On European oyster beds the greatest spatfalls of 0. edulis un doubtedly occur in the warmer localities and in the warmer sea sons, and probably a higher temperature (not less than 63°F.) than that required for spawning (59-60°F.) is more favourable, if not actually necessary, for spatting. Other oysters are probably similar to 0. edulis in this respect. Very good spatting seasons occur sporadically for example, 1913, 1914 and 1921 were excep tionally good seasons in the Thames estuary. In the same locality bad and moderate spatting seasons have occurred on the average about twice as frequently as good or very good seasons. Similar fluctuations occur in all parts of the world.
In times of scarcity when the spatfall has failed for a longer succession of years than usual, and when the adults on the beds have been reduced beyond a certain minimum, par ticular beds have gradually died out, as in the Firth of Forth, the German bight, and elsewhere. In open waters, beds die out more easily from these causes than in enclosed estuarine waters. On the other hand, new oyster beds sometimes occur off the coast when swarms of larvae have been carried long distances sea wards by tidal currents, and found congenial ground. It is in this manner that deep-sea beds are mostly formed. By careful study of situations it is possible to start new oyster beds artificially by importing a species appropriate to a locality. Of the unknown factors favourable for a good spatting probably food and the physical condition of the water are most important.
Oysters are said to be fat when the body is naturally large, but largeness of body may be caused either by a well-developed reproductive organ only, or by accumulation of reserve food-products. Fatness during the summer or breeding time is mainly of the former type. Fattening by accumulation of reserve products occurs mainly at the close of the breeding sea son, when food may be taken in faster than it can be utilized. The food-reserves consist largely of glycogen (animal starch), which is semi-liquid in the living animal and stored in the large bladder-cells found in most parts of the body. Abundance of food is dependent ultimately upon the chemical constituents of the sea-water, of which phosphates and nitrates are the most important.
Shell growth in oysters occurs in spurts, and one or more fringes of new shell may be added during one period of growth. In the European oyster in some localities, e.g., Fal estuary, shell growth occurs twice a year, in spring and autumn.
The rate of growth of the shell varies locally and from season to season, so that it is impossible to fix any special size for any special age. In the case of the European oyster, if the larva settles in early summer, it may grow into a spat i to r sin. at the end of the growing period of that year, but if larvae settle late in the year they may finish growth in their first summer at a size of even i inch. The average size of natural spat on oyster beds, however, at the end of the first summer lies between -I ands inch. (Under artificial conditions spat may grow much larger in the first summer, but have then usually extremely thin fragile shells.) In the same way average sizes at the end of the second summer of growth are about i to I -On.; in the third summer, about II- to 2in. ; fourth summer, average 2i to 21in., fifth summer, average 2i to 2-1-in. with a range to about 4 inches.
The average size at a given age will usually be higher on the shallower than on the deeper beds in an estuary, but is probably very high on deep-sea beds. American, Portuguese and tropical oysters grow much more rapidly than the European species; for example, the American species is said to reach a length of in seven months in South Carolina.
The oyster's natural enemies are more numerous in early than late stages of life. The larva is eaten by small fishes, jelly-fishes, other bivalves, sea-squirts (Ascidians), worms, anemones and small crustaceans, all of which are usually common on oyster beds. Spat, young and old oysters are attacked by borers which bore a hole through the shell and eat the oyster through the hole (especially Murex in Europe, Xymene in Australia, Urosalpinx in America). Fifty per cent of a spatfall may be destroyed where borers are abundant. Crabs (Carcinus in Europe, Scylla in Australia) attack weak oysters, especially the small ones, and do great damage. Large fishes with special crushing teeth devour large and small oysters (Myliobatis, Europe and Australia, Pogonias in America). Starfishes (es pecially Asterias species in Europe and America) consume at times quantities of stock, and a flat-worm is reported to be a pest on certain beds in America. The shell is bored into and used for protection by the sponge, Cliona, and often in large numbers by the worm Polydora. Competitors for food are numerous, such as all bivalves, especially mussels and, in general, all animals which feed in fundamentally the same way as the oyster.