Mussel, a bivalve mollusk. Mussels are found in oceans and in fresh water. Marine mussels are usually 3 to 10 inches (7.5 to 25 cm) long. They have dark, oval or elongated shells. These mussels secrete numerous threadlike filaments with which they attach themselves to rocks along the coasts of Europe and North America. Freshwater mussels grow up to five inches (13 cm) in length. They have oval shells from which they extend a fleshy foot. These mussels are found in rivers and lakes in Europe and North America. The zebra mussel, a freshwater species native to Europe, grows up to one inch (2.5 cm) in length. It has become a pest in the Great Lakes and in the Mississippi River basin, where large numbers clog water-intake pipes of factories and water-treatment plants.
Mussels are eaten raw or cooked in various ways. They are also used as bait.
Marine mussels belong to the family Mytilidae; most freshwater mussels to the family Unionidae. The zebra mussel is Dreissena polymorpha of the family Dreissenidae.