Fish can be divided into two broad categories on the basis of their breeding behavior. Egg-layers, which form the majority, practice external fertilization, while in live bearers the eggs are fertilized in the female's body and emerge fully formed young. Some species show little regard for their offspring and cannibalize their own eggs or fry. Others display surprisingly high levels of parental care.
When fish mate, it is known as spawning. In egg-layers, this involves the male releasing sperm into the water at roughly the same time that the female expels her eggs nearby, so that fertilization occurs outside the females body. This is a rather haphazard method of reproduction, and not surprisingly, a significant proportion of the eggs fail to become fertilized. In order to maximize the chances of fertilization occurring, some egg-layers attempt to maneuver their genital openings as close to each other as possible before spawning occurs, and there is occasionally contact between the fish, such as embraces with the fins.
Reproduction in live bearers:
During mating, sperm are transferred into the female's body in the form of a sperm packet. This dissolves within about 15 minutes, freeing the sperm. Any sperm that fail to fertilize immediately do not die off, as happens in the mammalian reproductive tract, but instead remain viable for the life of the female, probably nourished by and output of sugars from the ovaries. This is why a female live bearer housed on her own can still give birth to successive broods of fry, using stored sperm from the past matings to fertilize the eggs. As a result, a number of males may be responsible for the young born in a single-brood.
Breeding strategies in egg-layers:
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 the eggs in the mouth for up to three weeks until they hatch, during which time the adult fish does not feed. The fry emerge from their parent's mouth when they are able to swim freely. Mouth brooding is best-documented in members of the cichlid family, but it also occurs in some bettas and catfish. These fish produce far fewer egs than other egg-layers, but their eggs stand a better chance of survival.
Survival of the fry:
Nor is it just cichlids that display parental care. A number of catfish also guard their eggs, as do various anabatoids, including the popular Siamese Fighting Fish. The male of this species constructs a special nest for the eggs by blowing air bubbles, and then guards the nest and watches over its fry during the immediate post-hatching period, when they are at their most vulnerable.
The more developed fry are when they emerge into their surroundings, the better their chances of survival. Mouth-brooding helps to give the fry of some egg-layers a head start in life. The female Mosquito fish, a livebearer, uses a different process, called superfetation, to improve the odds for her young at varying stages of growth. As a result, Least Killiefish fry are proportionately larger and better developed than those of similar live bearers when they are born.
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This is an article post submitted by: Silver Dollars