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 — gastropods (also called univalves) such as the abalone and snail; bivalves like the clam and oyster; and cephalopods such as the octopus and squid.