mollie or molly, New World fish of the genus Mollienesia, in the same family as the guppy (see killifish). Mollies 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 mollie ( M. formosa ). This species, which ranges as far N as Texas, consists only of females, which copulate with males of other mollie species; the male does not contribute to the heredity of the all-female offspring. The sailfin mollie, 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 mollie species, although mottled black sailfin mollies sometimes occur in nature. Mollies are classified in the phylum Chordata, subphylum Vertebrata, class Osteichthyes, order Cyprinodontiformes, family Poeciliidae.