Shellfish are the most widely known type of non-fish seafood. Some shellfish examples include lobster, crab, shrimp, clams, scallops, and oysters. Despite the name, shellfish are not actually fish. Biologically, they differ substantially. Fish have fins, backbones, and gills. Shellfish is an umbrella term for all aquatic animals that have some kind of a shell, such as crustaceans or mollusks.