The mosquitofish (Gambusia affinis) is a species of freshwater fish in the family Poeciliidae, native to the Mississippi River (USA) and its tributary waters from southern Indiana and Illinois to the Gulf Coast and parts of northeastern Mexico. It is commonly known by its generic name, gambusia, and sometimes called the western mosquitofish to distinguish it from the closely related eastern mosquitofish (G. holbrooki), which according to ITIS is actually a subspecies of G. affinis, rather than its own species; this has complicated classification of G. affinis). The name "mosquitofish" was given because the diet of this fish predominantly consists of large amounts of mosquito larvae (as well as other invertebrate larvae); an adult female can consume hundreds in a day. Hardy to a variety of temperatures, salinities and oxygen levels, mosquitofish have spread through many parts of the world in introductions attempting to reduce mosquito populations. Although this biocontrol did play a major role in containing malaria in South America, Russia and the Ukraine in the 1920s, they are now recognized the Global Invasive Species Database as one of the world’s 100 worst invasive species. This voracious, aggressive predator has extirpated and eliminated many native species, is extremely hard to eradicate, and is now a pest in fresh and brackish waters around the world. Mosquitofish are small; females reach an overall length of 7 centimeters and males at a length of 4 centimeters (1.6 in). Like all other New World members of this family, G. affinis gives birth to live young.

(Froese and Pauly 2011; ITIS 2011; IUCN/SSC Invasive Species Specialist Group 2010; Wikipedia 2012)