Shellfish is a broad term that refers to any type of aquatic invertebrate that has a shell, such as clams, oysters, mussels, and scallops. Crustaceans are a type of arthropod that have a segmented body and jointed appendages, such as shrimp, crabs, and lobster. Bivalves are a type of mollusk that have two hinged shells, such as clams, oysters, and mussels.
So, not all shellfish are crustaceans, and not all shellfish are bivalves. There are many different types of shellfish, and they can be classified into different groups based on their physical characteristics and evolutionary relationships.