Oyster

When an oyster is carrying white embryos or larvae it is said to be whitesick; when carrying grey larvae, greysick, and when bluish-grey to black, blacksick.

Method II.

This can be illustrated by the Canadian and American oyster (more fully described by Stafford, in the ref erence given below) but with little alteration this may be applied to the rock oyster, 0. cucullata, the Portuguese oyster and the gigantic Japanese oyster.

Each individual is normally either male or female. So far as is known there is no change of sex. Both sexes shed their sexual products directly into the sea, where haphazard fertilization oc curs. A good sized individual may shed at one spawning upwards of so to 6o million eggs. The egg, which is small (about 5o 11), develops into an embryo and larva in 8 to 12 hours. Within about two days the larva grows a complete bivalve shell, which in a period of two weeks or more reaches a size of about 38oµ before cementing itself to some object in the sea. After settling, the larva rapidly grows shell and develops adult organs to become a spat in the same way as the European oyster larva.

Artificial fertilization can easily be performed in this type of oyster by mixing ripe eggs and sperm in sea-water, provided clean conditions are ensured and excess of sperm avoided. In the 0. edulis type artificial fertilization is more difficult.

Food and Mode of Feeding.

The main food of the oyster consists of microscopic plants, diatoms and peridinians, although microscopic animals probably always contribute an important portion. Decaying sea-weeds with their dependent bacteria have also a definite but unknown food-value. Oysters do not, however, feed all day long, nor at all times of the year. The American oyster does not feed late at night and in early morning, and relatively little on the ebb tide, little or no feeding occurs in late autumn and winter; below a temperature of about 4° C. (39°.2 F) this oyster practically ceases to feed. Other oysters probably behave similarly but the temperature minimum varies for each species.

Oysters obtain their food by means of the gills, which are used at one and the same time as a water pump, food-collector and food-transporter. This living machine draws water into the

shell into the inhalant chamber, and expels it from the exhalant chamber. The gills constitute the sole partition between these two chambers, and sieve off all the particles in the water drawn through the shell-space. The food particles arrested on the sur face of the gill are collected into minor channels situated on the radial markings and from these are transported mainly to a large channel on the free edge of each gill leaflet. As the food-particles pass into these larger channels they are rolled into a sausage shaped mass by mixing with a sticky fluid, mucus, and then passed to the lips or palps, which may either direct it into the mouth to be eaten, or pass it on to the mantle whence it is eventually rejected from the shell. Food-masses containing much heavy material, such as sand or mud, are rejected, others eaten. All these manifold functions are performed by rows of microscopic whip-like organs (cilia) beating in sets rhythmically and in co ordination. Throughout these operations mucus is manufactured and thrown out to entrap and retain the captured particles, and to permit of their easy transport in bulk. Unwanted material in the inhalant chamber is carefully collected together and literally blown out of the shell by sudden closing of the valves.

Breeding.—It has been seen that breeding comprises the two distinct operations of spawning and spatting. In the European and the American oysters, it is found that the beginning of breed ing varies according to the warmth of the season and of the locality (see Table II). Probably other species behave similarly. The European species spawns little when the sea-temperature falls below about F., and the American, Canadian and Portuguese forms appear to require a temperature round about 68° F. for successful spawning, the European oyster tends in estuaries to spawn on the spring tides, and in 1925 spawned especially on the full-moon tides in the Fal estuary.