molly or mollie, New World fish of the genus Poecilia, which includes the guppy or rainbow fish, Poecilia reticulata. Mollies, which are related to the killifishes, are found from the E and central United States to Argentina. Top-living fish, they are found in fresh or brackish water, where they feed on algae. Fertilizing internally and giving birth to live young, mollies are of great interest to geneticists because of the reproductive peculiarities of one species, the Amazon molly (P. formosa). This species, which ranges as far N as Texas, consists only of females, which copulate with males of other molly species; the male does not contribute to the heredity of the all-female offspring. The sailfin molly, or sailfin (M. latipinna), is found in fresh- and saltwater from South Carolina to Mexico. Sailfins are olive-green with black markings and brightly colored fins. The saillike dorsal fins of the male make it an especially popular aquarium fish. Solid black mollies are artificially bred from any of several molly species, although mottled black sailfin mollies sometimes occur in nature. Mollies are classified in the phylum Chordata, subphylum Vertebrata, class Actinopterygii, order Cyprinodontiformes, family Poeciliidae.

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