Species Description: The eastern mosquitofish (Gambusia holbrooki) is closely related to G. affinis, and has previously been classified as subspecies G. affinis. Some authorities (e.g., ITIS) still consider G. affinis and G. holbrooki to be subspecies of the same organism.
The mosquitofish, Gambusia affinis, is a small, robust-bellied fish. The relatively large head is flattened on the upper surface, and the small mouth is superior (upturned) and protrusible. The eyes are large relative to the body. Dorsal and caudal fins are rounded and no lateral line is visible (IGGS 2006).
The body is usually greenish olive to brown above, grey-blue on the sides, and silvery-white below. The body has a characteristic diamond or net pattern formed by dark pigment at the scale margins. Small black dots are also usually present on the body and tail. A small dark bar below the eye also aids in identification, and a black peritoneum (abdominal cavity lining) can often be observed through the belly in living specimens. Melanistic (black or nearly black) individuals are common in some populations. The dorsal fin is single and has only soft rays. Individuals typically have 7-9 soft dorsal rays, 8-10 anal fins, and 29-32 lateral line scales (Hoese and Moore 1977, Robins et al 1986, IGGS 2006).
The species is sexually dimorphic, with adult males being considerably smaller than females and also possessing a gonopodium, an elongated anal fin that functions as an intromittent organ for sperm transfer during mating. Mature females have a distinct gravid spot located on the posterior abdomen above the rear of the anal fin (Hoese and Moore 1977, GLAVCD undated).
Potentially Misidentified Species: As previously noted, Gambusia affinis and Gambusia holbrooki are sufficiently similar in appearance and behavior to cause considerable taxonomic difficulties, and these may or may not be distinct species. In south Florida, Gambusia affinis co-occurs with the congeneric mangrove gambusia, G. rhizophorae. The latter species is not known from as far north as the IRL however (Ray et al. 1986).
The characteristic net-like scale pattern and the poster origin of the dorsal fin relative to the anal fin are typically sufficient to distinguish Gambusia from the co-occurring poeciliid, the sailfin molly, Poecilia latipinna (Hoese and Moore 1977).
Poeciliid fishes can generally be differentiated from the potentially similar looking cyprinodontids (the killifishes) by the presence of either a gravid spot (mature females) or an intromittent organ (occurring on mature males) on the former.
Regional Occurrence: Gambusia affinis is native to fresh/low-salinity waters of the eastern and southeastern US and Gulf of Mexico, from New Jersey to central Mexico (Hoese and Moore 1977). They may be found in fresh water as far inland as Illinois (Ray et al. 1986).
Mosquitofish now also occur throughout much of the world as a result of intentional and non-intentional introductions beginning approximately 100 years ago. Intentional introductions have largely been for purposes of mosquito control, although G. affinis is not considered to be any more effective against mosquitoes than most native mosquito-eating species (ISSG 2006).
IRL Distribution: Gambusia affinis is a common inhabitant of lower salinity portions of the lagoon and associated upland freshwater systems. Salt marshes, mosquito impoundments, and mesohaline seagrass beds are among the habitats utilized by this species.
Age, Size, Lifespan: Female mosquitofish reach a maximum total length of around 6-7 cm while males may reach around 4 cm (FishBase 2004, GLAVCD undated), although most individuals are somewhat smaller.
The maximum age reported for the species is 3 years (FishBase 2004).
Abundance: While population densities in some portions of the IRL may be limited by salinity, Minckley et al. (1991) suggest that Gambusia affinis may be "possibly the single most abundant freshwater fish in the world".
Reproduction: Accomplished through the use of the male gonopodium. An early description of mating behavior is that of Collier (1936). The male faces forward and swings the gonopodium forward, initiating brief contact between the gonopodial tip and the oviductal opening and transferring sperm to the female.
Viable sperm can be stored by females for several months, and the stored sperm in any female is likely derived from multiple matings with a variety of males (Medlen 1951).
Male mosquitofish mature at approximately 1 month/21 mm SL, and females at 28 mm/6 weeks (McDowall 2000).
Embryology: Females typically brood around 60 young, but large individuals may carry 300 or more (McDowall 2000).
