Fish and Shellfish

Seafood can be categorized into two very distinct groups: fish and shellfish. Fish possess a backbone to sense changes in their aquatic environment, gills that enable them to breathe oxygen, and limbs in the form of fins. Shellfish, on the other hand, possess an exoskeleton, or shell. Shellfish include crustaceans, such as shrimp and lobsters, and mollusks, such as clams and oysters. Sea urchins fall under the category of shellfish but are echinoderms and are related to sand dollars, starfish, and sea cucumbers, the latter being the other echinoderm consumed by humans.