Dictionary.net

Any aquatic animal whose external covering consists of a shell, either testaceous, as in oysters, clams, and other mollusks, or crustaceous, as in lobsters and crabs.

Any aquatic animal whose external covering consists of a shell, either testaceous, as in oysters, clams, and other mollusks, or crustaceous, as in lobsters and crabs.

Aquatic invertebrates belonging to the crustacean or mollusk families and used as food.

An invertebrate water animal having a shell, as a clam, lobster, mollusk, etc.

A fish or an aquatic animal with an external shell.