THE ORGANS OF REPRODUCTION
=The Germ-cells.=--In most fishes the germ-cells are produced in large
sacs, ovaries or testes, arranged symmetrically one on either side of
the posterior part of the abdominal cavity. The sexes are generally but
not always similar externally, and may be distinguished on dissection
by the difference between the sperm-cells and the ova. The ovary
with its eggs is more yellow in color and the contained cells appear
granular. The testes are whitish or pinkish, their secretion milk-like,
and to the naked eye not granular.
[Illustration: FIG. 91.--Sword-tail Minnow, male, _Xiphophorus helleri_
Heckel. The anal fin modified as an intromittent organ. Vera Cruz.]
In a very few cases both organs have been found in the same fish, as
in _Serranus_, which is sometimes truly hermaphrodite. All fishes,
however, seem to be normally dioecious, the two sexes in different
individuals. Usually there are no external genital organs, but
in some species a papilla or tube is developed at the end of the
urogenital sinus. This may exist in the breeding season only, as in the
fresh-water lampreys, or it may persist through life as in some gobies.
In the Elasmobranchs, cartilaginous claspers, attached to the ventral
fins in the male, serve as a conduit for the sperm-cells.
=The Eggs of Fishes.=--The great majority of fishes are oviparous, the
eggs being fertilized after deposition. The eggs are laid in gravel or
sand or other places suitable for the species, and the milt containing
the sperm-cells of the male is discharged over or among them in the
water. A very small quantity of the sperm-fluid may impregnate a large
number of eggs. But one sperm-cell can enter a particular egg. In
a number of families the species are ovoviviparous, the eggs being
hatched in the ovary or in a dilated part of the oviduct, the latter
resembling a real uterus. In some sharks there is a structure analogous
to the placenta of higher animals, but not of the same structure or
origin. In the case of viviparous fishes actual copulation takes place
and there is usually a modification of some organ to effect transfer of
the sperm-cells. This is the purpose of the sword-shaped anal fin in
many top-minnows (_Pæciliidæ_), the fin itself being placed in advance
of its usual position. In the surf-fishes (_Embiotocidæ_) the structure
of part of the anal fin is modified, although it is not used as an
intromittent organ. In the Elasmobranchs, as already stated, large
organs of cartilage (claspers) are developed from the ventral fins.
[Illustration: FIG. 92.--White Surf-fish, viviparous, with young,
_Cymatogaster aggregatus_ Gibbons. San Francisco.]
In some viviparous fishes, as in the rockfishes (_Sebastodes_) and
rosefishes (_Sebastes_), the young are very minute at birth.
[Illustration: FIG. 93.--_Goodea luitpoldi_ (Steindachner). A
viviparous fish from Lake Patzcuaro, Mexico. Family _Pæciliidæ_. (After
Meek.)] In others, as the surf-fishes (_Embiotocidæ_), they are
relatively large and few in number. In the viviparous sharks, which
constitute the majority of the species of living sharks, the young are
large at birth and prepared to take care of themselves.
[Illustration: FIG. 94.--Egg of _Callorhynchus antarcticus_, the
Bottle-nosed Chimæra. (After Parker and Haswell.)]
The eggs of fishes vary very much in size and form. In those sharks
and rays which lay eggs the ova are deposited in a horny egg-case, in
color and texture suggesting the kelp in which they are laid. The eggs
of the bullhead sharks (_Heterodontus_) are spirally twisted, those of
the cat-sharks (_Scyliorhinidæ_) are quadrate with long filaments at
the angles. Those of rays are wheelbarrow-shaped with four "handles."
One egg-case of a ray may sometimes contain several eggs and develop
several young. The eggs of lancelets are small, but those of the
hagfishes are large, ovate, with fibres at each side, each with a
triple hook at tip. The chimæra has also large egg-cases, oblong in
form.
[Illustration: FIG. 95.--Egg of the Hagfish, _Myxine limosa_ Girard,
showing threads for attachment. (After Dean.)]