The gestation period has been experimentally determined to average around 24 days, a timeframe that is in close agreement with field observations of pond-raised animals (Krumholz 1948). Most fry in the experimental investigation had absorbed the maternal yolk before being liberated, but some broods contained animals that had not completely absorbed the yolk sacs.
Temperature: Mosquitofish are subtropical to warm-temperate, and typically occur at temperatures between 12-29°C (FishBase 2004). Individuals inhabiting ponds have been observed to reduce activity and "hibernate" in deeper water during winter months (GLAVCD undated).
Salinity: The livebearer family (Poecilidae) is principally a freshwater family, and only mosquitofish and sailfin mollies invade marine habitats in the southeastern US. Gambusia affini is euryhaline in habit, and is common in salt marshes and other low-salinity coastal habitats. It is generally found at lower salinities than the sailfin molly (Hoese and Moore 1977).
Dissolved Oxygen: Laboratory experiments by Cech et al. (1985) revealed that mosquitofish utilized aquatic surface breathing (i.e., expoiting the oxygen-saturated surface layer of water) under hypoxic conditions, commencing such behavior in a facultative manner at oxygen levels of 20-65 torr and in an obligatory manner below 20 torr at 20°C.
Trophic Mode: Mosquitofish feed primarily on zooplankton, small insects and insect larvae, and detrital material (ISSG 2006). As the common name suggests, they are voracious consumers of mosquito larvae. All sizes and ages of mosquitofish feed on mosquito larvae, and a large female mosquitofish can consume hundreds of larvae a day (GLAVCD undated). Rajasekharan and Chowdaiah (1972) demonstrated that G. affinis could discern between different species of simultaneously presented mosquito larvae and preferentially consumed certain species based on a number of factors facilitating prey capture, including size, vertical position in the water column, and the tendency of larvae to clump in groups.
Baber and Babbitt (2004) indicate Gambusia
Mosquitofish, particularly where it occurs as a non-native, also prey heavily on the eggs and young of co-occurring fish species.
Predators: Britton and Moser (1988) report that G. affinis represents a significant portion of the diets of four species of Camargue (French Mediterranean) herons they studied. The authors also indicate that female fish are preferentially consumed over the smaller males, in apparent concordance with optimal foraging theory.
Parasites: Gambusia affinis has been discovered to be one of several primarily freshwater fishes that serve as intermediate hosts of nematodes of genus Falcaustra, adults of whom typically infest reptile or amphibian hosts (Moravec et al. 1995).
Habitats: Gambusia affinis occurs in a variety of freshwater and in protected brackish environments. It preferentially occupies vegetated habitats, including salt marsh and seagrass beds (Ray 1986). It is benthic and non-migratory in habit and is most often encountered in standing or slow-flowing waters (FishBase 2004, IGGS 2006).
Preference experiments by Casterlin and Reynolds (1977) revealed that mosquitofish selectively occupied areas with subsurface vegetation but avoided floating cover that restricted access to the water surface.
Activity Time: Lined sole are typically active in the evening hours, spending much of the daytime hours buried in shallow sand.
Invasion History: Gambusia affinis is among the most widely introduced fish species worldwide, and a number of countries have reported negative environmental consequences in the wake of mosquitofish introduction (Fishbase 2004).
Although effective as a mosquito control agent and widely introduced for this purpose, mosquitofish have been found to compete with or displace indigenous fish and to otherwise disrupt habitats. For example, McDowall (1990) reports that selective predation by mosquitofish can alter zooplankton, insect and crustacean communities.
G. affinis has been nominated as among the 100 "World's Worst" invaders by the Invasive Species Specialist Group (ISSG 2006).
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Britton RH and ME. 1982. Size specific predation by herons and its effect on the sex-ratio of natural populations of the mosquito fish Gambusia affinis Baird and Girard. Oecologia 53:146-151.
Casterlin ME and WW Reynolds. 1977. Aspects of habitat selection in the mosquitofish Gambusia affinis. Hydrobiologia 55:125-127.
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FishBase. 2004. Species profile: Gambusia affinis Mosquito fish. Available online.
Greater Los Angeles County Vector Control District (GLAVCD). Undated. Mosquitofish fact sheet. Available online.
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Medlen AB. 1951. Preliminary observations on the effects of temperature and light upon Gambusia affinis. Copeia 1951:148-152.
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