CHAPTER XI

CRUSTACEA IN RELATION TO MAN

The Crustacea come into relation with human life in the most obvious and direct way in the case of those species that are used for food. The number of species so used in various parts of the world is very large, almost the only necessary condition being that the species shall be sufficiently large and abundant to make it worth while to fish for it.

As most of the larger Crustacea belong to the Decapoda, it is this order that supplies practically all the edible species, almost the only exceptions being a few Barnacles which are eaten in various parts of the world. Thus the sessile Barnacle _Balanus psittacus_, found on the coasts of Chili, and growing to a length of 9 inches by 2 or 3 inches diameter, is, according to statements quoted by Darwin, "universally esteemed as a delicious article of food," and the pedunculate _Pollicipes cornucopia_ is used for food on the coasts of Brittany and Spain.

By far the most valuable of all the edible Crustacea are the European and American Lobsters (_Homarus gammarus_ and _H. americanus_). The former is found on the coasts of Europe from Norway to the Mediterranean, living mostly a short distance below low-water mark wherever the bottom is rocky. At some places, as for instance at Worthing, Lobsters are common on a sandy bottom, but as a rule they seem to prefer localities where the crevices of a rough hard bottom afford abundance of shelter. They are usually caught in traps known as "Lobster pots" or "creels," which vary in construction in different localities. In some cases they are made of wicker-work, hemispherical in shape, with a funnel-shaped opening on top, so devised as to permit the Lobsters to enter easily, while preventing their escape. Another form is semi-cylindrical, with a framework of wood covered with netting or with wooden spars, and having two funnel-shaped entrances at the sides. These traps are baited with pieces of fish, preferably stale, and are sunk in suitable places, each attached by a line to a buoy or float.

Important Lobster fisheries are carried on in Norway, Scotland, England, Ireland, Heligoland, and other parts of the coasts of Northern Europe. In the South the Lobster fishery is of less importance, other large Crustacea, especially the Spiny Lobster, being more abundant and more highly esteemed.

The American Lobster, as already mentioned, closely resembles the European species, the chief difference being in the form of the rostrum (see Fig. 9, p. 32). It is found on the Atlantic coast from Labrador to Cape Hatteras, but it is not abundant south of New Jersey. The canning of Lobsters is a very important industry in Newfoundland, the Maritime Provinces of Canada, and the Northern New England States.

The only other species of the genus _Homarus_ (_H. capensis_) is found at the Cape of Good Hope, but it is of small size and is of no economic importance.

The European Lobster rarely reaches a weight of 10 pounds, although individuals of 14 pounds weight have been caught. In America, there are authentic records of Lobsters weighing 20 and even 23 pounds.

The bad effects of over-fishing have become apparent of late years, especially on the American coast, in the reduced average size of the Lobsters caught rather than in a diminution of the total yield of the fishery. Numerous experiments in legislation have been made with a view to checking the depletion of the fishing-grounds, but in no case with conspicuous success. A "close time" for the spawning Lobsters has often been tried, but the fact that the female carries the eggs attached to her body for nearly a year after spawning makes it quite impossible to give effective protection by this means. In most Lobster-fishing districts a minimum size is fixed by law, below which it is illegal to take or sell Lobsters, and in many cases also the capture of females carrying spawn, or, as it is termed, "in berry," is prohibited.

The so-called "Norway Lobster" (_Nephrops norvegicus_--Plate XXX.), the "Dublin Prawn" of the London fishmongers, is a smaller and much less valuable species than the common Lobster. It may be recognized at once by its long and slender claws, furnished with rows of tubercles or blunt spines, and by the sculptured markings on the somites of the abdomen. When alive it is of an orange colour, beautifully marked with red and white. It differs considerably in its habits from the common Lobster, living at a considerably greater depth (30 to 60 fathoms in Norway), and on a muddy bottom. It is generally taken by trawling, and is captured in large quantities by trawlers fishing in various parts of the North Sea. Since it must be cooked soon after it is caught, and cannot easily be brought to market alive like the common Lobster, only a small number of those actually caught are made use of. Formerly most of those sold in London were caught in the Irish Sea (whence the name of "Dublin Prawn"), but the North Sea is now the chief source of supply. The species is found in suitable localities from Norway to the Mediterranean, and is especially abundant in the Adriatic, where it is caught and sold in Venice and elsewhere under the name of "Scampo."

