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.
[Illustration: Fig. 82.--Mouth of Larva of Petromyzon
branchialis.]
[Illustration: Fig. 83.--Mouth of Petromyzon fluviatilis.
_mx_, Maxillary tooth; _md_, Mandibulary tooth;
_l_, Lingual tooth; _s_, Suctorial teeth.]
[Illustration: Fig. 84.--Armature of præoperculum of young Caranx
ferdau. (Magnified.)
I. Of an individual, 1¼ inch long. II. Of an individual, 2 inches
long.]
[Illustration: Fig. 85.--Tholichthys osseus. Six times the
natural size.]
[Illustration: Fig. 86.--Tholichthys-stage of Heniochus (?).]
[Illustration: Fig. 87.--Tholichthys-stage of Pomacanthus (magn.)
Atlantic.]
[Illustration: Fig. 88.--Young Chætodon citrinellus (30 mill.
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).
[Illustration: Fig. 89.--Young Sword-fish (Histiophorus), 9 mill.
long. Atlantic. (Magn.)]
[Illustration: Fig. 90.--Young Sword-fish (Histiophorus), 14
mill. long. South Atlantic. (Magn.)]
[Illustration: Fig. 91.--Young Sword-fish (Histiophorus), 60
mill. long. Mid-Atlantic.]
[Illustration: Fig. 92.--Xiphias gladius, young, about 8 inches
long.]
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.
[Illustration: Fig. 93.--“Ostracion boops” (much magnified).]
[Illustration: 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.
[Illustration: Fig. 95.--Leptocephalus.]
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.
[See Kölliker, Zeitschr. wiss. Zool. iv. 1852, p. 360; and
Carus, Ueber die Leptocephaliden. Leipz. 1861. 4to.]
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.
* * * * *
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.
[Illustration: Fig. 96.--Chimæra colliei ♂, west coast of
North America. A. Front view of head. B. Palate. _a_, Peritoneal
aperture; _b_, Nostrils; _c_, Vomerine teeth; _d_, Mandibular
teeth; _e_, Palatine teeth; _f_, Claspers.]