Shellfish are water animals with hard outer shells instead of bones. This group includes creatures like clams, oysters, crabs, lobsters, shrimp, and mussels. Despite the name, shellfish aren't actually fish at all: they're invertebrates, meaning they have no backbone.

Shellfish fall into two main categories. Crustaceans like crabs and lobsters have jointed legs and can move around. Mollusks like clams and oysters often stay in one place and filter their food from the water. Both types have been important food sources for humans throughout history, and many cultures have traditional shellfish dishes.

Some people have shellfish allergies, which means their bodies react badly to eating these animals. This is different from a fish allergy: someone allergic to shellfish can sometimes eat regular fish, and someone allergic to fish can sometimes eat shellfish safely.