Molluscs are shellfish that have a hard inedible shell. They are classified into three other subgroups—gastropods or univalves, bivalves, and cephalopods.

Mollusks: Defined as soft-bodied invertebrates with calcium carbonate shells, examples of commonly enjoyed mollusks are clams, mussels, scallops and oysters. So, to elaborate on our description above with a specific example, both crabs and scallops are shellfish, but only crabs are crustaceans.

Mollusks are soft-bodied invertebrates that exist within hard calcium carbonate shells. There are around 100,000 species in this category, including snails and slugs. The ones we eat, like in the crustacea category, come from the sea: clams, mussels, scallops, oysters and cockles, among others.

There are two groups of shellfish: crustaceans (such as shrimp, prawns, crab and lobster) and mollusks/bivalves (such as clams, mussels, oysters, scallops, octopus, squid, abalone, snail). Allergy to crustaceans is more common than allergy to mollusks, with shrimp being the most common shellfish allergen for both children and adults.

Crustaceans are aquatic animals that have jointed legs, a hard shell, and no backbone. Examples include crab, crayfish, lobster, prawns, and shrimp. Most molluscs have a hinged two-part shell and include clams, mussels, oysters, and scallops. It can also include various types of octopus, snail, and squid.