mussel, edible freshwater or marine bivalve mollusk. Mussels are able to move slowly by means of the muscular foot. They feed and breathe by filtering water through extensible tubes called siphons; a large mussel filters 10 gal (38 liters) of water per day. The close-fitting shells protect the mussel from desiccation and enable it to live high up on the shore. Most marine mussels belong to the single family, Mytilidae. They are widespread and are especially abundant in cooler seas. They form extensive, crowded beds, anchoring themselves by the byssus, a secretion of strong threads. The blue mussel grows up to 3 in. (7.6 cm) and is common along the Atlantic coast; the smaller hooked mussel has a more southerly range. The horse mussel, found in deeper waters, grows to 6 in. (15 cm) in length.
Freshwater mussels, most of which belong to families the order Unionoida, are not closely related to the marine species. They are chiefly of two kinds: the large, dark-shelled burrowing mussels, a source of pearls and of mother-of-pearl; and the tiny "fingernail clams" found on the bottoms of clear pools and brooks. Freshwater mussels of the family Unionidae, sometimes called clams, pass through a parasitic larval state, living on the fins, gills, and bodies of fishes.
The thumbnail-sized zebra mussel, Dreissena polymorpha, is a small freshwater mussel native to Europe that was introduced in the 1980s into the Great Lakes. Lacking natural predators, it has proliferated and spread widely in North America, clogging intake pipes at water and power facilities and disrupting native ecosystems. The larger quagga mussel, D. rostriformis bugensis, also native to Europe, was first found in the Great Lakes in 1989 and has largely supplanted the zebra mussel there. Quagga populations are also found in the Colorado and upper Missisippi river basins and in associated waterways. Zebra and quagga mussels are more closely related to the true clams and cockles than other mussels.
Mussels are classified in the phylum Mollusca, class Pelecypoda or Bivalvia.