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 on the body 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).
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