The Spiny Lobster, Rock Lobster, or Sea-crawfish (_Palinurus vulgaris_--Plate V.), is common on the south and south-west coasts of the British Islands, becoming rare in the north, although specimens have been found as far north as Orkney, and there is a single record of the species from the West of Norway. It is far less commonly used for the table in this country than in France, where it is known as "Langouste" and is very highly esteemed.

Various species of Spiny Lobsters belonging to the same family (Palinuridæ) as the European species are found in different parts of the world. In tropical countries the species of _Panulirus_ are commonly used for food (for example, _P. interruptus_ in California and _P. fasciatus_ in India), as are species of _Jasus_ in South Africa, Australia, and New Zealand. Recently a consignment of Spiny Lobsters (_Jasus lalandii_) was sent to the London market from the Cape, but it appears that the experiment was not altogether successful.

Belonging to the same tribe (Nephropsidea) as the Lobsters are the fresh-water Crayfishes. The English Crayfish (_Astacus pallipes_) is common in many rivers as far north as Lancashire, and in some parts of Ireland, but is not found in Scotland. It is not much esteemed for the table, and although small numbers are sent to Billingsgate, chiefly from Leicestershire, they are said to be used only for garnishing dishes. The same species occurs on the Continent of Europe, chiefly in the west and south (France, Germany, Switzerland, Spain, Italy, and the Balkan Peninsula). It is known in France as "Écrevisse à pattes blanches" (from the whitish colour of the under-side of the large claws), and in Germany as "Steinkrebs," and is distinguished, among other characters, by the shape of the rostrum (Fig. 77, B), which has a tooth on each side close to the point. Far more important as an article of food is the larger _Astacus fluviatilis_, the "Écrevisse à pattes rouges" or "Edelkrebs," which is found in France, Germany, Austria, Southern Sweden, Russia, etc. In this species the under-side of the large claws is generally of a fine red colour, and the rostrum (Fig. 77, A) has a pair of side-teeth about the middle of its length, and a long slender point. The red-clawed Crayfish is an important article of commerce on the Continent, and is sent to the London market in considerable numbers, chiefly from Germany and South-West Russia. In France it is cultivated for the market in "Crayfish farms" on a large scale.

A species of Crayfish (_A. leptodactylus_) occurring in the Lower Danube and in other rivers flowing into the Black Sea sometimes finds its way to the London market, although it is less valued than the red-clawed species. It is distinguished by its long and slender claws, by the spiny edges of the rostrum, and by other characters. A fourth species (_A. torrentium_), occurring chiefly in Central Europe, is very closely allied to _A. pallipes_, and, like it, is of little value for the table.

Within the last thirty years the Crayfish fisheries of Western Europe have suffered heavily from outbreaks of an epidemic disease which has all but exterminated these animals in certain districts. In this country it is said to be responsible for the almost complete disappearance of Crayfish from localities where they were formerly plentiful, as, for instance, in the neighbourhood of Oxford. The cause of the disease is believed to be a protozoan parasite belonging to the group Myxosporidia.

In other parts of the world it does not seem that the fresh-water Crayfishes are of much importance as an article of food. Some species of _Cambarus_ are so used to a limited extent in the United States, and the gigantic _Astacopsis serratus_ (Plate XX.) is known as the "Murray River Lobster" in the markets of Sydney and Melbourne.

The Decapods of the suborder Natantia comprise a large number of edible species, generally known as Shrimps and Prawns. The Common Shrimp, _Crangon vulgaris_ (Fig. 78), which is plentiful on the British coasts wherever the bottom is sandy, is about two or three inches long, and when alive is of a translucent greyish colour speckled with brown. It differs from most of the Natantia in having the body somewhat flattened from above downwards, and the rostrum very short. When boiled, it is of a reddish-brown colour, and from this it is sometimes known as the "Brown Shrimp." On many parts of the coast the Shrimp fishery is of considerable importance. Most often the Shrimps are caught by means of a large bag-net attached to a semicircular hoop with a long handle, and pushed over the surface of the sand by a fisherman wading in the water at ebb-tide.

