REPRODUCTION
Most fishes are egg-layers, but many bear living young. Live-bearing fishes may be
ovoviviparous, in which the eggs essentially simply hatch within the female, or
viviparous, in which the unborn young are supplied nourishment through the mother's
tissues. In some ovoviviparous fishes the embryo develops in the egg while the egg is
still within its follicular covering within the ovary, and ovulation (or release of the
egg) and birth occur at the same time. In other ovoviviparous forms the eggs are released
from the protective follicles into the cavity of the hollow ovary, where development
continues. In some viviparous fishes the walls of the egg follicle are in intimate contact
with the embryo, supplying it with nourishment. In the viviparous sharks, a part of the
oviduct, or egg channel, is developed into a uterus, where the modified yolk sacs of the
young are closely joined to pockets within the uterus.
In live-bearing fishes and in some egg-layers, fertilization occurs internally, and
methods have been evolved for introducing the sperm into the female's body. In sharks the
pelvic fins of the male are modified into intromittent organs called myxoptergia, and in
the male topminnows the anal fin is modified into a similar-functioning intromittent organ
called the gonopodium.
At least three modes of reproduction--heterosexual, hermaphroditic, and
parthenogenetic--are found in fishes. In the most common form, heterosexual reproduction,
there are separate male and female parents, but even here there is considerable variation.
In some live-bearing fishes, the female is able to store sperm for up to 8 or even 10
months, and this sperm is used to fertilize new batches of eggs as they develop. In some
cases, a female may carry sperm from several males at once.
In hermaphroditic reproduction, a single fish is both male and female, produces both eggs
and sperm (either at the same time or at different times), and mates with other similar
hermaphroditic fishes. External self-fertilization occurs in one hermaphroditic fish,
which sheds egg and sperm simultaneously. In another, internal self-fertilization may
occur. In certain fishes there is a time sequence of hermaphroditism, young fishes
reversing their sex as they grow older.
In parthenogenetic reproduction, unfertilized eggs develop into embryos. This is known to
exist in at least one fish species, Poecilia formosa, of the Amazon River; however, even
though development proceeds without fertilization in some of these females, mating with a
male is still required to stimulate egg development.
Parental care also shows great diversity. Some fishes, like the Atlantic herring, form
huge schools of males and females and freely shed their eggs and sperm (milt), and then
abandon the eggs. Other fishes build nests and care for both the eggs and newly hatched
young. Others have evolved methods of carrying the eggs with them, commonly in their
mouths, but also in gill cavities or in special pouches on the body.
Distribution - Anatomy - Circulation - Respiration - Air Breathing
Body Temperature - Water
Balance - Swimming - Gas Bladder
Lateral Line System - Evolution