Mussel (0. Eng. muscle, Lat. musculus, diminutive of mus, mouse, applied to small sea fish and mussels), a term applied in England to two families of Lamellibranch Molluscs - the marine Mytilacea, of which the edible mussel, Mytilus edulis, is the representative; and the fresh-water Unionidae, of which the river mussel, Unio pictorum, and the swan mussel, Anodonta cygnea, are the common British examples. It is not obvious why these fresh-water forms have been associated popularly with the Mytilacea under the name mussel, unless it be on account of the frequently very dark colour of their shells. They are somewhat remote from the sea mussels in structure, and have not even a common economic importance.
The sea mussel (Mytilus edulis) belongs to the second order of the class Lamellibranchia, namely the Filibranchia, distinguished by the comparatively free condition of the gillfilaments, which, whilst adhering to one another to form gillplates, are yet not fused to one another by concrescence. It is also remarkable for the small size of its foot and the large development of two glands in the foot - the byssus-forming and the byssus-cementing glands. The byssus is a collection of horny threads by which the sea mussel (like many other Lamellibranch or bivalve molluscs) fixes itself to stones, rocks or submerged wood, but is not a permanent means of attachment, since it can be discarded by the animal, which, after a certain amount of locomotion, again fixes itself by new secretion of byssus from the foot. Such movement is more frequent in young mussels than in the full-grown. Mytilus possesses no siphonal tube-like productions of the margin of the mantle-skirt, nor any notching of the same, representative of the siphons which are found in its fresh-water ally, the Dreissensia polymorpha. Mytilus edulis is an exceedingly abundant and widely distributed form. It occurs on both sides of the northern Atlantic and in the Mediterranean basin. It presents varieties of form and colour according to the depth of water and other circumstances of its habitat. Usually it is found on the British coast encrusting rocks exposed at low tides, or on the flat surfaces formed by sandbanks overlying clay, the latter kind of colonies being known locally as "scalps." Under these conditions it forms continuous masses of individuals closely packed together, sometimes extending over many acres of surface and numbering millions. The readiness with which the young Mytilus attaches itself to wicker-work is made the means of artificially cultivating and securing these molluscs for the market both in the Bay of Kiel in North Germany and at the mouth of the Somme and other spots on the coast of France.
Natural scalps are subject to extreme vicissitudes: an area of many acres may be destroyed by a local change of current producing a deposit of sand or shingle over the scalp, or by exposure to frost at low tide in winter, or by accumulation of decomposing vegetable matter. The chief localities of natural scalps on the British coast are Morecambe Bay in Lancashire and the flat eastern shores, especially that of the Wash of Lincoln, and similar shallow bays. These scalps are in some cases in the hands of private owners, and the Fisheries Department has made arrangements by which some local authorities, e.g. the corporation of Boston, can lease layings to individuals for the purpose of artificial cultivation.
The sea mussel is scarcely inferior in commercial value to the oyster. In 1873 the value of mussels exported from Antwerp alone to Paris to be used as human food was £280,000. In Britain their chief consumption is in the deep-sea line fishery, where they are held to be the most effective of all baits. Twenty-eight boats engaged in haddock-fishing at Eyemouth used between October 1882 and May 1883 920 tons of mussels (about 47,000,000 individuals), costing nearly £1800 to the fishermen, about one-half of which sum was expended on the carriage of the mussels. The quantity of mussels landed on Scottish coasts has decreased in recent years owing to the decline in the line fisheries. In 1896 the quantity was over 243,000 cwts., valued at £14,950; in 1902 it was only 95,663 cwts., valued at £5976.. In the statistics for England and Wales mussels are not separately distinguished. Many thousand tons of mussels are wastefully employed as manure by the farmers on lands adjoining scalp-producing coasts, as in Lancashire and Norfolk, three half-pence a bushel being the price quoted in such cases. It is a curious fact, illustrative of the ignorant procedure and arbitrary fashions of fisher-folk, that on the Atlantic seaboard of the United States the sea mussel, Mytilus edulis, though common, is not used as bait nor as food. Instead, the soft clam, Mya arenaria, a Lamellibranch not used by English or Norwegian fishermen, though abundant on their shores, is employed as bait by the fishermen to the extent of 14 million bushels per annum, valued at £120,000. At the mouth of the river Conway in North Wales the sea mussel is crushed in large quantities in order to extract pearls of an inferior quality which are occasionally found in these as in other Lamellibranch molluscs (Gwyn Jeffreys).
Mytilus edulis is considered of fair size for eating when it is 2 in. in length, which size is attained in three years after the spat or young mussel has fixed itself. Under favourable circumstances it will grow much larger than this, specimens being recorded of 9 in. in length. It is very tolerant of fresh water, fattening best, as does the oyster, in water of density 1014 (the density of the water of the North Sea being 1026). Experiments made by removing mussels from salt water to brackish, and finally to quite fresh water show that it is even more tolerant of fresh water than the oyster; of thirty mussels so transferred all were alive after fifteen days. Mytilus edulis is occasionally poisonous, owing to conditions not satisfactorily determined.
The fresh-water Mussels, Anodonta cygnea, Unio pictorum, and Unio margaritiferus belong to the order Eulamellibranchia of Lamellibranch Molluscs, in which the anterior and posterior adductor muscles are equally developed. An account of the anatomy of Anodon is given in the article Lamellibranchia. Unio differs in no important point from Anodonta in internal structure. The family Unionidae, to which these genera belong, is of world-wide distribution, and its species occur only in ponds and rivers. A vast number of species arranged in several genera and sub-genera have been distinguished, but in the British Islands the three species above named are the only claimants to the title of "fresh-water mussel." Anodonta cygnea, the Pond Mussel or Swan Mussel, appears to be entirely without economic importance. Unio pictorum, the common river mussel (Thames), appears to owe its name to the fact that the shells were used at one time for holding water-colour paints as now shells of this species and of the sea mussel are used for holding gold and silver paint sold by artists' colourmen, but it has no other economic value. Unio margaritiferus, the pearl mussel, was at one time of considerable importance as a source of pearls, and the pearl mussel fishery is to this day carried on under peculiar state regulations in Sweden and Saxony, and other parts of the continent. In Scotland and Ireland the pearl mussel fishery was also of importance, but has altogether dwindled into insignificance since the opening up of commercial intercourse with the East and with the islands of the Pacific Ocean, whence finer and more abundant pearls than those of Unio margaritiferus are derived.
In the last forty years of the 18th century pearls were exported from the Scotch fisheries to Paris to the value of £Ioo,000; round pearls, the size of a pea, perfect in every respect, were worth £3 or £4. The pearl mussel was formerly used as bait in the Aberdeen cod fishery.
LITERATURE. - For an account of the anatomy of Mytilus edulis the reader is referred to the treatise by Sabatier on that subject (Paris, 1875). The essay by Charles Harding on Molluscs used for Food or Bait, published by the committee of the London International Fisheries Exhibition (1883), may be consulted as to the economic questions connected with the sea mussel. The development of this species is described by Wilson in Fifth Ann. Rep. Scot. Fish. Board (1887). (E. R. L.; J. T. C.)
