Oyster, a marine acephalous mollusk, of the lamellibranchiate order and genus ostrea (Linn.). The shells are very irregular, inequi-valve, and lamellated, the right or upper shell being the smaller and flatter and moving forward with age, leaving a lengthening groove for the ligament exposed along the beak of the adhering valve, which is the left and lower, the deeper, and more capacious, and attached to foreign bodies by a calcareous growth from the shell itself. The shells are so variable in surface and shape that it would be difficult to describe them, and for the same reason it is almost impossible strictly to define the limits of the species; there is only one adductor muscle to hold the valves together, and the small ligament at the hinge is inserted into a little depression on each side, without teeth or projecting plates. The animal is very simple; the mantle has a double fringe, and its lobes are widely separated, united only near the hinge; there is no vestige of foot; respiration is effected by means of vascular gills or membranous plates attached to the inner surface of the mantle, to which water is brought by the ceaseless action of vibratile cilia; the mouth is jawless and toothless, but is provided with short labial processes separate from the gills for selecting food, consisting of minute particles brought to it by the respiratory currents; the intestine is comparatively short, with a few convolutions; the ventricle of the heart lies upon the rectum.
By most writers oysters have been considered hermaphrodite, but according to Siebold and others they are of separate sexes, though the females vastly preponderate, and are ovoviviparous; they are sensible of light, as is known by their closing the valves when reached by the shadow of an approaching boat, and have numerous short, pedunculated, yellowish brown eyes between the fringes for more than a third of the length of the mantle. The adult oyster has no power of locomotion, and the only signs of vigorous movements are in the expulsion of the respiratory currents, the excrements, and the sperm or ova by the sudden closing of the valves and the contraction of the mantle; but it is said they can turn themselves if placed upside down, and the sensibility of the fringes and labial pro-cesses is acute. The eggs are expelled in a white, greasy, viscid fluid, called "spats" by the fishermen, which adhere to submarine bodies, and to each other, by their development forming the immense banks found upon some coasts, the old ones being destroyed by the pressure of the new; fecundation is effected through the medium of the water, which conveys the sperm to the ova; the eggs are to a certain extent developed within the cavity of the mantle about the gills'; to this cavity also the floating ova of some of the smaller Crustacea gain access, and here the little, soft, yellowish white crab (pinnotheres) is often developed to a considerable size; this last is a parasitic inhabitant of the oyster shell, and is not a portion of its food, as the softness of the mouth of the latter does not admit of its attacking any resisting substance.
Oysters are found in almost all seas, usually in from two to six fathoms of water, and never at a great distance from the shore; they are especially fond of tranquil waters or the gulfs formed by the mouths of great rivers; they cannot live in fresh water, but some species remain dry during the greater part of every tide; the tree oysters (0. parasitica and polymorpha), which attach themselves to mangrove and other bushes in the tropics, enclose within the shells a sufficient quantity of water to keep up the respiratory currents; this faculty, possessed more or less by all the family, renders practicable their transportation to great distances. They have been highly esteemed as food from the times of the Greeks and Romans to the present day; they are of easy digestion, but not very nutritious, and act rather as a provocative to appetite than as satisfying food; they are eaten all the year round, except in the months of May, June, July, and August, which is the spawning season; and they are good even then.
The common oyster of Europe (0. edu-lis, Linn.), abundant on the coasts of Great Britain and France, occurs in large banks or beds, sometimes extending for miles, usually on rocky bottoms; from about the middle of August to the middle of May they are dredged from the bottom by a kind of iron rake drawn by a boat under full sail, several hundreds being taken at a single haul; these are transferred to artificial beds or parks, where they are preserved for sale, continually growing in size and improving in flavor. The growth of the oyster is slow, it being only as large as a half dollar at the end of four to six months, and twice that size at the end of a year; in artificial beds the growth is usually slower, the full size not being attained till the fifth to the seventh year. The west coast of Scotland and the Hebrides have the best oysters of the British coasts, and here in sheltered bays they acquire the green color so esteemed by the epicure, and supposed to be due to confervae and similar colored growths in the breeding places; other English beds extend from Graves-end on the Thames along the Kent coast, and in the estuaries of the Colne and other rivers along the Essex coast.
