A mussel is a small edible bivalve shellfish. There are saltwater and freshwater (family Unionidae) varieties, and they come in a variety of sizes.
One species, the European zebra mussel (Dreissena polymorpha) is causing a serious problem in North America. From its first discovery in American waters in 1988 it has spread to a large number of waterways disrupting the ecosystem, displacing the local unoinoida mussels, and damaging harbours, boats, power plants etc.
Mussels are used as the host animal for the cultivation of freshwater pearls.
Also see clam.
Not to be confused with muscle.