CHAPTER XIII.. Growth and Variation of Fishes.

GROWTH AND VARIATION OF FISHES.

Changes of form normally accompanying growth (after absorption of the vitelline sac) are observed in all fishes; but in the majority they affect only the proportional size of the various parts of the body. In young fishes the eyes are constantly larger than in adult relatively to the size of the head; and again, the head is larger relatively to that of the body. Changes amounting to metamorphosis have been hitherto observed in Petromyzon only. In the larval condition (Ammocætes) the head is very small, and the toothless buccal cavity is surrounded by a semicircular upper lip. The eyes are extremely small, hidden in a shallow groove; and the vertical fins form a continuous fringe. In the course of three or four years the teeth are developed, and the mouth changes into a perfect suctorial organ; the eyes grow; and the dorsal fin is divided into two divisions. In Malacopterygians and Anacanths the embryonal fringe from which the vertical fins are developed, is much longer persistent than in Acanthopterygians. A metamorphosis relating to the respiratory organs, as in Batrachians, is indicated in the class of Fishes by the external gills with which fœtal Plagiostomes (Fig. 58, p. 136) and the young of some Ganoids, viz. the Protopterus and Polypterus, are provided.

mx, Maxillary tooth; md, Mandibulary tooth; l, Lingual tooth; s, Suctorial teeth.]

I. Of an individual, 1¼ inch long. II. Of an individual, 2 inches long.]

One of the most extraordinary changes by which, during growth, the form and position of several important organs are affected, occurs in Flat-fishes (Pleuronectidæ); their young are symmetrically formed, with a symmetrical mouth, and with one eye on each side, and, therefore, keep their body in a vertical position when swimming. As they grow they live more on the bottom, and their body, during rest, assumes a horizontal position; in consequence, the eye of the lower side moves towards the upper, which alone is coloured; and in many genera the mouth is twisted in the opposite direction, so that the bones, muscles, and teeth are much more developed on the blind side than on the coloured. In a great number of other Teleostei certain bones of the head show a very different form in the young state. Ossification proceeds in those bones in the direction of lines or radii which project in the form of spines or processes; as the interspaces between these processes are filled with bone, the processes disappear entirely, or at least project much less in the older than in the younger individuals (Fig. 84). The young of some fishes may be armed with a long powerful præopercular or scapular spine, or show a serrature of which nothing remains in the adult fish except some ridges or radiating lines. These processes seem to serve as weapons of defence during a period in the life of the fish in which it needs them most. In not a few instances a portion of this armature is so much developed that the disappearance of its most projecting parts with the growth of the fish is not only due to its being surrounded by other bone, but, partially at least, caused by absorption. The Carangidæ, Cyttidæ, Squamipinnes, Xiphiidæ, offer instances of such remarkable changes. A fish, described as Tholichthys osseus (Fig. 85), is probably the young of a Cyttoid, the suprascapula, humerus, and præoperculum forming enormously enlarged plates. In the fish Fig. 86 those bones appear still enlarged, and the frontals develop a remarkably long and curved horn above the orbit. In the Tholichthys-stage of Pomacanthus (specimens 10 millimetres long, Fig. 87), the frontal bone is prolonged into a straight lancet-shaped process, nearly half as long as the body; the suprascapular and præopercular processes cover and hide the dorsal and ventral fins. The plates attached to the shoulder-girdle remain persistent until the young fish has assumed the form of the adult; thus they are still visible in young Chætodon citrinellus, 30 millimetres long, in which the specific characters are already fully developed.--The Sword-fishes with ventral fins (Histiophorus) belong to the Teleosteans of the largest size; in young individuals, 9 millimetres long (Fig. 89), both jaws are produced, and armed with pointed teeth; the supraorbital margin is ciliated; the parietal and præoperculum are prolonged into long spines; the dorsal and anal fins are a low fringe, and the ventrals make their appearance as a pair of short buds. When 14 millimetres long (Fig. 90) the young fish has still the same armature of the head, but the dorsal fin has become much higher, and the ventral filaments have grown to a great length. At a third stage, when the fish has attained to a length of 60 millimetres, the upper jaw is considerably prolonged beyond the lower, losing its teeth; the spines of the head are shortened, and the fins assume nearly the shape which they retain in mature individuals. Young Sword-fishes without ventral fins (Xiphias) undergo similar changes; and, besides, their skin is covered with small rough excrescences longitudinally arranged, which continue to be visible after the young fish has assumed the form of the mature in other respects (Fig. 92).

