Oyster

OYSTER. Many marine animals are popularly called oysters, but most of these are only distantly related to the genus of true oysters, Ostrea. The different kinds of true oysters differ among themselves in shape, size and ornamentation of the shell and in their biological characters. Their nearest allies are the fan-shells (Pinna) and Lima. Window-pane oysters, pearl-oysters (Mar garitifera) and crow-oysters are distant allies. All belong to the class Lamellibranchia (q.v.) of the Mollusca (q.v.).

At the present time oysters occur mainly between tidemarks or in shallow water, especially in estuaries, off the coasts of all continents except in the polar seas. A few species (including Ostrea cochlear) have been dredged from deep water down to 4,300 feet. As fossils, oysters occur from the Triassic onwards; the type form begins in the Jurassic. More than 500 supposed species of fossil oysters have been described but probably many of them are mere varieties.

Kinds.—Although more than Doc) species have been described on shell-characters, it is probable that the total will diminish as more is discovered about the different forms. The most im portant species of economic value are given in the following table.

So far as is known, all oysters reproduce by one of the two following methods.

Method I.

This may be illustrated by the European oyster, 0. edulis, other oysters which exhibit similar or the same sex forms being the Australian mud oyster, the British Columbian oyster and the Japanese shallow-water oyster, 0. densilamellosa.

Sex and Sex-change.—Individuals are normally at one instant either male or female, but they change from male to female, and back again many times during life. Hermaphrodites, however, do occur in varying proportions, but usually function as females.

Spawning.

The Sperms have a "head" about in. or 2 ,u, and a tail 20 to 30 /I, and are shed directly into the sea; females collect the sperms from the sea-water and store them. The female functioning individual spawns its eggs from the body, but retains them inside the shell where they develop into tiny motile bivalve larvae before they leave the parent. These larvae are frequently

incorrectly called "spat" (see below). An individual three to four years old spawns about soo,000 eggs, while one seven to eight years old may spawn about I millions or more in a season.

Development.

As the eggs, which are about 15oµ in diameter, are extruded they are fertilized, and begin to develop at once into embryos. The embryos begin swimming slightly when three to four days old. At the age of four to five days the shell begins to form and the embryo by a stronger development of its swim ming organs passes into the larval stage.

Up to this time the embryos have remained white in colour, but as the shell grows larger the larva becomes grey and then passes through various shades of blue-grey and slate and, finally, becomes purplish-black when the shell is fully developed at a size of i8o to 210/2. As the shell develops the cilia grow stronger. At this stage (at an age of about ten days) the larva leaves the parent to swim freely in the sea, although largely at the mercy of winds and tides. It swims about for a week to ten days or more, feeding and growing to a size of about 270µ before settling down to a permanent sedentary life.

The larva may "settle" on a great variety of objects at any level in the water. It attaches itself by cementing the left valve of the shell to the object, loses its swimming organs, and quickly develops rudimentary adult organs, being then known as a spat. New shell material is deposited at the edge of the larval shell in a few hours and the shell thereby also soon assumes the adult form. The settling of larvae on objects in the sea is called a spatfall. Studies of spatfalls there is a tendency of the larvae to settle in situations where the light is subdued. It is probable that the larvae of tropical species settle more easily than those of temperate species.