The family Poeciliidae is comprised of mostly small-bodied fishes (of which Belonesox is the largest) that have internal fertilization by means of the male gonopodium (modified anal rays 3-5) and bear live young. Poeciliids are generally the most dominant fishes in lowland fresh and brackish waters of Middle America and the West Indies, ranging from the eastern U.S.A. to northeastern Argentina. Many popular aquarium fishes (e.g., Swordtails, Guppies, Mollies) are poeciliids, as well as the ubiquitous Mosquitofish (Gambusia spp.).