OYSTER. The use of this name in the vernacular is equivalent to that of Ostrea (Lat. from Gr. d irpEov, oyster, so called from its shell, 66TEOV, bone, shell) in zoological nomenclature; there are no genera so similar to Ostrea as to be confounded with it in ordinary language. Ostrea is a genus of Lamellibranch Molluscs. The degeneration produced by sedentary habits in all lamellibranchs has in the oyster reached its most advanced stage. The valves of the shell are closed by a single large adductor muscle, the anterior adductor being absent. The muscular projection of the ventral surface called the foot, whose various modifications characterize the different classes of Mollusca, is almost entirely aborted. The two valves of the shell are unequal in size, and of different shape; the left valve is larger, thicker and more convex, and on it the animal rests in its natural state. This valve, in the young oyster, is attached to some object on the sea-bottom; in the adult it is sometimes attached, sometimes free. The right valve is flat, and smaller and thinner than the left. In a corresponding manner the right side of the animal's body is somewhat less developed than the left, and to this extent there is a departure from the bilateral symmetry characteristic of Lamellibranchs.
The organization of the oyster, as compared with that of a typical lamellibranch such as Anodon (see Lamellibranchia), is brought about by the reduction of the anterior part of the body accompanying the loss of the anterior adductor, and the enlargement of the posterior region. The pedal ganglia and auditory organs have disappeared with the foot, at all events have never been detected; the cerebral ganglia are very minute, while the parieto-splanchnic are well developed, and constitute the principal part of the nervous system.
According to Spengel, the pair of ganglia near the mouth, variously called labial or cerebral, represent the cerebral pair and pleural pair of a gastropod combined, and the parietosplanchnic pair correspond to the visceral ganglia, the commissure which connects them with the cerebro-pleural representing the visceral commissure. Each of the visceral ganglia is connected or combined with an olfactory ganglion underlying an area of specialized epithelium, which constitutes the olfactory organ, the osphradium. The heart and pericardial chamber in the oyster lie along the anterior face of the adductor muscle, almost perpendicular to the direction of the gills, with which in Anodon they are parallel. In Anodon and the majority of lamellibranchs the ventricle surrounds the intestine; in the oyster the two are quite independent, the intestine passing above the pericardium. The renal organs of the oyster were discovered by Hoek to agree in their morphological relations with those of other lamellibranchs.
The generative organs of the oyster consist of a system of branching cavities on each side of the body lying immediately beneath the surface. All the cavities of a side are ultimately in communication with an efferent duct opening on the surface of the body a little above the line of attachment of the gills. The genital opening on each side is situated in a depression of the surface into which the renal organ also opens. The genital products are derived from the cells which line the cavities of the genital organs. The researches of Hoek have shown that in the same oyster the genital organs at one time produce ova, at another spermatozoa, and that consequently the oyster does not fertilize itself. How many times the alternation of sex may take place in a season is not known. It must be borne in mind that. in what follows the species of the European coasts, Ostrea edulis, is under consideration. The ova are fertilized in the genital duct, and before their escape have undergone the earliest stages of segmentation. After escaping from the genital aperture they find their way into the infra-branchial part of the mantle cavity of the parent, probably by passing through the suprabranchial chamber to the posterior extremity of the gills, and then being conducted by the inhalent current caused by the cilia of the gills into the infra-branchial chamber. In the latter they accumulate, being held together and fastened to the gills by a white viscid secretion. The mass of ova thus contained in the oyster is spoken of by oyster fishers as "white spat," and an oyster containing them is said to be "sick." While in this position the ova go through the earlier stages of development. At the end of a fortnight the white spat has become darkcoloured from the appearance of coloured patches in the developing embryos. The embryos having then reached the condition of "trochospheres" escape from the mantle cavity and swim about freely near the surface of the water among the multitude of other creatures, larval and adult, which swarm there. The larvae are extremely minute, about ff in. long and of glassy transparency, except in one or two spots which are dark brown. From the trochosphere stage the free larvae pass into that of "veligers." How long they remain free is not known; Huxley kept them in a glass vessel in this condition for a week. Ultimately they sink to the bottom and fix themselves to shells, stones or other objects, and rapidly take on the appearance of minute oysters, forming white disks 2 o in. in diameter. The appearance of these minute oysters constitutes what the fishermen call a "fall of spat." The experiment by which Hoek conclusively proved the change of sex in the oyster was as follows. In an oyster containing white spat microscopic examination of the genital organs shows nothing but a few unexpel]ed ova. An oyster in this condition was kept in an aquarium by itself for a fortnight, and after that period its genital organs were found to contain multitudes of spermatozoa in all stages of development.
