A group of mosquitofish, Gambusia affinis. Males are much smaller than females. Second fish from the bottom is a male. This small, live bearing fish is famous as a voracious consumer of mosquito larva and for this reason has been widely stocked in suitable climates worldwide as a means of mosquito control. Very often, in addition to devouring mosquitos, this aggressive species has caused the decline of native fish species that had filled a similar niche. It is native to the Atlantic and Gulf drainages of the US from NJ to Mexico and the Mississippi Basin from IN and IL south to the Gulf of Mexico. Photo Dallas County, TX.