Common name for Gambusia affinis (Baird & Girard 1853), a small (3-6 cm.), viviparous fish that ranges from northern Mexico through Texas and Lousiana northward to Nebraska and Illinois. They have been introduced in many tropical and subtropical habitats around the world to control mosquito populations, often with disasterous consequences for native fish; see the writeup for Gambusia. Their current taxonomic classification is:
Superorder: Acanthopterygii (spiny-rayed fishes)
Species: affinis (Baird & Girard 1853)
These fish are related to guppies (both are in the family Poeciliidae) and share the same general body shape. They are much less colorful, however: they are typically olive gray with silvery bellies. The scales are edged in black. A black stripe runs from the eye down the lower edge of the operculum. They are very similar to the Eastern Mosquitofish, Gambusia holbrooki, but can be distinguished by counting the rays in the dorsal fin: 6-7 rays in G. affinis, 8-9 rays in G. holbrooki. (Aren't you glad that someone bothered to do this?)
G. affinis are easily kept in aquariums and thrive in fresh or brackish water in temperatures between 20C and 28C. Like guppies, breeding requires no effort other than providing hiding places (such as floating plants) for the young. The young are born in five to eight weeks after fertilization, and broods range from 10 to 80.