According to Milne-Edwards,[222] in the great oyster parks on the coasts of Calvados, the oysters are educated to keep their shells closed when out of water, and so retain water enough inside to keep their gills moist, and arrive at their destination in good condition. As soon as an oyster is taken out of the sea, it closes its shells, and keeps them closed until the shock of removal has passed away, or perhaps until the desirability of a fresh supply of water suggests itself. The men take advantage of this to exercise the oysters, removing them from the sea for longer and longer periods. In time this has the desired effect; the well-educated mollusc learns that it is hopeless to ‘open’ when out of the water, and so keeps his shell closed and his gills moist, and his general economy in good condition.
Oysters have been known to live entirely out of water for a considerable time. Prof. Verrill once noticed[223] a large cluster of oysters attached to an old boot, hanging outside a fish-shop in Washington. They had been taken out of the water on about 10th December, and on 25th February following some of the largest were still alive. It was noticed that all those which survived had the hinge upward and the ventral edge downward, this being the most favourable position possible for the retention of water within the gill-cavity, since the edge of the mantle would pack against the margins of the shell, and prevent the water from leaking away.
Such a succulent creature as the oyster has naturally many enemies. One of the worst of these is the ravenous Starfish, or Five-finger. His omnivorous capacities are well described by a clever writer and shrewd observer of nature: “Here is one doubled up like a sea-urchin, brilliant of hue, and when spread out quite 16 inches in diameter; where, and oh where, can you obtain a prey? The hoe we carry is thrust out and the mass dragged shorewards, when the rascal disgorges two large dog-whelks he has been in the process of devouring. We feel a comfortable glow of satisfaction to think that this enemy of our oyster-beds is also the enemy of our other enemy, this carnivorous borer. Here, quite close alongside, is another, only inferior in size, and we drag him ashore likewise, to find that the fellow has actually had the courage and audacity to suck the contents out of a large horse-mussel (Modiola), the strong muscle alone remaining undevoured. We proceed along but a short way when we meet with still another in the curled-up condition in which they gorge themselves, and as we drag it shorewards the shell of a Tapes pullastra drops from the relaxing grasp of the ogre. Slowly the extended stomach returns to its place, and the monster settles back to an uncomfortable after-dinner siesta on an exposed boulder; for the starfish wraps its turned-out stomach around the prey it has secured, in place of attempting to devour the limey covering in which most of its game is protected. Once the mouth of the shell is enclosed in the stomach of the starfish, the creature soon sickens, the hinge-spring relaxes its hold, and the shell opening permits the starfish to suck out the gelatinous contents, and cast free the calcareous skeleton.”[224]
According to other observers the starfish seizes the oyster with two of his fingers, while with the other three he files away the edge of the flat or upper valve until the points of contact with the round valve are reduced almost to nothing; then he can introduce an arm, and the rest is easy work. Others suggest that the starfish suffocates the oyster by applying two of its fingers so closely to the edge of the valves that the oyster is unable to open them; after a while the vital powers relax and the shell gapes. The Rev. J. G. Wood holds[225] that the starfish pours a secretion from its mouth which “paralyses the hinge muscle and causes the shell to open.” Sometimes in a single night a whole bed of oysters will be totally destroyed by an invasion of starfish. Another dreaded enemy is the ‘whelk,’ a term which includes Purpura lapillus, Murex erinaceus, Buccinum undatum, and probably also Nassa reticulata. All these species perforate the shell with the end of their radula, and then suck out the contents through the neatly drilled hole. Skate fish are the cause of terrible destruction in the open beds, and a scarcely less dangerous visitant is the octopus. Crabs crush the young shells with their claws, and are said to gather in bands and scratch sand or mud over the larger specimens, which makes them open their shells. Yet another, and perhaps unconscious, foe is found in the common mussel, which takes up room meant for the young oysters, grows over the larger individuals, and harbours all sorts of refuse between and under its closely packed ranks. Cliona, a parasitic sponge, bores in between the layers of the oyster’s shell, pitting them with tiny holes (corresponding to its oscula), and disturbing the inmate, who has constantly to construct new layers of shell from the inside. Weed, annelids, ‘blubber,’ shifting sand or mud, sewage or any poisoning of the water, are seriously harmful to the oyster’s best interests. A very severe winter is often the cause of wholesale destruction in the beds. According to the Daily News of 26th March 1891, the Whitstable oyster companies lost property to the value of £30,000 in the exceptionally cold winter of 1890–91, when, on the coast of Kent, the surface temperature of the sea sank below 32°, and the advancing tide pushed a small ice-floe before it. Two million oysters were laid down in one week of the following spring, to make up for the loss. During the severe winter of 1892–93 extraordinary efforts were made at Hayling I. to protect the oysters from the frost. Twenty million oysters were placed in ponds for the winter, and a steam-engine was for days employed to keep the ponds thawed and supplied with water, while large coal and coke fires were kept burning at the edge of the ponds.[226] On the other hand, the unusually warm and sunny summer of 1893 is said to have resulted in the finest fall of spat known in Whitstable for fifty years.[227]
