MUSSEL, the name given to certain aquatic bivalve molluscs of the class Lamellibranchia. In its most regular usage it is ap plied to the marine Mytilidae, of which the edible mussel, Mytilus edulis, is the most familiar example, and to the fresh-water genus Anodonta. The members of the large fresh-water genus Unio are sometimes called mussels.
The marine mussels (Mytilus, Modiolaria, Modiola) belong to the sub-order Mytilacea of the order Filibranchia. The edible mussel, Mytilus edulis, has a wide distribution. It is found on both sides of the Atlantic and in the Mediterranean, and on the eastern Atlantic seaboard its range extends as far south as Rio de Oro in Morocco. Its habitat is towards low-tide mark, where it lives at tached to rocks or on consolidated shingle or sand-banks. It is usu ally found in large numbers, such mussel-beds sometimes covering several acres and containing millions of individuals.
The Mytilidae have an inequilateral shell, the apex or umbo ap pearing to be displaced to one end when compared with that of a cockle, for example. This is due to the imperfect development of the anterior end of the shell. The foot is provided with a bys sus, a bunch of hairs secreted in a glandular cavity of the foot. From this cavity the hairs are extruded and harden on contact with the sea-water, forming a means of attaching the mussel to a rock or stone. This fixation is not, however, permanent ; the ani mal can discard the byssus and anchor itself afresh in a new situa tion. Such a means of anchorage and the faculty of occasionally changing position are highly important in animals which live, as the Mytilidae do, in the tidal zone where gales tend to shift the sand or shingle on which they live, and where the animal is liable to be swept away from its feeding-ground or buried beneath the debris of the shore.
The edible mussel, Mytilus edulis, is of considerable economic importance as human food and as bait for edible fishes. In 1922,
1,888 tons of mussels were delivered at Billingsgate market in Lon don. It is very little eaten in the United States. M. edulis is con sidered of fair size for eating when it is 2in. long, a size attained in three years after the young mussel has settled down. Under favourable conditions it will attain a much greater size. The de gree of salinity of the water has a considerable effect on the size of the shell. For example, those mussels which live in the compara tively fresh water of the Baltic sea are often a quarter or one-fifth the size of the North sea forms. Nevertheless, it seems to thrive best in water having a salinity lower than that of normal sea-water. In Great Britain the chief mussel beds are in Morecambe bay (Lancs.) and in the Wash.
There are many genera in the family Mytilidae, which has a wide distribution. They include Modiolaria, Lithodomus (the date mussel), which bores into rock, Modiola and Myrina.
The fresh-water mussels are members of the family Unionidae (order Eulamellibranchia) and include the genera Anodonta, Unio, Quadrula, etc. They are mainly found in rivers and lakes. They burrow in the soft mud of the bottom and, living thus in more tranquil conditions than the sea-mussels, have only a weakly de veloped byssus. The developmental history of these mussels is unique among molluscs, as the young larva (glochidium) under goes part of its development as a parasite on fish. The members of the genus Unio and its near allies are of considerable economic importance in the United States, where they are cultivated for their pearly shell, which is used in button-making. At one time the pearls obtained from these animals were widely sought in Europe; but this industry has decayed owing to the introduction of the Orient pearls of Ceylon, etc.