A broad term for all aquatic animals that have a shell of some kind. Shellfish are separated into two basic categories crustaceans and mollusks. Crustaceans include crabs, crayfish, lobster and shrimp. Mollusks are divided into three groups Mopods (also called univalves) such as the abalone and snail, Bivalves like the calm and oyster; and Cephalopods such as the octopus and squid).