Page 63 - Encyclopedia of Aquarium and Pond Fish, 3rd Edition

REPRODUCTIVE BEHAVIOR 61

SEXING FISH
In some species, there are obvious differences between the sexes.
This is called sexual dimorphism. Male livebearers, for example, are
smaller than females and possess a gonopodium. In many tropical
species, males tend to be more colorful and have more elaborate
fins, which they use for ritual display during courtship. However,
many of the fish sold in aquatic stores are immature, so it will not
be possible to determine their gender at the time of purchase. Other
distinguishing features, such as a specific breeding coloration, may
When livebearers, such as these External fertilization is the most
mollies, mate, the gonopodium of typical method of fish reproduction. become obvious only when the fish are ready to spawn. Likewise,
the male enters the female’s genital Here, a male angelfish releases his an egg-laying female’s genital pore is not visible until just before
pore and transfers a tiny sperm sperm over eggs that a female has spawning, when her body is also likely to be swollen with eggs.
packet that contains up to 3,000 just laid on a leaf. It is unlikely that all In Viejita Dwarf Cichlids (Apistogramma
individual sperm. the eggs will be fertilized. viejita), it is easy to tell the sexes apart. Female
Males are significantly larger than females
fertilize eggs immediately do not die off, as happens in the and have longer fins. They also have a very
different coloration.
mammalian reproductive tract, but instead remain viable for
the life of the female, probably nourished by an output of Male
sugars from the ovaries. This is why a female livebearer
housed on her own can still give birth to successive broods of
fry, using stored sperm from past matings to fertilize the eggs.
As a result, a number of males may be responsible for the
young born in a single brood.
In some livebearers, including guppies (see pp.165–167), the
embryos are nourished by their yolk sac as the eggs develop
in their mother’s body. As she gives birth, the fry hatch from
their eggs. In the One-Sided Livebearer (Jenynsia lineata),
placentalike links form between the body of the female and
the embryos. The mother provides nutrients through cords
that develop between the ovarian wall and either the anus
or mouth and gills of the young fish.

Breeding strategies in egg-layers
Since livebearers nurture their young through the most
vulnerable early stages of life, they do not need to produce
as many offspring as egg-layers. An egg-laying female may
lay hundreds or thousands of eggs, of which only a small
proportion survive and hatch. Although the incubation
period is brief—often about 36 hours—the eggs are at
great risk during this time. Eggs are eagerly devoured
by fish (sometimes even the parents) and other aquatic
animals, since they provide a rich source of protein. Some
egg-layers deposit their eggs in relatively inaccessible places
where they are more likely to escape the attentions
of predators, such as in caves, on the underside of leaves, or
even, in the case of Lamprologus cichlids (see p.155), in empty
snail shells. Others guard their eggs until they hatch.
To protect their eggs from the dangers of incubating in the
open, a number of species exhibit a behavior called mouth-
brooding. After spawning, one or the other of the pair carries Haplochromid cichlids are mouth-brooders. After spawning, the female
nips at “egg spots,” shown here, on the male’s anal fin. As she does so, she
the eggs in the mouth for up to three weeks until they hatch, swallows his sperm, ensuring that the eggs in her mouth become fertilized.

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