Shellfish are divided into two classes: Crustacea, including the lobster, crab, shrimp, prawn, and mollusca, including the oyster, mussel, cockle and periwinkle. Shellfish generally abound in comparatively shallow water near the shore.

Perhaps the most important members of the crustacea are the various minute, pelagic copepoda, of which incalculable myriads form an important constituent of the plankton in all seas. These copepoda live upon the diatoms and other microscopic, marine vegetable life floating at the surface of the sea. The most important edible members of the crustacea are the lobster and the shrimp.

The lobster is found along the coasts of the North Atlantic and Mediterranean, particularly along the European coasts from Norway to the Mediterranean, and off North America from Labrador to Cape Hatteras, The lobster lives in shallow water at about 12 fathoms depth, and frequents a rocky bottom. The lobster’s eggs remain attached to the female until the larvae hatch out. From 10,000 to 12,000 eggs are carried in this way by a female lobster. She protects them from the ravages of fish that will otherwise consume them as food, and by keeping them constantly irrigated with fresh sea-water she promotes their healthy life and development. The eggs may take as long as twelve months to hatch, and although “berried” lobsters are seen in greatest numbers in the spring they are also captured at all seasons of the year.

When hatched the young lobster larvae leave their mother and float up to the surface water, where they develop for a time among the plankton. During the larval period the lobster is a free and active swimmer.

The young larvae are consumed in large quantities by fish such as herring, mackerel and sprat, especially during the summer months when they are most abundant. While developing into a complete lobster it passes through at least three distinct changes of form. When the larva has attained the length of about 3/5 in. it already possesses many of the characteristic features of the adult. Soon afterwards, it sinks to the sea bottom and gradually grows into a complete adult. During the growth of the lobster it frequently casts its shell and grows a new one. Growth only takes place when the shell is cast and while the new shell is hardening. During the first few weeks of its life the lobster casts its shell about once a week, but this casting happens less and less frequently as the lobster grows older. The new shell is formed beneath the old one, and although at first quite soft rapidly hardens when the old one has been cast off. Most adult lobsters cast their shells in July, August and September.

A lobster grows slowly, and when from 9 to 10 ins. long is probably from four to five years old. It becomes mature when about 6 ins. long—that is when about three years old.

The lobster is usually caught in creels or “pots” baited with portions of stale fish—generally flounder, skate, eels, etc. Lobster fisheries tend to deteriorate in value very rapidly. Owing to the lobsters’ keen sense of smell, the method of capture by means of creels or pots is very efficient, so that the lobsters are caught in great numbers, with the result that the fishery soon shows signs of exhaustion, the average size of the lobster caught becoming smaller. The lobster fishery is entirely confined to the shallow water near the shore, and can only be replenished and maintained by the young lobsters that hatch out in that neighbourhood. Large quantities of lobster spawn are destroyed every year when berried lobsters are caught. It is estimated that, on an average, 30 per cent of the lobsters caught are berried females. The fishermen either remove the spawn and throw it back into the sea—where, of course, it almost certainly becomes fish food—or sell it to be used in making certain special sauces.

Various attempts have been made by legislation in different countries to prevent the capture of berried females, and so protect the lobster spawn, but, since berried females are found all the year round and comprise about 30 per cent of all the lobsters captured, it is practically impossible to prohibit the capture of berried lobsters without seriously penalizing the fishermen.

A better policy would be to hatch lobster eggs in large numbers artificially, and when the young lobsters are well established add them to the natural stock. This is actually done on a large scale and with excellent results in America and Norway.

In Europe lobsters are generally sent to market in a fresh state, but in America they form the basis of an extensive canning industry. In 1913 over 2,500,000 lobsters were captured round the coasts of Great Britain and Ireland, the total value of the fish being more than £110,000.

Shrimping is one of the most important methods of inshore fishing, and gives employment to a large number of fishermen round our coasts. The shrimp is found on sandy or muddy ground in shallow water near the coast. A female shrimp, like the lobster and the crab, carries its eggs under its tail.

