A type of fish that is distinguished from other fish by their shell. The shell is a type of external skeleton that gives protection and structural support to the shellfish. Their shells range in degree of hardness, which divides them into two classifications. The crustaceans have a softer shell, except for the hard-shell crab, and the mollusks that have a harder shell. The crustaceans include lobsters, crabs, shrimp, crayfish and spiny lobsters. The mollusks include conch, snails, clams, mussels, oysters, scallops and squid.