gills are the most important of all the organs which regulate the composition of the blood in fishes. For life to continue, this must be kept remarkably constant, and the slightest variation may lead to severe illness or death. Thus, oxygen, carbon dioxide, and some salts are exchanged in the gills, sometimes through the skin, and waste products such as uric acid or urea and excess salts, acids, or alkalis are passed out in the urine by the kidneys.

It is generally believed that life originated in the seas, at a time when they were bout one-third as saline as they now are and when there was more potassium and less magnesium. The composition of the seas at that time—hundreds of millions of years ago—was supposedly much like that of the blood of present-day animals which then resembled sea water and has since remained virtually the same while the seas have steadily become saltier.

Fishes have to do one of two things. If the water in which they are swimming is saltier than their own blood, as in marine fishes, they must work continually to prevent themselves from being dehydrated and "pickled." This they do by excreting a lot of salt and comparatively little water. They swallow water and the salts it contains, and they pass a fairly dilute urine, most of the salt being excreted by special cells in the gills. If the water is fresh, they must work to prevent themselves from blowing up with water, for their saltier blood and tissues will normally tend to "bog up" with absorbed water. So in this case the fishes produce a copious stream of very dilute urine and, by means of other special gill cells, actively absorb such salt, as is present in the surrounding water.

Some fishes can do either of these and can live in salt or fresh water, especially if not too suddenly plunged from one to the other. Others cannot tolerate such a change and can live only in one type of water, with a tolerance to minor variations which differs from species to species. The presence of calcium has been found to influence this ability, and marine fishes can sometimes stand transfer to brackish or practically fresh water as long as it is hard, i.e., calcium-containing.

Thus, Scatophagus argus, a marine species often kept in fresh-water aquaria, may be plunged directly into hard fresh water with little or no obvious discomfort, but in soft fresh water it dies, either rapidly or after a period of decline. Other species such as eels or salmon are indifferent even to calcium content. At the other extreme are the pelagic marine fishes like herring, which are extremely sensitive to changes in salinity and cannot stand appreciable dilution of the sea water in which they live. However, most fishes are fairly tolerant of small variations. Most fresh-water fishes can take up to about 1/4 strength marine water, i.e., approaching the salinity of their own blood, whereas most sea-water fishes can live comfortably in % strength marine water, but not much less. In terms of specific gravity, with fresh water at 1.000, and sea water at 1.025 to 1.030, the majority of fresh-water species can stand salt up to a reading of about 1.0075, and the majority of marine species can stand dilution down to about 1.017. In percentages, full-strength sea water contains about 3.7% of dissolved solids, most of which is sodium chloride (common salt).

A salt solution of about 37 grams per liter or 5 1/2 ounces per gallon is equivalent in saltiness to sea water. Fresh-water species may thus be expected to stand up to about 1 ounce per gallon, and some, such as guppies and mollies, can take more and be gradually acclimatized to sea water itself.

Temperature Tolerance and Adaptability

In general, fishes are adapted to the temperature variations of their natural surroundings and not much more. If a fish comes from a seasonally variable environment, in which it may stew in summer or freeze in winter, then, like the common carp, it can stand these ex tremes, but it cannot usually stand a sudden change from one to the other. If it comes from the rock pools of the seashore, like the goby, it may stew in an exposed small pool and then be flushed out by an incoming tide at a temperature 20° to 30°F. below that of the pool. The goby can stand these extremes.

If, like the majority of marine fishes, the fish comes from a very constant environment in which it can move about so that even the slow changes in temperature can be avoided, it is likely to be very touchy about the temperature of its surroundings and therefore inadaptable.

The fresh-water aquarium fishes tend to come from fairly still waters which may undergo fairly wide changes in temperature but which do not change suddenly. When these waters are tropical, their temperature does not drop below 65° to 70°F., but it may rise to over 100°F., and therefore tropical fishes can stand this kind of range. In the aquarium they tend to be less happy at the extremes of their natural tolerance than in the wild, and it is usual to try to keep the temperature within a range of, say, 70° to 80°F., with as short a period as possible beyond these limits, particularly downward.

The sudden exposure to a change in temperature of more than 2° to 3°F. downward or 5° to 6°F. upward is likely to cause shock, followed by disease. For complete safety, it is best to avoid rapid change of more than 2°F. in either direction or a slower change at a rate of more than about 5°F. per hour covering a total change of 5°F. downward and 15°F. upward. A greater change than these will result in damage to most species, unless several days are taken up in achieving it. The usual symptoms of chill are a very characteristic slow, weaving motion like slow-motion swimming without getting anywhere, often called "shimmies," and the development of a disease called white spot, or ich, which is caused by an organism frequently present in the water but which does not usually gain a hold in healthy fishes. The condition and its cure are described in Chapter XIII. The usual symptoms of heat shock are gasping respiration, surface hugging, and sometimes lack of balance, so that the fish turns slowly onto its side, plunges to an upright position, and then repeats the process. The respiratory signs may occur in fishes which simply lack oxygen, but after too sudden an increase in temperature they will occur even in the presence of sufficient oxygen.