The breeding season of the European oyster lasts from May to September. The rate of growth of the young oyster is, roughly speaking, an inch Of diameter in a year, but after it has attained a breadth of 3 in. its growth is much slower. Professor Mobius is of opinion that oysters over twenty years of age are rare, and that most of the adult Schleswig oysters are seven to ten years old.
The development of the American oyster, 0. virginiana, and of the Portuguese oyster, 0. angulata, is very similar to that of 0. edulis, except that there is no period of incubation within the mantle cavity of the parent in the case of these two species. Hence it is that so-called artificial fertilization is possible; that is to say, fertilization will take place when ripe eggs and milt are artificially pressed from the oysters and allowed to fall into a vessel of sea-water. But if it is possible to procure a supply of spat from the American oyster by keeping the swarms of larvae in confinement, it ought to be possible in the case of the European oyster. All that would be necessary would be to take a number of mature oysters containing white spat and lay them down in tanks till the larvae escape. This would be merely carrying oyster culture a step farther back, and instead of collecting the newly fixed oysters, to obtain the free larvae in numbers and so insure a fall of spat independently of the uncertainty of natural conditions. This method has been tried several times in England, in Holland and in France, but always without permanent success.
Natural beds of oysters occur on stony and shelly bottoms at depths varying from 3 to 20 fathoms. In nature the beds are liable to variations, and, although Huxley was somewhat sceptical on this point, it seems that they are easily brought into an unproductive condition by over-dredging. Oysters do not flourish in water containing less than 3% salt; and hence they are absent from the Baltic. The chief enemies of oysters are the dog-whelk, Purpura lapillus, and the whelk-tingle, Murex erinaceus, which bore through the shells. Starfishes devour large numbers; they are able to pull the valves of the shell apart and then to digest the body of the oyster by their everted stomach. Cliona, the boring sponge, destroys the shells and so injures the oyster; the boring annelid Leucodore also excavates the shell.
The wandering life of the larvae makes it uncerain whether any of the progeny of a given oyster-bed will settle within its area and so keep up its numbers. It is known from the history of the Liimfjord beds that the larvae may settle 5 m. from their place of birth.
The genus Ostrea has a world-wide distribution, in tropical and temperate seas; seventy species have been distinguished. Its nearest allies are Pinna among living forms, Eligmus among fossils. For the so-called pearl-oyster see Pearl.
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Oysters are more valuable than any other single product of the fisheries, and in at least twenty-five countries are an important factor in the food-supply. The approximate value of the world's oyster crop approaches f4,000,000 annually, representing over 30,000,000 bushels, or nearly Io billion oysters. Not less than 150,000 persons are engaged in the industry, and the total number dependent thereon is fully half a million.. The following table shows in general terms the yearly oyster product of the world: - United States. - The oyster is the chief fishery product in the United States. The states which lead in the quantity of oysters taken are Maryland, Virginia, New York, New Jersey and Connecticut; the annual value of the output in each of these is over $ I ,000,000. Other states with important oyster interests are Rhode Island, North Carolina, Louisiana and California. The oyster fisheries give employment to over 56,000 fishermen, who man 4000 vessels, valued at $4,000,000, and 23,000 boats, valued at $1,470,000; the value of the 1 i,000 dredges and 37,000 tongs, rakes and other appliances used is $365,000. The quantity of oysters taken in 1898 was 26,853,760 bushels, with a value of $12,667,405. The output of cultivated oysters in 1899 was about 9,800,000 bushels, worth $8,700,000.
Oyster banks of some importance exist in the Gulf of St Lawrence and on the coast of British Columbia. All of the grounds have suffered depletion, and cultural methods to maintain the supply have been instituted. The oyster output of the Dominion has never exceeded 200,000 bushels in a single year, and in 1898 was 134,140 bushels, valued at $217,024.
The natural oyster beds of Great Britain and Ireland have been among the most valuable of the fishery resources, and British oysters have been famous from time immemorial. The most important oyster region is the Thames estuary, the site of extensive planting operations. The present supply is largely from cultivated grounds. Important oyster-producing centres are xx.14a
Whitstable, Colchester and Brightlingsea. The oysters landed on the coasts of England and Wales in 1898 numbered 35,809,000, valued at £122,320, and in 18 99, 3 8 ,97 8, 000, valued at £143,841. The Scottish fishery has its centre at Inveraray and Ballantrae, and in 1905 yielded 218,000 oysters, valued at £865. Public oyster grounds of Ireland in 1903 produced 2,532,800 oysters, valued at £5030. The fishery is most extensive at Wicklow, Queenstown, Ballyheige, Galway and Moville. Planting is carried on in seven counties; the oysters taken from cultivated beds in 1903 numbered 2,687,500 oysters, valued at £5420.
