A clam. Clams are filter-feeding bivalves with a two-part shell that they can close firmly to protect their soft bodies. They live in both freshwater and marine environments and are found all over the world. Most clams have a single genome, but some species, such as the Pacific oyster, have two genomes. Scientists believe that the Pacific oyster's two genomes may have evolved from a hybridization event between two different clam species.