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Poeciliidae (live-bearer, guppy; subclass Actinopterygii, order Atheriniformes) A large family of small, freshwater and brackish-water fish that generally inhabit freshwater streams, although several species can be found in saline coastal marshes. They all have an elongate body form, a single, soft-rayed dorsal fin on the middle of the back, with the anal fin opposite in mid-ventral position. The tail fin is always rounded, and the pelvic fin often small. There are about 138 species, occurring in Central and S. America.