Shellfish is a colloquial and fisheries term for exoskeleton-bearing aquatic invertebrates used as food, including various species of mollusks, crustaceans, and echinoderms.
Although most kinds of shellfish are harvested from saltwater environments, some are found in freshwater. Shellfish are among the most common food allergens.
Despite the name, shellfish are not fish. Most shellfish are low on the food chain and eat a diet composed primarily of phytoplankton and zooplankton. Many varieties of shellfish, and crustaceans, in particular, are actually closely related to insects and arachnids; crustaceans make up one of the main subphyla of the phylum Arthropoda. Mollusks include cephalopods (squids, octopuses, cuttlefish) and bivalves (clams, oysters), as well as gastropods (aquatic species such as whelks and winkles; land species such as snails and slugs).