The British beds are kept up by careful culture and by the introduction of broods from all quarters; since 1872 several varieties of American oysters have been introduced, but the planting is still an experiment, and it is said that the change of sea deteriorates their quality. Not many years ago the beds of France were nearly exhausted; in 1858 M. Coste recommended plans for their restoration, and since then the parks in the bays of St. Brieuc and Arcachon, and the isle of Re, restocked by broods from Cancale and other sources, have become enormous; and the successful culture is yearly extending along the entire Atlantic and Mediterranean coasts of France. The Danish coast is well supplied with beds. The Neapolitan lake Fusaro is the great oyster park of Italy. - The species most esteemed in America are the Virginian oyster (0. Virginiana, Lister) and the northern oyster (0. boreah's, Lam.). In the 0. Virginiana the shell is elongated and narrow, and the beaks pointed and not much curved; the surface of the smaller and upper valve when not worn presents everywhere leaf-like scales of a leaden color, and a lengthened pyramidal hinge ridge along the beak; the muscular impression is nearly central, and of a dark chestnut or violet color; it often measures 12 to 15 in. in length, but is rarely more than 3 in. wide.
This is the common oyster from Chesapeake bay southward; it is sometimes found in the vicinity of Boston, and also at the mouth of the river St. Lawrence; it multiplies so rapidly on some of the low shores of the southern states as to offer impediments to navigation, and to change the course of tidal currents. In the 0. borealia the shell is more rounded and curved, with the beaks short and considerably curved; the surface is very irregular, presenting loosely arranged layers of a greenish color, with the margins more or less scalloped; the muscular impression is dark violet, and the interior chalky or greenish white; a common size is 5 or 6 in. long, but it grows to the length of a foot and to a width of 6 in. This is the common New York oyster, said also formerly to have been abundant in Massachusetts bay. Boston market is supplied principally from artificial beds derived from the Virginia and New York oysters; the flats in the vicinity of our large maritime cities are generally thickly beset with poles, indicating the localities of oyster beds. The principal sources of supply are the Chesapeake bay, the coast of New Jersey, and Long Island sound.
Formerly the northern beds were almost wholly kept up by restocking them with seed oysters from Chesapeake bay and from the Hudson river; but of late years the spat is secured at spawning time, and new ground in the vicinity is brought under cultivation, till the area of oyster beds in Long Island sound is now computed by miles rather than by acres, and it is yearly extending. With constantly improving methods of culture, means are also devised for protecting the oyster to some extent from its natural enemies, and for transporting oysters to the remotest parts of the country. No trustworthy statistics can be given of the oyster area or annual product, or the amount of money invested, or number of men and vessels engaged in the business; the wholesale trade of New York alone is estimated at $25,000,000 a year, and that of Chesapeake bay is probably nearly as great. - More than 60 species of oysters are described in various parts of the world; those of tropical climates have generally a less delicious flavor than the natives of temperate zones. About 200 species of fossil oysters, from the time of the ammonites to the present epoch, are known.
The family ostreadoe, of which the oyster is the type, contains also the genus anomia, translucent, pearly white within, attached to rocks and weeds by a calcareous plug passing through a hole or notch in the right valve, the same species presenting a great variety of shapes from acquiring the form of the surface to which they are attached. Allied genera are placuna, like the P. sella or Hungarian saddle, and P. placenta of the Chinese seas, so transparent as to be used for glass in windows; the pecten or scallop shell; and the spondylus or thorny oyster. The pearl oyster is axicula margaritifera, belonging to another family. (See Pearl).
Oysters at different Stages of Growth.