The industry owes its importance to the attention given to oyster cultivation. In the fishery on public grounds in 1896 only 6370 fishermen were engaged, employing 1627 vessels and boats, valued at 1,473,449 francs, and apparatus worth 211,495 francs, while only 13,127,217 kilograms of oysters were taken, or about 320,000 bushels, valued at 414,830 francs. In the parks, claires and reservoirs the private culture of oysters has attained great perfection. Fully 40,000 men, women and children are employed, and the output in 1896 was 1,536,417,968 oysters, worth 1 7,537,77 8 francs. The principal centre is Arcachon.
The oyster industry has passed from the hands of the fisherman into those of the oyster culturist. The oyster being sedentary, except for a few days in the earliest stages of its existence, is easily exterminated in any given locality; since, although it may not be possible for the fishermen to rake up from the bottom every individual, wholesale methods of capture soon result in covering up or otherwise destroying the oyster banks or reefs, as the communities of oysters are technically termed. The main difference between the oyster industry of America and that of Europe lies in the fact that in Europe the native beds have long since been practically destroyed, perhaps not more than 6 or 7% of the oysters of Europe passing from the native beds directly into the hands of the consumer. It is probable that 60 to 75% are reared from the spat in artificial parks, the remainder having been laid down for a time to increase in size and flavour in shoal waters along the coasts. In the United States, on the other hand, from 30 to 40% are carried from the native beds directly to market. The oyster fishery is everywhere, except in localities where the natural beds are nearly exhausted, carried on in the most reckless manner, and in all directions oyster grounds are becoming deteriorated, and in some cases have been entirely destroyed. At present the oyster is one of the cheapest articles of diet in the United States; and, though it can hardly be expected that the price of American oysters will always remain so low, still, taking into consideration the great wealth of the natural beds along the entire Atlantic coast, it seems certain that a moderate amount of protection would keep the price of seed oysters far below European rates, and that the immense stretches of submerged land especially suited for oyster planting may be utilized and made to produce an abundant harvest at much less cost than that which accompanies the complicated system of culture in vogue in France and Holland.
The simplest form of oyster culture is the preservation of the natural oyster-beds. Upon this, in fact, depends the whole future of the industry, since it is not probable that any system of artificial breeding can be devised which will render it possible to keep up a supply without at least occasional recourse to seed oysters produced under natural conditions. It is the opinion of almost all who have studied the subject that any natural bed may in time be destroyed by overfishing (perhaps not by removing all the oysters, but by breaking up the colonies, and delivering over the territory which they once occupied to other kinds of animals), by burying the breeding oysters, by covering up the projections suitable for the reception of spat, and by breaking down, through the action of heavy dredges, the ridges which are especially fitted to be seats of the colonies.' The 1 Even Huxley, the most ardent of all opponents of fishery legislation, while denying that oyster-beds had been permanently annihilated by dredging, practically admitted that a bed may be reduced to such a condition that the oyster will only be able to recover its former state by a long struggle with its enemies and competition - in fact that it must re-establish itself much in the same way as they have acquired possession of new grounds in Jutland, a process which, according to his own statement, occupied thirty years (Lecture at the Royal Institution, May 11th, 1883, printed with additions in the English Illustrated Magazine, i. pp. 47-55, 112-121).
immense oyster-beds in Pocomoke Sound, Maryland, have practically been destroyed by over-dredging, and many of the other beds of the United States are seriously damaged. The same is doubtless true of all the beds of Europe. It has also been demonstrated that under proper restriction great quantities of mature oysters, and seed oysters as well, may be taken from any region of natural oyster-beds without injurious effects. Parallel cases in agriculture and forestry will occur to every one. Mdbius, in his most admirable essay Die Auster and Die Austernwirthschaft, has pointed out the proper means of preserving natural beds, declaring that, if the average profit from a bed of oysters is to remain permanently the same, a sufficient number of mother oysters must be left in it, so as not to diminish the capacity of maturing. He further shows that the productive capacity of a bed can only be maintained in one of two ways: (I) b y diminishing the causes which destroy the young oysters, in which case the number of breeding oysters may safely be decreased; this, however, is practicable only under such favourable conditions as occur at Arcachon, where the beds may be kept under the constant control of the oyster-culturist; (2) by regulating the fishing on the natural beds in such a manner as to make them produce permanently the highest possible average quantity of oysters. Since the annual increase of half-grown oysters is estimated by him to be four hundred and twenty-one to every thousand full-grown oysters, he claims that not more than 42% of these latter ought to be taken from a bed during a year.
