Gambusia affinis (Baird & Girard 1853) is a species of freshwater fish.
- Family: Poeciliidae
- Order: Cyprinodontiformes
- Other Names: Top, minnow, its reputation as a mosquito eater is responsible for one of its common names (mosquitofish).
They are a small and stout dull grey, robust fish with a rounded tail and a terminal and upward-pointing mouth adapted for feeding at the water's surface. In these features and their small size they resemble the tropical guppies, which belong to the same taxonomic family (the 'live-bearers', or Poeciliidae). Mature females measure 50 - 60 mm, and males reach only around 25 - 35 mm long.
Females can reach sexual maturity in only six to eight weeks, and they may bear three to four broods of young in a single season. The first may number only a dozen, but later broods include 60 to 100 young. Females store sperm in their reproductive tract for up to two months and give birth to live offspring. Live-bearing gives their young a much higher survival rate than in most species of egg-laying fish, which typically suffer from egg predation.
Under favourable conditions, Gambusia live two to three years. Estimates of their breeding potential have therefore demonstrated an incredible ability for this species to multiply and dominate new habitats into which they have been introduced. Their success in a new environment is almost guaranteed by their rapid maturation, by breeding several times a year, and producing broods of around 50 advanced live young. Individual populations have been recorded expanding from 7,000 to 120,000 in five months.