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Gambusia affinis (Baird & Girard 1853)
  • Family: Poeciliidae
  • Order: Cyprinodontiformes
  • Other Names: Top, minnow, its reputation as a mosquito eater is responsible for one of its common names (mosquitofish).

These fish are native to the watershed of the Gulf of Mexico, where it has long been known that they feed readily on the aquatic larvae and pupae[?] stages of mosquitoes. They are remarkably hardy, surviving in waters of very low oxygen saturations, high salinities (including twice that of seawater!), and high temperatures; they can even survive in waters up to 42 oC for short periods. For these reasons, this species may now be the most widespread freshwater fish in the world, having been introduced as a biocontrol to tropical and temperate countries in both hemispheres, and then spreading further both naturally and through even further introductions.

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 from which family they also belong (the 'live-bearers', or Poeciliidae). Mature females measure 50 - 60 mm, and males to 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. Being a live-bearer their young encounter a much greater survivorship of young than do egg-laying fishes 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 their new habitats by sheer numbers in which they have been introduced. Their own 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.

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