The Plectognaths show no less extraordinary changes: an extraordinary form taken in the South Atlantic, and named Ostracion boops, is considered by Lütken to be the young of a Sun-fish (Orthagoriscus). In very young more advanced Sun-fishes (18 to 32 millimetres) the vertical diameter of the body exceeds, or is not much less than, the longitudinal; and small conical spines are scattered over its various parts. The caudal fin is developed long after the other vertical fins.

Similar changes take place in a number of other fishes, and in many cases the young are so different that they were described as distinct genera: thus Priacantichthys has proved to be the young of Serranus, Rhynchichthys that of Holocentrum, Cephalacanthus of Dactylopterus, Dicrotus of Thyrsites, Nauclerus of Naucrates, Porthmeus of Chorinemus, Lampugus of Coryphæna, Acronurus of Acanthurus, Keris of Naseus, Porobronchus of Fierasfer, Couchia of Motella, Stomiasunculus of Stomias, etc.

The fins are most frequently subject to changes; but, whilst in some fishes parts of them are prolonged into filaments with age, in others the filaments exist during the early life-periods only; whilst in some a part of the dorsal or the ventral fins is normally developed in the young only, in others those very parts are peculiar to the mature age. The integuments are similarly altered: in some species the young only has asperities on the skin, in others the young are smooth and the old have a tubercular skin; in some the young only have a hard bony head; in others (some Siluroids) the osseous carapace of the head and neck, as it appears in the adult, is more or less covered with soft skin whilst the fish is young.

In not a few fishes the external changes are in relation to the sexual development (Callionymus, many Labyrinthici, Cyprinodonts). These secondary sexual differences show themselves in the male individual, only when it commences to enter upon his sexual functions, and it may require two or more seasons before its external characteristics are fully developed. Immature males do not differ externally from the old female. The male secondary sexual characters consist principally in the prolongation of some of the fin-rays, or of entire fins; and in Salmonidæ in the greater development of the jaw-bones. The coloration of the male is in many fishes much brighter and more variegated than that of the female, but in comparatively few permanent (as in some Callionymus, Labrus mixtus); generally it is acquired immediately before and during the season of propagation only, and lost afterwards. Another periodical change in the integuments, also due to sexual influence and peculiar to the male, is the excrescence of wart-like tubercles on the skin of many Cyprinoids; they are developed chiefly on the head, but sometimes extend over the whole body and all the fins.

With regard to size, it appears that in all Teleosteous fishes the female is larger than the male; in many Cyprinodonts the male may be only one-sixth or even less of the bulk of the female. The observations on the relative size of the sexes are few in Palæichthyes, but such as have been made tend to show that, if a difference exists at all, the male is generally the larger (Lepidosteus). In the Rays (Raja) the sexes, after they have attained maturity, differ in the development of dermal spines and the form of the teeth, the female being frequently much rougher than the male. There is much variation in this respect in the different species; but the males are constantly distinguished by an oblong patch of erectile claw-like spines on each pectoral fin, and by having the teeth (all, or only a portion) pointed, and not obtuse, like those of the females. In Sharks no secondary sexual differences have been observed; the male Chimæridæ (see Fig. 96, p. 184), possess a singular comb-like cartilaginous appendage on the top of the head, which can be erected or depressed into a groove, both the appendage and the anterior part of the groove being armed with hooklets. The use of this singular organ is not known.

The majority of Teleostei are mixogamous--that is, the males and females congregate on the spawning-beds, and the number of the former being in excess, several males attend to the same female, frequently changing from one female to another. The same habit has been observed in Lepidosteus. Gastrosteus is truly polygamous, several females depositing their ova into the same nest, guarded by one male only. Some Teleostei (Ophiocephalus), and probably all Chondropterygians, are monogamous; and it is asserted that the connection between the pair is not merely temporary, but lasts until they are separated by accident. Monogamous are probably also all those Teleosteans which bring forth living young, and those, the males of which, for the attraction of the female, are provided with appendages, or ornamented with a bright coloration.

