The mosquitofish is a member of the large Poeciliidae family of livebearers, which is closely related to killifish or cyprinodonts, differing from them mainly in bringing forth its young alive, rather than laying eggs.
Also known as the North American topminnow or the western mosquitoﬁsh, this species is famous as the numberone scourge of mosquito larvae. Although there are other larvae-eating species of fish, the mosquitofish tolerates salinity and pollution levels that would kill most other species, and it produces up to 1,500 young in its lifetime.
Native to the southeastern United States, the mosquitoﬁsh has been introduced to suitable warm waters around the world since 1905, when it was experimentally introduced to Hawaii and virtually eliminated mosquitoes. As a result, Gambusia affinis affinis is the widest-ranging freshwater fish on earth (other species of mosquitofish have not been as successfully introduced). It has most recently been introduced in many places to help control West Nile virus.
Female mosquitofish are about 2 inches long, and the males are only half as large. The anal ﬁn of the male is modified to form an intermittent organ for introducing sperm into the female. A mature female may produce three or four broods during one season, sometimes giving birth to 200 or more young at a time. This ﬁsh is easily raised in aquariums and is not sensitive to temperature variations, but it does not adjust well to living with other ﬁsh.
Although it has been highly effective at controlling malarial mosquitoes, the mosquitoﬁsh is not a panacea. Mosquitofish larvae cannot survive without water (as mosquito larvae can), they do not control mosquitoes in places with abundant surface vegetation to hide mosquito larvae, they may consume the young of forage and game species, and they can have adverse effects on indigenous ﬁsh species.