The Schleswig-Holstein oyster-beds are the property of the state, and are leased to a company whose interest it is to preserve their productiveness. The French beds are also kept under government control. Not so the beds of Great Britain and America, which are as a general rule open to all corners,' except when some close-time regulation is in force. Huxley has illustrated the futility of "close-time" in his remark that the prohibition of taking oysters from an oyster-bed during four months of the year is not the slightest security against its being stripped clean during the other eight months. "Suppose," he continues, "that in a country infested by wolves, you have a flock of sheep, keeping the wolves off during the lambing season will not afford much protection if you withdraw shepherd and dogs during the rest of the year." The old close-time laws were abolished in England in 1866, and returned to in 1876, but no results can be traced to the action of parliament in either case. Huxley's conclusions as regards the future of the oyster industry in Great Britain are doubtless just as applicable to other countries - that the only hope for the oyster consumer lies in the encouragement of oyster-culture, and in the development of some means of breeding oysters under such conditions that the spat shall be safely deposited. Oyster culture can evidently be carried on only by private enterprise, and the problem for legislation to solve is how to give such rights of property upon those shores which are favourable to oyster culture as may encourage competent persons to invest their money in that undertaking. Such property right should undoubtedly be extended to natural beds, or else an area of natural spawning territory, should be kept under constant control and surveillance by government, for the purpose of maintaining an adequate supply of seed oysters.
The extension of the area of the natural beds is the second step in oyster culture. As is well known to zoologists, and as has been very lucidly set forth by Mdbius, the location of oyster banks is sharply defined by absolute physical conditions. Within certain definite limits of depth, temperature and salinity, the only requirement is a suitable place for attachment. Oysters cannot thrive where the ground is composed of moving sand or where mud is deposited; consequently, since the size and number of these places are very limited, only a very small percentage of the young oysters can find a resting-place, and the remainder perish. Mdbius estimates that for every oyster brought to 1 Connecticut has greatly benefited its oyster industry by giving to oyster-culturists a fee simple title to the lands under control by them.
market from the Holstein banks, 1,045,000 are destroyed or die. By putting down suitable "cultch" or "stools" immense quantities of the wandering fry may be induced to settle, and are thus saved. As a rule the natural beds occupy most of the suitable space in their own vicinity. Unoccupied territory may, however, be prepared for the reception of new beds, by spreading sand, gravel and shells over muddy bottoms, or, indeed, beds may be kept up in locations for permanent natural beds, by putting down mature oysters and cultch just before the time of breeding, thus giving the young a chance to fix themselves before the currents and enemies have had time to accomplish much in the way of destruction.
The collection of oyster spat upon artificial stools has been practised from time immemorial. As early as the 7th century, and probably before, the Romans practised a kind of oyster culture in Lake Avernus, which still survives to the present day in Lake Fusaro. Piles of rocks are made on the muddy bottoms of these salt-water lakes, and around these are arranged circles of stakes, to which are often attached bundles of twigs. Breeding oysters are piled upon the rookeries, and their young become attached to the stakes and twigs provided for their reception, where they are allowed to remain until ready for use, when they are plucked off and sent to the market. A similar though ruder device is used in the Poquonock river in Connecticut. Birch trees are thrown into the water near a natural bed of oysters, and the trunks and twigs become covered with spat; the trees are then dragged out upon the shore by oxen, and the young fry are broken off and laid down in the shallows to increase in size. In 1858 the methods of the Italian lakes were repeated at St Brieuc under the direction of Professor P. Coste, and from these experiments the art of artificial breeding as practised in France has been developed. There is, however, a marked distinction between oyster-culture and oyster-breeding.
In considering the oyster-culture in France it is necessary to distinguish the centres of production from the centres of rearing or fattening. The chief centres or regions of oyster production are two, (i) Arcachon, (2) Brittany. The basin of Arcachon has an area of about 38,000 acres at high water, and only about 15,000 acres are under water at low tide. The water is salter than the sea. At the beginning of the 19th century there were only natural oyster beds in the basin, and these produced 75 million oysters per annum. But in the middle of the century the natural beds had been almost exhausted and the system of government control, letting "parks" to private tenants, and artificial cultivation was instituted. Certain beds in the basin are reserved and kept under government control. Cultch is placed upon them every year, and gathering of oysters upon them is allowed only at intervals of two or more years, when the authority thinks they are sufficiently stocked to permit of it. These beds supply spat for the private cultivators. The latter collect the spat on tiles: these are made of earthenware and concave on one side. One of the most important points in the system is the coating of the tiles with lime. It is necessary to detach the young oysters from the tiles when they are nearly a year old (detroquage): this could not be done without destroying the oysters if they were attached directly to the surface of the tile. The coating of lime or mortar is soft and brittle, and consequently the young oysters can easily be detached with a stout knife. The method of liming the tiles (chaulage) consists in dipping them into a liquid mixture of lime and water. Sometimes lime only is used, sometimes equal quantities of lime and sand, or lime and mud. Often it is necessary to repeat the dipping, and for the second coat hydraulic lime may be employed.
