Some of the most commonly kept fish in the freshwater aquarium are the livebearers. These fish are so called because they give birth to live young rather than laying eggs as is typical of most fish species. Male livebearers have a modified anal fin that allows them to internally fertilise a female fish of the same species. In swordtails, platies, mollies, guppies & gambusia this modified anal fin is called a gonopodium. In other livebearing species like the goodeids & halfbeaks, the modified anal fin is called an andropodium. During internal fertilisation the males impregnate the females with spermatophores. The female can store and use these sperm packets to fertilise several batches of eggs in her abdomen when environmental conditions are good. This has the advantage that if a male is not available later on the females can still produce several more batches of young ensuring the species future survival.
male mollies showing gonopodium (modified anal fin)