The western mosquitofish (Gambusia affinis) is a species of freshwater fish, also known commonly, if ambiguously, as simply mosquitofish or by its generic name, gambusia, or by the common name gambezi. There is also an eastern mosquitofish (G. holbrooki).
Mosquitofish are small in comparison to other fish, with females reaching an overall length of 7 centimeters (2.8 in) and males at a length of 4 centimeters (1.6 in). 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. 
Several counties in California distribute mosquito fish at no charge to residents with man-made fishponds and pools as part of their Mosquito Abatement programs.  The fish are made available to residents only and are to be used only on their own property, not introduced into natural habitat.
Mosquitofish were introduced directly into ecosystems in many parts of the world as a biocontrol to lower mosquito populations which in turn negatively affected many other species in each distinct bioregion.
Fertilization is internal, the male secreting milt into the genital aperture of the female through his gonopodium. Within 16 to 28 days after mating, the female will give birth to about 60 young. The males reach sexual maturity within 43 to 62 days. The females, if born early in the reproductive season, reach sexual maturity within 21 to 28 days; females born later in the season reach sexual maturity in six to seven months.
Mosquitofish are small, dull grey, with a large abdomen, and have rounded dorsal and caudal fins and an upturned mouth towards the surface. Sexual dimorphism is pronounced; mature females reach a maximum overall length of 7 centimeters (2.8 in), while males reach only 4 centimeters (1.6 in). Sexual dimorphism is also seen in the physiological structures of the body. The anal fins on adult females resemble the dorsal fins, while the anal fins of adult males are pointed. This pointed fin, referred to as a gonopodium, is used to deposit milt inside the female. Adult female mosquitofish can be identified by a gravid spot they possess on the posterior of their abdomen. Other species considered similar to G. affinis include Poecilia latipinna, Poecilia reticulata, and Xiphophorus maculatus; it is commonly misidentified as the eastern mosquitofish.
Naming and taxonomy
The mosquitofish is a member of the family Poeciliidae of order Cyprinodontiformes. The genus name Gambusia is derived from the Cuban Spanish term gambusino, meaning "useless". The common name, mosquitofish, is derived from their diet, which consist of large amounts of mosquito larvae. Classification of the western mosquitofish has been difficult due to their similarity to the eastern mosquitofish, and according to ITIS (Integrated Taxonomic Information System), G. holbrooki (eastern mosquitofish) is an invalid taxonomic name and is rather a subspecies of G. affinis.
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.
The native range of the mosquitofish is from southern parts of Illinois and Indiana, throughout the Mississippi River and its tributary waters, to as far south as the Gulf Coast in the northeastern parts of Mexico. They are found most abundantly in shallow water protected from larger fish. Mosquitofish can survive 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 (108 °F) for short periods. Because of their notable adaptability to harsh conditions and their global introduction into many habitats for mosquito control they have been described as the most widespread freshwater fish in the world.
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. Mosquitofish 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 and in Clark County, Nevada, mosquitofish were bred in aquariums so that people could stock stagnant pools of water with the mosquitofish to reduce the number West Nile virus cases.
Reproduction of the mosquitofish starts with the male arranging the rays of the gonopodium (modified anal fin) into a slight tube. The male mosquitofish will use this tubular fin to secrete milt into the female's genital aperture in the process of fertilization internal. The female's genital aperture is located just behind the anal fin and is an opening for the milt to fertilize the ova within the ovary. Mosquitofish are within the infraclass teleostei and as all teleosts, mosquitofish lack a uterus so production of oocytes and gestation occur within the ovary of a female mosquitofish. Inside the female, sperm from multiple males can be stored to later fertilize more ova. Based on laboratory experiments, the female mosquitofish is believed to be vitellogenic in nature during spring when the average temperature reaches about 14 °C (57 °F), and then the oocytes finish maturing when the average temperature reaches about 18 °C (64 °F). Then late in the summer when the photoperiod is less than 12.5 hours long, the next clutch of oocytes lose vitellogenesis. In one reproductive season a female may fertilize, with stored milt, 2 to 6 broods of embryos, with the size of the brood decreasing as the season progresses.
Mosquitofish have a 16 to 28 day gestation period. Mosquitofish are lecithotrophic, which means during gestation, nutrients are provided to the embryos by a yolk sac. If the gestation period is shorter, each newborn will at birth still have a yolk sac connected through a slit located on the ventral side of the body wall. Brood size of females depends on the size of the given female; larger females are more capable of a larger brood quantity than smaller females. Most females, though, have a brood quantity of about 60 young. Mosquitofish are viviparous, which means after the gestation of a brood, the female will have live birth. In most cases, the newborn brood will have an equal male to female ratio.
After birth, newborn mosquitofish are about 8 millimeters (0.31 in) to 9 millimeters (0.35 in) in length. As juveniles, they grow at a rate of about 0.2 millimeters (0.0079 in) per day. Growth rates of juvenile mosquitofish reach their peak when the water temperature is within a range of 24 °C (75 °F) to 30 °C (86 °F). As temperatures rise above or dip below this range, growth rates decrease. Temperatures at or above 35 °C (95 °F) are typically lethal, while growth stops when temperatures are at or below 10 °C (50 °F).  For male mosquitofish, sexual maturity is reached in about 43 to 62 days. Female mosquitofish reach sexual maturity in about 21 to 28 days if born early within the reproductive season. The lifespan of a mosquitofish averages less than a year and the maximum is about 1.5 years. However, mosquitofish kept as pets can live much longer, with owners reporting lifespans of over three years. Male mosquitofish lifespans are considerably shorter than the hardier females.
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