The western mosquitofish (Gambusia affinis) belongs to the order Cyprinodontiformes and family Poeciliidae. The name “mosquitofish” is appropriate for this fish because its diet sometimes consists of large numbers of mosquito larvae. The family contains about twenty-one genera and 160 species. There are eight other freshwater species of Gambusia similar in appearance in North America north of Mexico, including the eastern mosquitofish (G. holbrooki), Amistad gambusia (G. amistadensis), Big Bend gambusia (G. gaigei), largespring gambusia (G. geiseri), San Marcos gambusia (G. georgei), Clear Creek gambusia (G. heterochir), Pecos gambusia (G. nobilis), and blotched gambusia (G. senilis). Many of these Gambusia spp. are endemic and afforded protection, as populations are either listed as threatened or endangered. The similar western mosquitofish ranges east of the Rocky Mountains in the Atlantic and Gulf Slope drainages from southern New Jersey to Mexico and the Mississippi River basin from central Indiana and Illinois south to Gulf. It was once considered a subspecies (G. affinis holbrooki) and is native to the Atlantic and Gulf Slope drainages from peninsular Florida to as far west as southern Alabama. In Arkansas, G. affinis occurs almost statewide, although it is rare in the uplands of the Ozark Mountains. It prefers vegetated ponds, lakes, drainage ditches, and backwaters and oxbows of sluggish streams and is less common in moderate gradient streams. In other parts of its range, it is often found in brackish or marine environments.
Gambusia affinis is a relatively small, terete, live-bearing (viviparous) fish with a large abdomen and rounded dorsal and caudal fins. Sexual dimorphism is very pronounced in mosquitofish. Females reach a total length of 71 mm (2.8 in.), while males are smaller, about 30 mm (1.2 in.). In addition to being about two times the size of the male and being heavier, the female can be distinguished by a gravid spot at the posterior of her abdomen and the absence of the male’s highly modified anal fin rays. Scales are relatively large in the lateral series and number twenty-eight to thirty-one. The dorsal fin rays number six (rarely seven) with fewer than ten anal rays. This fish is dull gray to olive or brown in dorsal and lateral coloration and white below with no bars or bands on the sides. The caudal fin is homocercal, a lateral line is absent, its head is flattened, and its terminal mouth is turned upward for surface feeding.
Mosquitofish are classified as omnivorous larvivorous (eating the larvae of insects) fish. Gambusia spp. typically eat algae, zooplankton, beetles, caddisflies, mayflies, mites, and fish fry, including its own progeny, and other invertebrates at the top of the water column; mosquito larvae may make up only a small portion of their diet or include hundreds of mosquito larvae in one day. In one study, maximum consumption rate of mosquitos in a single day by one mosquitofish was observed to be from 42% to 167% of its own body weight.
Mosquitofish were deliberately introduced directly into ecosystems in many parts of the world as biocontrol to lower mosquito populations, which, in turn, harmed many other fish and amphibian species in each distinct bioregion. In the United States, mosquitofish were introduced in various states, and the earliest record of interstate transportation was in 1905, from Texas to Hawaii, in the erroneous belief they would be better at eating mosquito larvae than native fish species were. Critical reviews of the scientific literature on mosquito control suggest ineffective and ill-advised tactics, and have not supported the opinion that Gambusia are particularly effective in reducing mosquito populations or in reducing the incidence of mosquito-borne diseases. In Australia, for example, mosquitofish are classified as a pest and may have exacerbated the mosquito problem in many areas by outcompeting native invertebrate predators of mosquito larvae. In addition, mosquitofish have been known to kill or injure other small fishes through predation and competition. They have been reported to often attack, nip at the fins of, and sometimes kill other species and even prey on eggs, larvae, and juveniles of various fishes and larval amphibians, including those of largemouth bass (Micropterus salmoides), common carp (Cyprinus carpio), and newts/salamanders and frogs; they are also known to prey on adults of smaller fishes.
Spawning occurs in warmer months, with young being born from March to October with a peak in April. Fertilization in mosquitofish is internal and viviparous; the male deposits milt into the genital aperture of the female through a modified pointed anal fin (gonopodium), and sperm are transferred in a spermatophore. Gravid female mosquitofish can be identified by possession of a gravid spot on the posterior of their abdomens. With a gestation period of sixteen to twenty-eight days in a single reproductive season, a given female is capable of multiple fertilizations, with stored milt for two to six broods of embryos, as the size of the brood decreasing as the season progresses. Females can produce fourteen to 226 embryos per brood depending on body size (larger females produce a larger brood), though most have a brood of about 60 young. The males are sexually mature within forty-three to sixty-two days, whereas the females, if born early in the reproductive season, can reach sexual maturity in only twenty-one to twenty-eight days. However, females born later in the season take longer, six or seven months, to reach sexual maturity.
Mosquitofish are known to be able to survive in relatively extreme environments, and are flexible in regards to low oxygen concentrations, high salt concentrations (up to twice that of sea water), and, for brief periods, temperatures of up to 42°C (108°F). However, they do not adapt well to extremely cold environments, selecting thermally fluctuating sites in summer and more thermally stable areas in winter. At extremely low dissolved oxygen levels, individual mosquitofish rise to the water surface and respire via a thin film of more oxygenated surface water. The lifespan of a mosquitofish is short, averaging less than a year, with a maximum of about one and a half years. However, mosquitofish kept as pets can live much longer, with owners reporting lifespans of over three years. Females tend to live longer than males.
There are a variety of protozoan and helminth parasites reported from G. affinis, including myxozoans, monogeneans, digenetic trematodes, cestodes, nematodes, acanthocephalans, and pentastomids. In addition, copepods of the genera Ergasilus and Lernaea infest G. affinis. A monogenean parasite, Salsuginus seculus, was reported in 2015 from the gills of western mosquitofish from Arkansas, Kansas, and Oklahoma. This monogenean had previously been reported on G. affinis from California, Louisiana, Texas, and the Bahama Islands. This is the only parasite, as of 2019, reported from G. affinis from Arkansas.
For additional information:
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Chris T. McAllister
Eastern Oklahoma State College
Last Updated: 09/26/2019