The tiles coated with lime are set out on the shore near the lowwater mark of spring tides, at the beginning of the spatting season. This is earlier in the south of France than in England: at Arcachon the collectors are put in position about the middle of June. Various methods are adopted for keeping the tiles in place and for arranging them in the position most favourable to the collection of spat. At Arcachon they are arranged in piles each layer being transverse to the one below, so that the space formed by the concavity of the tile is kept open. A wooden frame-work often surrounds the heap of tiles to prevent them being scattered by the waves.
In the following season, about April, the young oysters, then from z to 1 in. in diameter, are separated or detroques. They may then be placed in oyster cases (caisses ostre'ophiles) or in shallow ponds (claires) made on the fore-shore. The cases are about 8 in. deep, made with a wooden frame-work, and galvanized wire netting top and bottom, the lid being hinged. These cases about 8 ft. by 4 ft. in dimensions are fixed on the fore-shore by means of short posts driven into the ground, so that they are raised about 9 in. or 1 ft. from the latter. The young oysters grow rapidly in these cases, and have to be thinned out as they grow larger. When they have been in the boxes a year they are large enough to be placed in the claires or simply scattered along the fore-shore.
In Brittany the chief seat of oyster production is the gulf of Morbihan, where the estuaries of numerous small rivers furnish fore-shores suitable to the industry. Here the prevalence of mud is one of the chief obstacles, and for this reason the tile-collectors are usually fastened together by wire and suspended to posts (tuiles en bouquets). The collectors are not set out before the middle of July. The natural beds from which the supply of spat is derived are reserved, but apparently are insufficiently protected, so that much poaching goes on.
These two regions of production, Arcachon and M orbihan supply young oysters for "relaying," i.e. rearing, not only to numerous places on the coast of France, but also to England, Ireland and elsewhere. Among rearing districts Marennes and La Tremblade are specially celebrated on account of the extensive system of claires or oyster ponds, in which the green oysters so much prized in Paris are produced. The irrigation of the claires is entirely under control, and the claires undergo a special preparation for the production of the green oysters, whose colour seems to be derived from a species of Diatom which abounds in the claires. In Holland the French system of oyster-culture is followed in the estuary of the Scheldt, with some modifications in detail. The tiles used are flat and heavy, and are placed on the foreshores in an oblique position resting on their edges and against each other. The tiles with the young oysters on them are placed in enclosures during the winter, and detroquage is carried out in the following summer.
In England the use of tiles has been tried on various occasions, in Cornwall on the river Fal, at Hayling Island and in Essex, but has nowhere become permanently established. The reasons for this are that the fall of spat is not usually very abundant, and the kind of labour required cannot be obtained at a sufficiently cheap rate. In many places oysters are simply imported from France and Holland and laid down to grow, or are obtained by dredging from open grounds. At Whitstable most of the stock is thus obtained, but cultch (i.e. dead shells) is here and elsewhere scattered over the ground to serve for the attachment of spat. The use of cultch as collector is a very ancient practice in England, and is still almost universally maintained. In the estuaries of Essex there are many private or semi-private oyster fisheries, where the method of culture is to dredge up the oysters in autumn and place them in pits, where they are sorted out, and the suitable ones are selected for the market. Just before the close season the young oysters and all the rest that remain are scattered over the beds again, with quantities of cultch, and in many cases the fishery is maintained by the local fall of spat, without importation. In some places where the ground is suitable cultch is spread over the foreshores also to collect spat. The genuine English "native" is produced in its greatest perfection in the Essex fisheries, and is probably the highest priced oyster in the world.
In addition to the literature quoted see also the following: Rapport sur les recherches concernant l'huitre et l'ostreiculture public par la Commission de la Societe Neerlandaise de Zoologie (Leiden, 1883-1884); P. Brocchi, Traite de l'ostreiculture (Paris, 1883); Bashford Dean, European Oyster Culture, Bulletin U.S. Fish Commission, vol. x. for 1890, vol. xi. for 1891; J. T. Cunningham, Report of the Lecturer on Fishery Subjects, in Report of Technical Instruction Committee of Cornwall (1899, 1900). (G. B. G.; J. T. C.)
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