The oysters of Japan are Ostrea cucullata, which occurs in shallow and moderately brackish or moderately salt water throughout the whole archipelago; and Ostrea gigas, a very large salt water species found in deep water. Many other species of Ostrea are found in temperate and tropical seas throughout the world.
Fossil Oysters. The oyster family appears to have had its origin in some imperfectly known forms, such as Ostrea nobilissima of the Carboniferous. The family is found also in the Permian. In the Triassic it is represented by a strongly plicated form, Alectryonia, which form becomes more prominent in the Jurassic and Cretaceous. There are also the common arcuate shells of Gryphæa and Exogyra in the Jurassic and Cretaceous. Ostrea itself is known in the Mesozoic, but it attained its maximum of size and abundance in the Tertiary. The sandy marls of this period in the Southeastern United States often contain great numbers of very large specimens of oysters, especially of two species, Ostrea Georgiana and Ostrea sellæformis. Consult White, C. A., “A Review of the Fossil Ostreidæ of North America and a Comparison of the Fossil with the Living Forms,” Annual Report of the United States Geological Survey, vol. iv. (Washington, 1883).
Oyster Culture. Owing to the exhaustion of the natural beds and their inability to supply the demand for oysters, it has been found necessary to resort to artificial methods of production, effecting (1) an increase in the number of eggs fertilized; (2) an increase in the surfaces available for fixation, and also of the number of spat attaching; (3) the saving of spat and young oysters which would naturally fall victims to enemies and adverse physical conditions; and (4) the utilization of barren bottoms and naturally unavailable food supplies. But a small part of the area under water suitable for oysters has been utilized by nature, mainly for lack of suitable bodies for the attachment of the young. In the United States such barren bottom is utilized by clearing it of all rubbish and either planting ‘cultch’ to collect the spat, or else young oysters (seed), that they may improve in size, shape, and quality under conditions safer and more favorable than in their original environment. In certain places either method may succeed, but commonly a locality is better adapted to one than the other.
The most suitable bottom for oyster culture consists of firm mud or of a firm substratum with a thin surface of soft mud, but stable sandy bottom is often used with success. Rocky bottom is usually deficient in food, loose sand drifts and covers the oysters, and very soft mud ingulfs and stifles them or produces inferior elongate stock. Mud naturally too soft may be utilized by distributing over it shells, sand, or other material, which, resting on or near the surface, furnishes a firm foundation upon which the growing oyster may repose in security. For spat-collecting it is frequently advantageous to use hard mud, gravel, or rocky bottom in shoal water, ill adapted to adult oysters from deficiency of food. The bottom being properly prepared and its boundaries marked with stakes or buoys, either system may be adopted to accord with circumstances. Generally seed-planting is more certain in its results and yields quicker returns to the grower. Seed-oysters vary from ‘blisters’ ½ inch in diameter to individuals almost ready for market, but ordinarily they are between 1 and 3 inches long. They are obtained from planters making a specialty of seed production or from natural beds, their cost varying from 10 cents to $1 per bushel, the larger culled stock, separate, well shaped, and free from rubbish, bringing higher prices and giving the best results. From 300 to 600 bushels of culled seed per acre are used, a larger quantity of ‘rough’ material being required, as much of it consists of old shells and debris. It is usually sowed broadcast with shovels from boats. Further attention, other than that required to keep the beds clean and free from enemies, is generally unnecessary, especially if culled seed has been used.
The system of spat-collecting is often extremely productive, though sometimes, for reasons not well understood, it results in complete failure. Spat will attach to almost any clean solid body, but certain materials, from their shape, structure, or cheapness, possess advantages which commend them. The most widely used and one of the best forms of cultch consists of the clean shells of the oyster itself. They are cheap, readily obtainable in all oyster regions, and, owing to their size and shape, can be used with success on bottom too soft for most other materials. The principal objection to them is that so many spat sometimes attach to a shell that they have no room for growth, and scallop (Pecten), jingle (Anomia), and other small fragile shells are sometimes preferable, as they catch the spat in smaller clusters and tend to break up as the oysters grow, but, owing to their lightness, they cannot be used in strong currents. The cost of oyster shells is from 2 to 5 cents per bushel, and sometimes they may be had for the hauling. Coarse gravel, pebbles, and crushed stone are used to a considerable extent in Long Island Sound and vicinity, but require a harder bottom than shells. The particles average about the size of a walnut or smaller, and as but few spat attach to each, the oysters are well shaped, less laborious to cull, and a larger proportion survive. This material costs from 5 to 8 cents per bushel and the cost of planting is about the same as of shells, ½ to 5 cents per bushel, according to local conditions.
Shells, stones, and gravel are distributed, like seed, from boats or scows. From 250 to 600 bushels per acre are used, soft bottom requiring more than hard. If there are extensive beds of adult oysters in the vicinity, and especially if the currents set from them to the spat-beds, they can be depended upon to supply the fry, but if not, adult oysters should be used in the proportion of 30 to 60 bushels per acre. The brood oysters should be planted several months before the spawning season, but the cultch should not be put down until spawning is about to begin, that it may be free from slime and sediment when the fry is ready to fix, even a thin coating of sediment being sufficient to suffocate the young oyster at that period.
Some planters allow the beds to remain unworked until the crop is ready to market, but to produce oysters of superior shape and quality, the clusters should be taken up and separated as soon as they can be culled without injury. It frequently happens that good localities for obtaining a set are not favorable to the production of marketable oysters, and in this case the culled young
