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 the 1950s, 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. Reproduction rates are highly dependent on temperature and ration level. As temperature increases from 20 to 30 °C, mean age at first reproduction decreases from 191 to 56 days, and brood size and mass of offspring increase significantly. Interbrood interval estimates at 25 and 30 °C are 23 and 19 days, respectively.
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.
- Whiteside, Bobby; Bonner, Timothy; Thomas, Chad; Whiteside, Carolyn. "Gambusia affinis western mosquitofish". Texas State University. http://www.bio.txstate.edu/~tbonner/txfishes/gambusia%20affinis.htm. Retrieved 25 October 2011.
- Wallus & Simon 1990, p. 175
- Masterson, J. "Gambusia affinis". Smithsonian Institution. http://www.sms.si.edu/irlspec/Gambusia_affinis.htm. Retrieved 21 October 2011.
- Alameda County Mosquito Abatement Program http://www.mosquitoes.org
- Mosquitofish. Santa Clara County Vector Control District
- Contra Costa County Mosquito and Vector Control District http://www.contracostamosquito.com/
- Kuntz, Albert (1913). "Notes on the Habits, Morphology of the Reproductive Organs, and Embryology of the Viviparous Fish Gambusia affinis". Bulletin of the United States Bureau of Fisheries (Department of Commerce) 33: 181–190. http://books.google.com/?id=E8QWAAAAYAAJ&pg=PA178-IA7.
- Rajkumar, R (1987). "Trophic microvilli of the belated embryos of Gambusia affinis (Baird and Girard) (Atheriniformes: Poeciliidae)". Journal of the Inland Fisheries Society of India Barrackpore 19 (1): 32–36. http://www.mendeley.com/research/trophic-microvilli-belated-embryos-gambusia-affinis-baird-girard-atheriniformes-poeciliidae-1/.
- "Gambusia affinis (fish)". Global Invasive Species Database. http://www.issg.org/database/species/ecology.asp?fr=1&si=126. Retrieved 21 October 2011.
- "Gambusia holbrooki Girard, 1859". ITIS. http://www.itis.gov/servlet/SingleRpt/SingleRpt?search_topic=TSN&search_value=165896&print_version=PRT&source=to_print. Retrieved 30 December 2011.
- Regional Office For The Eastern Mediterranean (2003). Use of Fish For Mosquito Control. World Health Organization. p. 15. http://www.emro.who.int/rbm/publications/fishmanual.pdf. Retrieved 2 January 2012.
- Nico, Leo; Fuller, Pam; Jacobs, Greg; Cannister, Matt (19 August 2009). "Gambusia affinis". USGS. http://nas.er.usgs.gov/queries/factsheet.aspx?SpeciesID=846. Retrieved 25 October 2011.
- Dionne, Michele (1985). "Cannibalism, Food Availability, and Reproduction in the Mosquito Fish (Gambusia affinis): A Laboratory Experiment". The American Naturalist 126: 16–23. doi:10.1086/284392. JSTOR 2461558.
- Krumholz, Louis (1944). "Northward Acclimatization of the Western Mosquitofish, Gambusia affinis affinis". Copeia 1944 (2): 82. doi:10.2307/1438757. JSTOR 1438757.
- "Гамбузия" (in Russian). Great Soviet Encyclopedia. Moscow. http://dic.academic.ru/dic.nsf/bse/77453/%D0%93%D0%B0%D0%BC%D0%B1%D1%83%D0%B7%D0%B8%D1%8F.
- "Aquatic Invasive Species: Gambusia affinis (Mosquito fish)". Washington Department of Fish and Wildlife. Washington Department of Fish and Wildlife. http://wdfw.wa.gov/ais/species.php?Name=gambusia_affinis. Retrieved 2 January 2012.
- Rupp, Henry (1995). "Adverse Assessments of Gambusia affinis". North American Native Fishes Association (NANFA). http://www.gambusia.net/ACmosquito.html. Retrieved 2 January 2012.
- Vinogradova 2000, p. 187
- Ilyin, Ivan. "История человека – история города Сочи" (in Russian). Объявления Сочи: История человека – история города Сочи / 135 лет со дня рождения Сергея Юрьевича Соколова. http://www.irrsochi.ru/news/121.html. Retrieved 8 November 2011.
- "В Сочи установлен памятник рыбке, спасшей местность от малярии" (in Russian). Кавказский узел. 26 June 2010. http://www.kavkaz-uzel.ru/articles/170779/. Retrieved 8 November 2011.
- "Врачу, спасшему Сочи от малярии, поставят памятник" (in Russian). ФедералПресс. 22 July 2010. http://fedpress.ru/federal/polit/society/id_193168.html. Retrieved 8 November 2011.
- Russel, Sabin (12 July 2008). "Heat wave adds to West Nile danger". San Francisco Chronicle (San Francisco, California: SFGate): p. B–1. http://www.sfgate.com/cgi-bin/article.cgi?f=/c/a/2008/07/11/BAH411N6UP.DTL. Retrieved 3 January 2012.
- Casal, Christine (March 23, 1993). "Reproduction of Gambusia affinis". Fish Base. http://www.fishbase.org/Reproduction/FishReproSummary.php?ID=3215&GenusName=Gambusia&SpeciesName=affinis&fc=216&StockCode=3411. Retrieved 29 December 2011.
- Koya, Y; Kamiya, E (2000). "Environmental Regulation of Annual Reproductive Cycle in the Mosquitofish, Gambusia affinis". The Journal of experimental zoology 286 (2): 204–11. doi:10.1002/(SICI)1097-010X(20000201)286:2<204::AID-JEZ12>3.0.CO;2-G. PMID 10617862.
- Schindler, Joachim; Hamlett, William (1993). "Maternal–embryonic relations in viviparous teleosts". Journal of Experimental Zoology 266 (5): 378–393. doi:10.1002/jez.1402660506.
- Campton, D. E.; Gall, G. A. E. (1988). "Growth and reproduction of the mosquitofish, Gambusia affinis, in relation to temperature and ration level: consequences for life history". Environmental Biology of Fishes 21 (1): 45–57. doi:10.1111/j.1095-8649.1988.tb05463.x.
- Grier, HJ; Grier, HJ (2010). "Oogenesis of microlecithal oocytes in the viviparous teleost Heterandria formosa". J. Morphol 272 (2): 241–57. doi:10.1002/jmor.10912. PMID 21210493.
- Wurtsbaugh, Wayne A.; Cech, Joseph J. (1983). "Growth and activity of juvenile mosquitofish: temperature and ratio effects". Transactions of the American Fisheries Society 112 (5): 653–660. doi:10.1577/1548-8659(1983)112<653:GAAOJM>2.0.CO;2.
- Campton, D. E.; Gall, G. A. E. (1988). "Effect of individual and group rearing on age and size at maturity of male mosquitofish, Gambusia affinis". Journal of Fish Biology 33 (2): 203–212. doi:10.1111/j.1095-8649.1988.tb05463.x.
- Vinogradova, Elena Borisovna (2000). Culex pipiens pipiens mosquitoes: taxonomy, distribution, ecology, physiology, genetic, applied importance and control. Pensoft Publishers. ISBN 978-954-642-103-6. http://books.google.com/books?id=770ImWlj3PsC&pg=PA187. Retrieved 29 April 2011.
- Wallus, Robert; Simon, T.P. (1990). Reproductive Biology and Early Life History of Fishes in the Ohio River Drainage. Taylor & Francis. ISBN 0-8493-1921-8.