The family Gymnuridae contains two genera and about 12 species. Their pectoral fins form a disc that is much broader than it is long, forming “wings” that give the family its common name, butterfly rays. These rays are marine and occasionally estuarine, found worldwide in tropical and subtropical seas. They live on the bottom in open sandy areas, where they feed on a variety of invertebrates, fishes, and other small organisms. Some bear a venomous spine on their short tails, but butterfly rays are considered far less dangerous than the long-tailed stingrays. Like other rays, butterfly rays are viviparous, giving birth to live young after nourishing them inside the uterus (see Development and Reproduction). (Allen and Robertson, 1994; Böhlke and Chaplin, 1968; Compagno, 1999; Hamlett and Koob, 1999; Helfman, Collete, and Facey, 1997; Last and Stevens, 1994; Moyle and Cech, 2000; Nelson, 1994; Wheeler, 1985; Wourms and Demski, 1993)