Shellfish is a term used to describe a variety of aquatic animals that have a hard outer shell, such as clams and crabs. Clams and crabs are two of the most popular types of SHELLFISH, and they are both found in many different parts of the world. Clams are bivalve mollusks that live in both saltwater and freshwater habitats, while crabs are crustaceans that live in both marine and brackish waters. Both clams and crabs are edible and are often used in various dishes.