PLATE XXXI
THE COMMON EDIBLE CRAB, Cancer pagurus. BRITISH. (MUCH REDUCED)
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Among British Crustacea, the next in importance to the Lobster as an article of food is the Edible Crab, Cancer pagurus ([Plate XXXI].), known in Scotland as the "Partan." Like the Lobster, it is found on rocky coasts in shallow water, and young specimens are often taken between tide-marks. It grows to a size of more than 10 inches across the shell, and may reach a weight of 12 pounds. The means used for its capture are the same as in the case of the Lobster, and the fishery is of considerable importance on many parts of the British coasts. On the other hand, a Connemara fisherman, who was using these Crabs for bait, received with incredulity the statement that they were good for human food!
The Shore Crab, Carcinus mænas ([Plate IX].), is not of much importance as food in this country, although it is recorded that fifty years ago great numbers were brought to the London market. On the shores of the Mediterranean and Adriatic, however, and especially in Venice, this species is regarded as a delicacy, particularly in the soft-shelled state after moulting.
On the Atlantic coast of North America, the most important edible Crustacean after the Lobster is the "Blue Crab" (Callinectes sapidus), one of the Swimming Crabs (Portunidæ). This is consumed in large quantities, especially in the soft-shelled state. Several other species of Crabs are eaten in America, including the little "Oyster Crab," a species of Pinnotheres living in the American Oyster. From its small size, and the difficulty of obtaining it in numbers, it is a very costly delicacy.
In the East Indies the most important edible Crabs are various species of Portunidæ, especially the large Scylla serrata and Neptunus pelagicus.
Except as food, the Crustacea are of very little direct use to man. Almost the only instance in which they are otherwise utilized is in the case of a species of sessile Barnacle (Balanus) which is cultivated in Japan for use as manure. The method of culture has been described by Professor Mitsukuri. Bunches of bamboo "collectors," like those used for the collection of oyster-spat, are fixed into the ground on tidal flats. After two or three months they are taken up, and the Barnacles with which they have become covered are beaten off and sold for use as manure.
Apart from their direct utility, however, the Crustacea are indirectly of great importance as providing a large part of the food-supply of marketable fishes. From this point of view, a study of the habits and distribution of the commoner species may be of practical value in throwing light on the migrations and other obscure points in the life-history of the fishes that prey upon them. As an example of this, we may refer to some investigations on the Mackerel fishery recently carried out by the naturalists of the Marine Biological Association at Plymouth. In the spring and early summer months the Mackerel migrate into inshore waters for the purpose of spawning. During this period the fish congregate in shoals at the surface of the sea, and are captured in drift-nets. The extent of this "shoaling" varies greatly from year to year, and determines whether the season shall be a profitable one for the fishermen or not. When shoaling, the fish feed exclusively on plankton, consisting largely of Copepoda, and it has been shown by Mr. G. E. Bullen that the fluctuations in the yield of the Mackerel fishery from year to year follow very closely the fluctuations in the abundance of the Copepod plankton on the fishing-grounds. The investigation has been carried a step farther by Dr. E. J. Allen, who points out that the abundance of Copepods is determined by the abundance of the Diatoms and other minute vegetable organisms of the plankton. These organisms are very largely influenced by the amount of sunshine during the period of their development in the earlier months of the year. Dr. Allen gives a diagram showing for each of seven years (1902-1908) the average number of hours of bright sunshine during the months of February and March in the South-West of England. With this he compares the number of fish caught in the month of May in each of these years by certain vessels engaged in the western Mackerel fishery. The correspondence between the two is very striking indeed, and justifies his conclusion that the amount of sunshine in the early months of the year determines the abundance of the vegetable life of the plankton, and through it of the Copepods and other animals which form the bulk of the plankton a little later in the year; and although there are doubtless other influences at work determining the success or failure of the fishery, it is largely a matter of the richness or poverty of the plankton harvest.