Fig. 94.—Young of Orthagoriscus, 18 and 32 mill. long. (Natural size.)
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.
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.[15] 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.