The Fishing Industry

by William E. Gibbs

All molluscs are attached tightly to the shell at one or two points, and cannot be removed from the shell alive. In the case of the bi-valves the animal is attached to the two shells by a muscle which draws the two valves of the bi-valve together. When this muscle is relaxed, for example in normal circumstances, when feeding at the bottom of the sea—the shell remains open. Some shellfish—notably the scallop—actually swim by opening and shutting the two valves of their shell.

The most important uni-valves are the periwinkle, the limpet and the whelk. Uni-valves possess a well-marked head and neck, a pair of eyes and a mouth. They are remarkable for the possession of a tongue, formed like a ribbon rasp, furnished on its upper surface with a large number of small teeth. The number and arrangement of these teeth differ in different species. With this ribbon rasp the uni-valve, for example a dog-whelk, can rasp a hole through the shell of an oyster and feed upon the contents.

Bi-valves do not possess a ribbon rasp, neither have they a projecting head, nor in most cases any eye. They possess a mouth, furnished with four flapper-like lips or gill plates. They feed on microscopic, floating plants that are drawn within their mouth by currents set up in the water by the rhythmic vibrations—from three to four hundred strokes per minute—of millions of hairs that hang down from soft plates supported under the protecting arch of the shell and called the “beard.” These currents of water not only bring food to the mouth of the bi-valve, but also irrigate the gill plates and so enable the animal to breathe. The oyster lies on the sea bottom with its muscle relaxed and its shell gaping.

A North European oyster acts alternately as female and male. It produces eggs—as many as a million in a season—and a fortnight after the eggs have been shed, the same oyster produces millions of spermatazoa, which form a cloud of fine dust in the water. These spermatazoa rapidly scatter in all directions, and, entering the tubular reproductive sacs of oysters that are producing eggs, fertilize them.

American and Portuguese oysters are definitely male and female, the eggs being discharged by the female and fertilized subsequently in the sea by the male.

The eggs remain attached to the parent’s gill plates, and in a day or so develop into minute, shell-less oysters. The parent oyster is then said to be “white-sick.” About two days later the young oysters have become dark-coloured and are found to have formed minute convex shells, rather like those of a cockle. The parent is then “black-sick.” A week later the young oysters escape and rise in thousands to the surface water, swimming by means of fine hairs or cilia that are attached to the upper edge of the shells. They are carried far and wide by tides and surface currents. Many are eaten by young fish and shrimps. As they grow the shells become heavier, and after a time they sink to the sea bottom. This is known as the “fall of spat.” If they fall on stony ground, where they will be well irrigated and nourished through the movement of the water, they will thrive. Many, however, fall on soft, unsuitable ground and perish.

The European oysters spawn in the summer (from May to September). They become mature in three years, are at their prime in from five to seven years, and rarely live longer than ten years.

Oysters are gathered from natural beds or from artificial grounds. The oyster breeders place movable tiles or frames for the spat to fall on. When the young have become affixed to these “stools” they are frequently carried away to develop in a different locality. The oysters are finally fattened in sea ponds or inlets that contain a large diatom population. At Marennes, on the west coast of France, the water in which the oysters are grown contains a particular blue diatom. After feeding upon these diatoms, the beard of the oyster becomes stained a bluish-green colour—the well-known “Marennes vertes” oysters.

A natural oyster bed is formed on stony ground free from mud and sand, so that the oyster, after becoming attached to a stone, is completely surrounded by clear sea-water. Oysters do not flourish in water containing less salt than ordinary sea-water. Thus, there are no oysters in the Baltic Sea.