Shellfish is a culinary term for aquatic invertebrates used as food: molluscs, crustaceans, and echinoderms. Both saltwater and freshwater invertebrates are considered shellfish. Shellfish is a misnomer, because these invertebrates are definitely not fish. The term finfish is sometimes used to distinguish ordinary (vertebrate) fish from shellfish. Some don't include shrimp, crab, or lobster in the category of "shellfish."(External Link) Molluscs commonly used as food include the clam, mussel, o… (More on Shellfish)