Without wishing to dogmatise on any point of oyster life, I think I can bring before my readers in a brief way a few interesting facts in the natural history of the edible oyster.
As is well known, there is a period every year during which the oyster is not fished; and the reason why our English oyster-beds have not been ruined or exhausted by overfishing arises, among other causes, from this fact of there being a definite close-time assigned to the breeding of the mollusc. It would be well if the larger varieties of sea produce were equally protected; for it is sickening to observe the countless numbers of unseasonable fish that are from time to time brought to Billingsgate and other markets, and greedily purchased. The fact that oysters are supplied only during certain months in the year, and that the public have a general corresponding notion that they are totally unfit for wholesome eating during May, June, July, and August (those four wretched months which have not the letter “r” in their names), has been greatly in their favour. Had there been no period of rest, it is almost quite certain that oysters would long ago—I allude to the days when there was no system of cultivation—have become extinct, so great is the demand for this dainty mollusc.
Oysters begin to sicken about the end of April, so that it is well that their grand rest commences in May. The shedding of the spawn continues during the whole of the hot months—not but that during that period there may be found supplies of healthy oysters, but, as a general rule, it is better that there should be a total cessation of the trade during the summer season, because were the beds disturbed by a search for the healthy oysters the spawn would be scattered and destroyed.
Oysters do not leave their ova, like many other marine creatures, but incubate them in the folds of their mantle, and among the laminæ of their lungs. There the ova remain surrounded by mucous matter, which is necessary to their development, and within which they pass through the embryo state. The mass of ova, or “spat” as it is familiarly called, undergoes various changes in its colour, meanwhile losing its fluidity. This state indicates the near termination of the development and the sending forth of the embryo to an independent existence, for by this time the young oysters can live without the protection of the maternal organs. An eminent French pisciculturist says that the animated matter escaping from the adults on breeding-banks is like a thick mist being dispersed by the winds—the spat is so scattered by the waves that only an imperceptible portion remains near the parent stock. All the rest is dissipated over the sea space; and if these myriads of animalculæ, tossed by the waves, do not meet with solid bodies to which they can attach themselves, their destruction is certain, for if they do not fall victims to the larger animals which prey upon them, they are unfortunate in not fixing upon the proper place for their thorough development.
Thus we see that the spawn of the oyster is well matured before it leaves the protection of the parental shell; and by the aid of the microscope the young animal can be seen with its shell perfect and its holding-on apparatus, which is also a kind of swimming-pad, ready to clutch the first “coigne of vantage” that the current may carry it against. My theory is, that the parent oyster goes on brewing its spawn for some time—I have seen it oozing from the same animal for some days—and it is supposed that the spawn swims about with the current for a short period before it falls, being in the meantime devoured by countless sea animals of all kinds. The operation of nursing, brewing, and exuding the spat from the parental shell will occupy a considerable period—say from two to four weeks. It is quite certain that the close-time for oysters is necessary and advantageous, for we seldom find this mollusc, as we do the herring and other fish, full of eggs, so that most of the operations connected with its reproduction go on in the months during which there is no dredging. As I have indicated, immense quantities of the spawn of oysters are annually devoured by other molluscs, and by fish and crustaceans of various sizes; it is well, therefore, that it is so bountifully supplied. On occasions of visiting the beds I have seen the dredge covered with this spawn; and no pen could number the thousands of millions of oysters thus prevented from ripening into life. Economists ought to note this fact with respect to fish generally, for the enormous destruction of spawn of all kinds must exercise a very serious influence on our fish supplies. I may also note that the state of the weather has a serious influence on the spawn and on the adult oyster-power of spawning. A cold season is very unfavourable, and a decidedly cold day will kill the spat.
Some people have asserted that the oyster can reproduce its kind in twenty weeks, and that in ten months it is full-grown. Both of these assertions are pure nonsense. At the age of three months an oyster is not much bigger than a pea; and the age at which reproduction begins has never been accurately ascertained, but it is thought to be three years. I give here one or two illustrations of oyster-growth in order to show the ratio of increase. The smallest, about the dimensions of a pin’s head, may be called a fortnight old. The next size represents the oyster as it appears when three months old. The other sizes are drawn at the ages of five, eight, and twelve months respectively. Oysters are usually four years old before they are sent to the London market. At the age of five years the oyster is, I think, in its prime; and some of our most intelligent fishermen think its average duration of life to be ten years.
In these days of oyster-farming the time at which the oyster becomes reproductive may be easily fixed, and it will no doubt be found to vary in different localities. At some places it becomes saleable—chiefly, however, for fattening—in the course of two years; at other places it is three or four years before it becomes a saleable commodity; but on the average it will be quite safe to assume that at four years the oyster is both ripe for sale and able for the reproduction of its kind. Let us hope that the breeders will take care to have at least one brood from each batch before they offer any for sale. Oyster-farmers should keep before them the folly of the salmon-fishers, who kill their grilse—i.e. the virgin fish—before they have an opportunity of perpetuating their race.