The above discussion applies to adult fishes. Their eggs and fry are usually much tougher and can stand a greater variability and a wider range of temperature than their parents. As the young fishes grow up, this tolerance is gradually lost. It also applies to involuntary changes which the fishes cannot avoid. In a pool or a tank, the water may be stratified, with quite large differences in temperature between top and bottom. The free swimming of fishes from one layer to another in these conditions is not harmful, probably because they do not stay long at any one fixed temperature and can tolerate short periods of immersion in hotter or colder strata. A day-to-night fluctuation in average temperature of 5° to 7°F. is also safe for most fresh-water
fishes but not for many marine or estuarine types. A fluctuation of 10° to 15°F. is dangerous and a common cause of otherwise unexplained trouble.

The accompanying table gives approximate temperature ranges of the various families of fishes, subdivided into smaller units where necessary. Books on aquarium fishes frequently list odd tolerance ranges such as 68° to 75°F. simply because they are derived from European continental references which are in degrees Centigrade. Their most common low temperature is 20°G, which is 68°F.; a temperature of 24°C. is 75°F. In this table ranges are given to the nearest 5°F. It will be seen that the commonest range of perfectly tolerable temperature for tropicals is 70° to 85°F. Most tropical fishes are un comfortable above or below this range, particularly below it. They are mostly in danger if much below 70°F. but not in severe danger above 85°F., unless it is prolonged for many days or the fishes are crowded.

These figures are mostly not the temperature range for breeding, which is usually but not always toward the upper part of the range, or at any rate above 75°F. Exceptions are most Cyprinidae, Corydoras, Oryzias latipes, and Macropodus opercularis. Breeding in very large tanks or ponds will often occur at lower temperatures.

Reproduction

Details of fish reproduction will be dealt with in the relevant later chapters, and remarks here are confined to a few general principles. The sexes are always separate, and, in the few cases in which it has been studied, sex determination is genetic, as in other vertebrates. Unlike most other vertebrates, however, the genetic sex may sometimes alter spontaneously, particularly in livebearers. When this occurs, the transformation is apparently always from female to male. Winge, who has studied the guppy extensively, has shown that it is possible to swing sex determination from one pair of chromosomes to another, so that this mechanism in the livebearers would seem to be in a primitive state.

Fertilization is usually external, at the moment of spawning, and fish sperm do not live long once they have been ejected into the water (about 60 seconds in the case of the trout). Some aquarium fishes are community spawners, such as Rasbora heteromorpha, but the majority will spawn in single pairs, even though in Nature they may spawn communally. The livebearing fishes were once thought to have true internal fertilization, but this is doubtful, since in some instances it has been shown that the male does not place sperm in the oviduct but merely shoots packets of them in the right general direction through the water.

The fertilized eggs usually hatch rapidly. Those of many characins hatch within 24 hours at about 75° to 80°F., most barbs within 40 hours, and goldfish in about 3 days at 80°F. but as long as a week at 60°F. Panchax varieties take 10 to 14 days, and, at the other extreme, the eggs of some of the fundulopanchax group may take several months. The young are usually very small, and require careful and special feeding, excepting larger young, such as those of most live- bearers. Typical numbers of eggs per spawning are 100 to 1000 in aquarium species, most of which should be fertile. This contrasts sharply with species like the cod, which lays some 9,000,000 eggs. Livebearers drop anything from half a dozen to 200 young, the larger numbers only from older females. There are persistent reports that particular strains or species produce a great preponderance of one sex.

The local strain of Barbus titteyct in Sydney, Australia, is generally agreed to produce anything from 90 to 99% females, and it was therefore some surprise that a purchase of 6 turned out to be all males. Very great caution is necessary before such opinions are accepted, because, unless the whole of a spawning is preserved until it can be sexed, there is the likelihood that the method used for culling selectively eliminates more or nearly all of one particular sex. This will be particularly likely if there is a sex difference in, say, size or in brightness of color, and the unwary owner culls the apparent runts and duller fishes. A spawning of glowlights (Hyphessobry con gracilis) was neatly divided into practically equal groups of males and females, when the intention was to throw out the less well-developed fishes (males). There is, in fact, every reason to suppose that about equal numbers of males and females will normally be produced, although it is quite possible for there to be a preferential death rate during growth.

There is also a common prejudice against early breeding, and it is even sometimes recommended that a fish like the glowlight should not be spawned before it is 2 years old. There seems to be no good reason for this. Early spawning may not yield as many eggs, but it certainly does not seem to impair later fertility or fecundity in any way. In fact, it is the writer's experience that early, frequent spawning is the way to get the more "difficult" species trained to their job. Typical records for young pairs are the following, taken from a pair of "Sumatra" barbs {Barbus tetrazona):

First spawning	40 young at 6 weeks
Second spawning	62 young at 6 weeks
Third spawning	325 young at 6 weeks

The spawnings were at fortnightly intervals.

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