Fish have babies, or reproduce, in various ways. The two main ways fish breed are laying eggs and bearing live young, both considered sexual reproduction. Sexual reproduction occurs when the sperm and egg fuse to form a zygote, or fertilized egg, whereas asexual reproduction occurs when an organism multiplies by dividing itself.
Advantages and Disadvantages of Sexual Reproduction
Sexual reproduction creates distinctive offspring different from the parent fish, whereas asexual reproduction produces identical offspring. Sexual reproduction is advantageous because it helps increase genetic diversity in the species. This genetic diversity can sometimes make the difference between survival and extinction for a species experiencing changing conditions. The disadvantages of sexual reproduction include the need to find a mate, exposure to predators, exposure to disease from the mate and a lower rate of population growth.
Some egg-laying fish drop their eggs in the water, where they are fertilized by sperm from the male; others drop their eggs after they're fertilized internally. Some egg-laying fish, such as trout, lay eggs in protected areas like plants, rocks and wood, where the eggs are left to hatch. Many egg-laying fish lay a large number of eggs and have nothing to do with the offspring. In these cases, laying a large number of eggs can be advantageous because usually only a few offspring survive.
Egg keepers lay their eggs and keep them with them until they hatch. Some fish, such as mouthbrooders, keep their eggs in their mouths, while others such as darters and catfish guard the places where they lay their eggs, males chasing off potential predators. Fish that keep or guard their eggs often produce a smaller number of offspring.
Some fish, such as sharks, rays, molly, gambusia, rockfish, guppies and surfperch give birth to live young. Live-bearing fish are internally fertilized by the male fish and carry the fertilized egg inside of them before giving birth to live offspring. Some species of sharks even nourish their unborn young via a placenta.
Some fish are hermaphrodites, meaning they have both male and female sex organs or change sexes. Sometimes these fish fertilize themselves. But since this act isn't reproduction by simple cellular division or replication, it still carries the classification of sexual reproduction.