Mussel is the common name used for several families of bivalve mollusks from saltwater and freshwater habitats. These groups have in common a shell whose outline is elongated and asymmetrical compared with other edible clams, which are often more or less rounded or oval. In addition, the word "mussel" is frequently used to mean the bivalves of the marine family Mytilidae, most of which live on exposed shores in the intertidal zone, attached using their strong byssal threads ("beard") to a firm substrate.
Humans have used mussels as food for thousands of years. About 17 species are edible, of which the most commonly eaten are Mytilus edulis, M. galloprovincialis, M. trossellus, and Perna canaliculus.
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.