Shellfish

shellfish ‘Any animal living in water whose outer cover is a shell, whether testaceous, as an oyster, or crustaceous, as a crab’ (OED).

On this basis it is correct to use the term for all single shells, bivalves (except the shipworm, once it has passed the juvenile stage), and crustaceans; and also for the sea urchin, for one cephalopod (the pearly nautilus), and for sea turtles. In practice the term is applied in a general way to the single shells, bivalves, and crustaceans. It is less satisfactory than the French fruits de mer (edible sea animals other than fish) but more convenient than the Italian frutti di mare (if lexicographers are correct in restricting the meaning of this to edible molluscs and sea urchins).