Shrimps are caught with a fine-meshed trawl net, drawn by a boat or by horse and cart, or with push nets or hose nets. One great objection to shrimping is that the shallow, sandy areas on which it takes place are much frequented by young fish—particularly dabs, plaice, soles, whiting and codling. Owing to the small mesh of the shrimp trawl, these small fish are captured in large numbers and are generally dead or dying when discharged from the net. Generally, the shrimps are separated from the small fish by riddling, and the smaller shrimps are then separated from the larger ones by a second riddling process, and are returned to the sea. The shrimps are thrown into boiling salt water, rapidly stirred for a few seconds, and spread out on the deck to cool. From three to four hauls are made per day, a good day’s fishing consisting of from 30 to 40 quarts of shrimps. Large numbers of shrimps are potted.

The other important group of shellfish is the mollusca. Molluscs, i.e. “soft creatures,” are essentially soft, mobile animals, protected by shells. They are classed as bi-valves, for example oyster and mussel, and uni-valves, for example limpet and whelk. There is no real difference between a bi-valve and a uni-valve, for what appear to be the two shells of the bi-valve are really one shell divided into two parts by a line of soft, uncalcified material which forms a hinge between the two halves of the shell; this hinge tends to keep the shell open, but the muscular action of the living animal inside keeps it closed when required.

With the exception of the mussel, very few shellfish actually live on the shore between the tide marks. Most of the seashore shells are brought by the sea from animals that lived in from 10 to 20 fathoms of water. The cockle lives buried in the sand, about an inch below the surface. The oyster lives on stones and shells below low-water mark.

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.

The chief enemies of the oyster are the dog-whelk that bores through the shell, and the starfish that pulls the valves apart and attacks the oyster inside.

The oyster is widely distributed in tropical and temperate seas all over the world. The approximate value of the annual oyster crop of the world is £4,000,000, representing a crop of 10 billion oysters.

In Europe up to 75 per cent of the oysters are reared from spat in artificial beds—not more than 7 per cent being “native.” In the United States, however, over 40 per cent are still obtained from natural beds.

The simplest form of oyster culture is the preservation of the natural bed. These beds are easily destroyed or made unproductive by over-dredging. Colonies are broken up. Other animals are admitted. Breeding oysters are covered up by stones and shells, and suffocated. Ridges suitable for the development of the spat are broken down.

After the beds have been properly protected and preserved the next stage is to extend the area of the natural beds. This involves a knowledge of the conditions of depth, temperature, salinity and character of bottom that are necessary to the successful growth of the oyster. Finally the productivity of an oyster “park” and the quality of its produce can be greatly improved by providing artificial “stools” for the reception and development of the spat. Many substances can be used for this purpose. The Romans used earthenware tiles, and similar tiles are used to this day in France. Brushwood, trees, stones and stakes, and old oyster shells (cultch) are also used.

The earthenware tiles used in France are hollowed on one side to receive the spat, and are coated with lime to facilitate the removal of the oysters when they are a year old. They are then from ¹/₂ to 1 inch in diameter, and are picked off the stools and placed on stands where they are thinned out from time to time as they grow.

The chief oyster fisheries in Britain are at Whitstable, Colchester and Brightlingsea. Nearly 40,000,000 oysters were gathered on the coasts of England and Wales in 1920, and were sold for about £250,000.

Perhaps the next most important edible bi-valve is the mussel. Frequently, mussel beds are situated near the mouth of rivers, and consequently tend to be contaminated by sewage. It has been established by various investigators—notably Dr. Klein and Professor James Johnstone—that mussels are able to cleanse themselves of sewage pollution in a comparatively short time if they are re-laid in sterilized water. Experiments on a large scale have been carried out with the mussel beds at the mouth of the Conway river since September, 1916. The mussels are gathered from the beds and placed about two deep on wooden grids in a large concrete cleansing tank of 40,000 gallons capacity. The mussels are first thoroughly hosed with water at high pressure to remove all adherent mud, etc. The tank is then filled with sterilized sea-water and the mussels are allowed to remain in it for 24 hours. During this period the mussels effectually free themselves from bacteria. The tank is then emptied, the mussels are hosed again, the tank is again filled with sterile water and after a further 24 hours is emptied. The mussels are once more flushed with the hose. After this treatment the mussels reach a high standard of purity. The scheme has proved to be a complete success, not only from a scientific point of view, but also as a commercial proposition. The sum of 1s. per bag of mussels (140 lbs.) is charged to the fishermen for this treatment.