What shellfish are not bivalves?

Here's the breakdown:

* Bivalves are a type of mollusk characterized by having two hinged shells (think clams, oysters, mussels, scallops).

* Shellfish is a broader term that includes any aquatic animal with a hard shell, regardless of its classification.

Therefore, shellfish that are NOT bivalves include:

* Crustaceans: These are arthropods (like insects) with a hard exoskeleton. Common examples include crabs, lobsters, shrimp, and crayfish.

* Gastropods: These mollusks have a single, spiral shell (like snails, whelks, and abalone).

Let me know if you have any other questions about shellfish!