Shellfish is a colloquial and fisheries term for exoskeleton-bearing aquatic invertebrates used as food, including various species of molluscs, crustaceans, and echinoderms. although most kinds of shellfish are harvested from saltwater environments, some are found in freshwater. in addition, a few species of land crabs are eaten, for example cardisoma guanhumi in the caribbean. 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...

Source: Wikipedia