The mosquitofish (Gambusia affinis) is a species of freshwater fish, also commonly, if ambiguously, known by its generic name, gambusia. It is sometimes called the western mosquitofish, to distinguish it from the eastern mosquitofish (G. holbrooki). Mosquitofish are small in comparison to other fish, with females reaching an overall length of 7 centimeters and males at a length of 4 centimeter. Females can be distinguished from males by their size and a gravid spot at the posterior of their abdomen. The name "mosquitofish" was given because the diet of this fish predominantly consists of large amounts of mosquito larvae, relative to body size. Mosquitofish were introduced to many parts of the world as a biocontrol to lower mosquito populations. However, such practices appear to be more detrimental to ecosystems than to mosquito population levels.

Based on diet, mosquitofish are classified as larvivorous fish. Their diet consists of zooplankton, small insects and insect larvae, and detritus material. Mosquitofish feed on mosquito larvae at all stages of life. Adult females can consume in one day hundreds of mosquito larvae. Maximum consumption rate in a day by one mosquitofish has been observed to be from 42%-167% of its own body weight. Mosquitofish have also shown cannibalistic behavior in laboratory experiments; however, whether these traits are hereditary is unknown.

Misquitofish are found most abundantly in shallow water protected from larger fish. They can survive in relatively inhospitable environments, and are resilient to low oxygen concentrations, high salt concentrations (up to twice that of sea water), and temperatures up to 42 °C (for short periods). Because of their notable adaptability to harsh conditions and ability to survive in many areas throughout the world, they are considered to be the most widespread freshwater fish. Another contributor to the mosquitofish's pan global occurrence is human intervention, as in biocontrol to lower mosquito populations.

Mosquitofish were intentionally introduced in many areas with large mosquito populations to decrease the population of mosquitoes by eating the mosquito larvae. However, most introductions were ill-advised; in most cases native fish had already proven to supply maximal control of mosquito population and introducing mosquitofish has been more harmful to indigenous aquatic life than to the mosquito population. Introductions outside the mosquitofish's natural range, can be harmful to the nonnative ecosystems. Mosquitofish have been known to kill or injure other small fish by their aggressive behavior and otherwise harm them through competition. They are now considered just slightly better at eating mosquitoes than at destroying other aquatic species. However, from the 1920s to 1950's, mosquitofish were a major factor in the eradication of malaria in South America, in southern Russia and in Ukraine. A somewhat famous example of mosquitofish eradicating malaria is on the coast of the Black Sea located near a city in Russia called Sochi. In Sochi, the mosquitofish is commemorated for eradicating malaria by a monument of the fish. In 2008, in some parts of California, mosquitofish were breed in aquariums so that people could stock stagnant pools of water with these fish to reduce the number West Nile virus cases.

The average lifespan for a mosquitofish averages less than a year and the maximum is about 1.5 years. Male mosquitofish lifespans are considerably shorter than the hardier females.
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