Mussels belong to the class Bivalvia within the phylum Mollusca. North American freshwater mussels are members of two families, Unionidae and Margaritiferidae within the order Unionoida. Approximately 300 species of freshwater mussels occur in North America with the vast majority concentrated in the Southeastern United States. Twenty-nine species, all in the family Unionidae, occur in South Carolina except for the Asian clam. Approximately 1000 species occur worldwide.