In the higher fishes the eggs are spherical, large or small according
to the species, and varying in the firmness of their outer walls. All
contain food-yolk from which the embryo in its earlier stages is fed.
The eggs of the eel (_Anguilla_) are microscopic. According to Günther
25,000 eggs have been counted in the herring, 155,000 in the lumpfish,
3,500,000 in the halibut, 635,200 in the sturgeon, and 9,344,000 in the
cod. Smaller numbers are found in fishes with large ova. The red salmon
has about 3500 eggs, the king salmon about 5200. Where an oviduct
is present the eggs are often poured out in glutinous masses, as in
the bass. When, as in the salmon, there is no oviduct, the eggs lie
separate and do not cohere together. It is only with the latter class
of fishes, those in which the eggs remain distinct, that artificial
impregnation and hatching is practicable. In this regard the value of
the salmon and trout is predominant. In some fishes, especially those
of elongate form, as the needle-fish (_Tylosurus_), the ovary of but
one side is developed.
[Illustration: FIG. 96.--Egg of Port Jackson Shark, _Heterodontus
philippi_ (Lacépède). (After Parker and Haswell.)]
=Protection of the Young.=--In most fishes the parents take no care
of their eggs or young. In some catfishes (_Platystacus_) the eggs
adhere to the under surface of the female. In a kind of pipefish
(_Solenostomus_), a large pouch for retention of the eggs is formed on
the belly of the female. In the sea-horses and pipefishes a pouch is
formed in the skin, usually underneath the tail of the male. Into this
the eggs are thrust, and here the young fishes hatch out, remaining
until large enough to take care of themselves. In certain sea catfishes
(_Galeichthys, Conorhynchos_) the male carries the eggs in his mouth,
thus protecting them from the attacks of other fishes. In numerous
cases the male constructs a rough nest, which he defends against all
intruders, against the female as well as against outside enemies.
The nest-building habit is especially developed in the sticklebacks
(_Gasterosteidæ_), a group in which the male fish, though a pygmy in
size, is very fierce in disposition.
In a minnow of Europe (_Rhodeus amarus_) the female is said to deposit
her eggs within the shells of river mussels.
=Sexual Modification.=--In the relatively few cases in which the sexes
are unlike the male is usually the brighter in color and with more
highly developed fins. Blue, red, black, and silvery-white pigment
are especially characteristic of the male, the olivaceous and mottled
coloration of the female. Sometimes the male has a larger mouth, or
better developed crests, barbels, or other appendages. In some species
the pattern of coloration in the two sexes is essentially different.
In various species the male develops peculiar structures not found in
the female, and often without any visible purpose. In the chimæra a
peculiar cartilaginous hook armed with a brush of enamelled teeth at
the tip is developed on the forehead in the male only. In the skates
or true rays (_Raja_) the pectoral fin has near its edge two rows
of stout incurved spines. These the female lacks. In the breeding
season, among certain fishes, the male sometimes becomes much brighter
by the accumulation of bright red or blue pigment accompanied by
black or white pigment cells. This is especially true in the minnows
(_Notropis_), the darters (_Etheostoma_), and other fresh-water species
which spawn in the brooks of northern regions in the spring. In the
minnows and suckers horny excrescences are also developed on head,
body, or fins, to be lost after the deposition of the spawn.
In the salmon, especially those of the Pacific, the adult male becomes
greatly distorted in the spawning season, the jaws and teeth being
greatly elongated and hooked or twisted so that the fish cannot shut
its mouth. The Atlantic salmon and the trout show also some elongation
of the jaws, but not to the same extent.
In those fishes which pair the relation seems not to be permanent, nor
is there anything to be called personal affection among them so far as
the writer has noticed.
There is no evidence that the bright colors or nuptial adornments of
the males are enhanced by sexual selection. In most species the males
deposit the sperm-cells in spawning-grounds without much reference to
the preference of the females. In general the brightest colors are not
found among viviparous fishes. None of the groups in which the males
are showily colored, while the females are plain, belong to this class.
The brightest colors are found on the individuals most mature or having
greatest vitality.