The freshwater Zebra mussels and their relatives ( family Dreissenidae ) live attached to rocks in a manner similar to marine mussels , but are classified with the Heterodonta , the taxonomic group including most bivalves referred to as `` clams '' . Freshwater Zebra mussels and their relatives in the family Dreissenidae are not related to previously mentioned groups , even though they resemble many Mytilus species in shape , and live attached to rocks and other hard surfaces in a similar manner , using a byssus . They are classified with the Heterodonta , the taxonomic group which includes most of the bivalves commonly referred to as `` clams '' .