Hybridism is another source of changes and variations within the limits of a species, and is by no means so scarce as has been believed hitherto; it is only apparently of exceptional occurrence, because the life of fishes is more withdrawn from our direct observation than that of terrestrial animals. It has been observed among species of Serranus, Pleuronectidæ, Cyprinidæ, Clupeidæ, and especially Salmonidæ. As in other animals, the more certain kinds of fishes are brought under domestication, the more readily do they interbreed with other allied species. It is characteristic of hybrids that their characters are very variable, the degrees of affinity to one or the other of the parents being inconstant; and as these hybrids are known readily to breed with either of the parent race, the variations of form, structure, and colour are infinite. Of internal organs the dentition, gill-rakers, pyloric appendages, are those particularly affected by such mixture of species.

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Some fishes are known to grow rapidly (in the course of from one to three years) and regularly to a certain size, growth being definitely arrested after the standard has been attained. Such fishes may be called “full-grown,” in the sense in which the term is applied to warm-blooded Vertebrates--the Sticklebacks, most Cyprinodonts, and many Clupeoids (Herring, Sprat, Pilchard) are examples of this regular kind of growth. But in the majority of fishes the rate of growth is extremely irregular, and it is hardly possible to know when growth is actually and definitely arrested. All seems to depend on the amount of food and the more or less favourable circumstances under which the individual grows up. Fishes which rapidly grow to a definite size are short-lived, whilst those which steadily and slowly increase in size attain to a great age, Teleosteans as well as Chondropterygians. Carp and Pike have been ascertained to live beyond a hundred years.

It is evident that such diversity and irregularity of growth in the same species is accompanied by considerable differences in the appearance and general development of the fish. No instance is more remarkable than that of the so-called Leptocephali, which for a long time have been regarded either as a distinct group of Fishes, or as the larval stages of various genera of fishes.

The Leptocephali proper are small, narrow, elongate, more or less band-shaped fishes, pellucid in a fresh state, but assuming a white colour when preserved in spirits, resembling a tapeworm, being quite as soft and flexible. The skeleton is entirely cartilaginous, or slight ossifications are only now and then visible, especially towards the end of the vertebral column. The latter is replaced by a chorda dorsalis which, in many specimens, is found to be divided into numerous segments. Neural arches are sometimes present in their rudimentary condition. The anterior end of the chorda passes into the cartilaginous base of the skull, the connection not being by means of joint and ligaments. Hæmal arches are found on the caudal portion. Ribs none. The skull, like the vertebral column, is nearly entirely cartilaginous. The basisphenoid, frontal, and jaw-bones are the first which may be distinguished, and the mandible has generally ossifications.

The muscles are generally not attached to the chorda, which is surrounded by a thick gelatinous mass, separating the lateral sets of muscles from each other. These muscles are attached to the external integument, each forming a thin flat angular band, the angle being directed forwards. However, specimens are frequently found in which the muscles are more developed, evidently at the expense of the gelatinous matter, which is diminished in quantity. They are attached to the chorda, and the entire fish has a more cylindrical form of the body (Helmichthys).

The nervous, circulatory, and respiratory organs are well developed. In those with a sub-cylindrical body the blood is red, in those with a flat body the blood-corpuscles show but rarely a faint coloration. There are four branchial arches, and in some (Tilurus) pseudobrauchiæ have been found. The gill-openings are more or less narrow. The nostrils are double on each side, and the posterior is close to the eye.

The stomach has a large blind sac, and in Leptocephalus two lateral cæca. The intestine is straight, running close to the abdominal profile, with a small appendix directed forward and a larger one directed backwards. The vent is nearly always very small, and, in preserved examples at least, cannot always be discovered. Its position is variable, even in examples entirely similar in other points. Air-bladder none. No trace of generative organs.

The vertical fins, when present, are confluent, with more or less conspicuous traces of rays; sometimes they are merely a fold of the skin, without any rays. Pectoral fins sometimes present, sometimes rudimentary, sometimes entirely absent. Ventrals none.

Most examples have series of round black dots along each side of the abdominal profile, along the lateral line, and sometimes along the dorsal fin. They remind us of the luminous organs of many Scopelidæ, Stomiatidæ, and other pelagic fishes, but are composed entirely of pigmentary cells.