The reproductive activity of the oyster is supposed to commence about the third year. Careful research has shown[228] that the sexes in the English oyster are not separate, but that each individual is male as well as female, producing spermatozoa as well as ova in the same gland. Here, however, two divergent views appear. Some authorities hold that the oyster does not fecundate its own eggs, but that this operation is performed by spermatozoa emitted by other specimens. It is believed that, in each individual, the spermatozoa arrive at maturity first, and that the ova are not produced until after the spermatozoa have been emitted; thus the oyster is first male and then female, morphologically hermaphrodite, but physiologically unisexual. Others are of opinion that the oyster does fecundate its own eggs, ova being first produced, and passed into the infrabranchial chamber—the ‘white-sick’ stage—and then, after an interval, spermatozoa being formed and fecundating these ova—the ‘black-sick’ stage. In this latter view the oyster is first female and then male, and is, both morphologically and physiologically, hermaphrodite. The old view, that ‘black-sick’ oysters are the male, and ‘white-sick’ the female, is therefore quite incorrect.
The ova, in their earliest stage, consist of minute oval clusters of globules floating in a transparent mucus. They pass from the ovary into the gills and folds of the mantle, and are probably fecundated within the excretory ducts of the ovary, before arriving in the mantle chamber. In this stage the oyster is termed ‘white-sick.’ In about a fortnight, as the course of development proceeds, the fertilised ova become ciliated at one end (the so-called veliger stage, p. 131), and soon pigment appears in various parts of the embryos, giving them a darker colour, which varies from grayish to blue, and thus the white-sick oyster becomes ‘black-sick.’ When the black spat emerge, they are still furnished with cilia for their free-swimming life. This is of very short duration, for unless the embryo finds some suitable ground on which to affix itself within forty-eight hours, it perishes. As the spat escapes from the parent oyster, which slightly opens its valves and blows the spat out in jets, it resembles a thick cloud in the water, and is carried about at the mercy of wind and tide. April to August are the usual spawning months, warm weather being apparently an absolute necessity to secure the adhering of the spat. A temperature of 65° to 72° F. seems requisite for their proper deposit. Thus on a fine, warm day, with little wind or tide running, the spat will fall near the parents and be safely secured, while in cold, blustering weather it will certainly be carried off to a distance, and probably be altogether lost. The number of young produced by each individual has been variously estimated at from 300,000 to 60,000,000. Either extreme seems enormous, but it must be remembered that besides climatal dangers, hosts of enemies—other Mollusca, fish, and Crustacea—beset the opening career of the young oyster.
As soon as the spat has safely ‘fallen,’ it adheres to some solid object, and loses the cilia which were necessary for its swimming life. It begins to grow rapidly, increasing from about 1/20 inch in diameter to about the size of a threepenny piece in five or six months, and in a year to one inch in diameter. Roughly speaking, the best guide to an oyster’s age is its size; it is as many years old as it measures inches across.
The oyster is at its prime at the age of five; its natural life is supposed to be about ten years. The rings, or ‘shoots’ on a shell are not—as is frequently supposed—marks of annual growth; cases have been noticed where as many as three ‘shoots’ were made during the year.
An oyster is furnished, on the protruding edges of the mantle, with pigmented spots which may be termed ‘visual organs,’ though they hardly rise to the capacities and organisation of real ‘eyes.’ But there is no doubt that they are sufficiently sensitive to the action of light to enable the oyster to apprehend the approach of danger, and close his doors accordingly. ‘How sensitive,’ notes Mr. W. Anderson Smith,[229] ‘the creatures are to the light above them; the shadow of the iron as it passes overhead is instantaneously noted, and snap! the lips are firmly closed.’
The geographical distribution of Ostrea edulis extends from Tränen, in Norway, close to the Arctic circle, to Gibraltar and certain parts of the Mediterranean, Holland, and N. Germany to Heligoland, and the western shores of Sleswick and Jutland. It occurs in Iceland, but does not enter the Baltic, where attempts to colonise it have always failed. Some authorities regard the Mediterranean form as a distinct species.