These fishes are found floating in the sea, frequently at a great distance from land. Their movements are slow and languid. The largest specimen of Leptocephalus observed was 10 inches, but specimens of that size are very rare.

Taking into account all the various facts mentioned, we must come to the conclusion that the Leptocephalids are the offspring of various kinds of marine fishes, representing, not a normal stage of development (larvæ), but an arrest of development at a very early period of their life; they continue to grow to a certain size without corresponding development of their internal organs, and perish without having attained the characters of the perfect animal. The cause by which this abnormal condition is brought about is not known; but it is quite within the limits of probability that fishes usually spawning in the vicinity of land sometimes spawn in the open ocean, or that floating spawn is carried by currents to a great distance from land; and that such embryoes, which for their normal growth require the conditions afforded by the vicinity of the shore, if hatched in mid-ocean, grow into undeveloped hydropic creatures, such as the Leptocephales seem to be.

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Abundance or scarcity of food, and other circumstances connected with the localities inhabited by fishes, affect considerably the colour of their muscles and integuments; the periodical changes of colour in connection with their sexual functions have been referred to above (p. 176). The flesh of many Teleostei is colourless, or but slightly tinged by the blood; that of Scombridæ, most Ganoids and Chondropterygians, is more or less red; but in badly-fed fishes, as well as in very young ones, the flesh is invariably white (anæmic). Many fishes, like the Salmonidæ, feed at times exclusively on Crustaceans, and the colouring substance of these Invertebrates, which by boiling and by the stomachic secretion turns red, seems to pass into the flesh of the fishes, imparting to it the well-known “salmon” colour. Further, the coloration of the integuments of many marine fish is dependent on the nature of their surroundings. In those which habitually hide themselves on the bottom, in sand, between stones or seaweeds, the colours of the body readily assimilate to those of the vicinity, and are thus an important element in the economy of their life. The changes from one set or tinge of colours to another may be rapid and temporary, or more or less permanent; in some fishes--as in the Pediculati, of which the Sea-Devil, or Lophius, and Antennarius are members--scarcely two individuals are found exactly alike in coloration, and only too frequently such differences in coloration are mistaken for specific characters. The changes of colours are produced in two ways: either by an increase or decrease of the black, red, yellow, etc., pigment-cells, or chromatophors, in the skin of the fish; or by the rapid contraction or expansion of the chromatophors which happen to be developed. The former change is gradual, like every kind of growth or development; the latter rapid, owing to the great sensitiveness of the cells, but certainly involuntary. In many bright-shining fishes--as Mackerels, Mullets--the colours appear to be brightest in the time intervening between the capture of the fish and its death: a phenomenon clearly due to the pressure of the convulsively-contracted muscles on the chromatophors. External irritation readily excites the chromatophors to expand--a fact unconsciously utilised by fishermen, who, by scaling the Red Mullet immediately before its death, produce the desired intensity of the red colour of the skin, without which the fish would not be saleable. However, it does not require such strong measures to prove the sensitiveness of the chromatophors to external irritation, the mere change of darkness into light is sufficient to induce them to contract, the fish appearing paler, and vice versa. In Trout which are kept or live in dark places, the black chromatophors are expanded, and, consequently, such specimens are very dark-coloured; when removed to the light they become paler almost instantaneously.

Total absence of chromatophors in the skin, or Albinism, is very rare among fishes; much more common is incipient Albinism, in which the dark chromatophors are changed into cells with a more or less intense yellow pigment. Fishes in a state of domestication, like the Crucian Carp of China, the Carp, Tench, and the Ide, are particularly subject to this abnormal coloration, and are known as the common Gold-fish, the Gold-Tench, and the Gold-Orfe. But it occurs also not rarely in fishes living in a wild state, and has been observed in the Haddock, Flounder, Plaice, Carp, Roach, and Eel.

It will be evident, from the foregoing remarks, that the amount of variation within the limits of the same species--either due to the natural growth and development, or to external physical conditions, or to abnormal accidental circumstances--is greater in fishes than in any of the higher classes of Vertebrates. The amount of variation is greater in certain genera or families than in others, and it is much greater in Teleosteans and Ganoids than in Chondropterygians. Naturally, it is greatest in the few species which have been domesticated, and which we shall mention in the succeeding chapter.