A variety of species are sold in England under the name of Prawns. The largest of the native species, to which the name of Common Prawn is perhaps most properly restricted, is _Leander serratus_. It grows to a length of over 4 inches, and has a long serrated rostrum extending beyond the antennal scales and curving upwards at the point. The first and second pairs of legs end in small pincer-claws. When alive the animal is very transparent, and beautifully marked with bands of brown and red on the body and limbs. A smaller species of the same genus (_L. squilla_), distinguished by the much shorter and straighter rostrum, and another very similar species of which the proper name appears to be _L. adspersus_ (often known as _L. fabricii_), are said to be sold on some parts of the English coast as "Cup Shrimps."

Much commoner, at least in the London market, than the species of _Leander_ is _Pandalus montagui_, often sold under the general name of Prawn, but sometimes called the "Pink Shrimp." This resembles _Leander serratus_ in having a long, serrated, up-curved rostrum, but differs from it strikingly in the form of the anterior pairs of feet. The first pair appear to the naked eye to have no pincer-claws, but to end in a sharp point, resembling the third maxillipeds, which are just in front of them. As a matter of fact, they do have pincers, but so minute that they can only be detected by microscopic examination. The feet of the second pair are unequal in length on the two sides, that on the left side being the longer, and are very slender. They end in small pincers, and examination with a pocket-lens will show that the carpus, or "wrist," and the segment below it (merus) are broken up into a large number of short segments, so that the limb is extremely flexible. When alive, the animal is even more handsomely marked than the Common Prawn.

A large species of Prawn is now imported to this country in considerable quantities from Norway. This is _Pandalus borealis_ (Fig. 79), a species closely allied to the last-named, but differing in the longer and more slender rostrum and in many other characters, as well as in its larger size (specimens have been recorded of 6 inches in total length). It also differs in its habitat, for while _P. montagui_ lives in shallow water, or even between tide-marks, _P. borealis_ occurs at depths of 30 to 60 fathoms in the Norwegian fjords. The recent development of the fishery for _P. borealis_ in Norway is a striking example of the practical value of zoological research. Until 1898 the species was hardly known except to zoologists, although a small fishery was carried on in the Drammen Fjord, near Christiania. The investigations of the naturalists employed by the Norwegian Department of Fisheries showed that the species existed in vast numbers in the deeper water of many of the fjords, and that it could be captured in abundance by means of a suitably-devised trawl-net. As a result, a very profitable fishery was established, and the "deep-water Prawns" are now not only largely consumed in Norway, but are exported in increasing quantities to the English and other markets.

In the warmer seas the large Prawns of the genus _Penæus_ are of considerable importance. Thus, in the Mediterranean countries, _Penæus caramote_ (Plate IV.) is highly esteemed for food, and _P. setifer_ and _P. brasiliensis_ are largely consumed in the Southern United States. _P. monodon_ and other species are eaten in India. An attempt has been made to send a species of the same genus (apparently _P. indicus_) in a frozen state from Queensland to the London market.

Numerous other species of Natantia are used for food in various parts of the world, but the only ones that need be further mentioned here are the River Prawns of the genus _Palæmon_, which are abundant in the fresh waters of most tropical countries, and sometimes grow to a very large size. They are generally distinguished by the fact that the legs of the second pair are very long, forming powerful pincer-claws. In the West Indies and Central America, _P. jamaicensis_ (Plate XXI.), which reaches a length of 10 inches exclusive of the great claws, is sold in the markets, while in India and elsewhere in the East _P. carcinus_, which grows to an even greater size, and other smaller species, are used for food. The fresh-water Prawns of the family Atyidæ, on account of their small size, are not of much importance from this point of view, but Professor Hickson states that the little _Caridina nilotica_, a very widely-distributed species, is eaten in Celebes.

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.

None of the Crustacea can be regarded as directly harmful to man. They have not the power of inflicting envenomed wounds which renders some other Arthropods, such as Scorpions, some Spiders, Centipedes, and Insects, formidable in spite of their small size; and although blood-curdling tales of the ferocity of the Land Crabs are to be found in the accounts of old voyages, even the largest of these is hardly an antagonist to be dreaded.

A considerable number of invertebrate animals, not of themselves noxious, are now known to be the indirect cause of much serious injury to human life by harbouring and disseminating organisms which produce disease. The progress of research is adding, almost every day, to the number of species known to be disease-carriers, and it is possible that in the future some Crustacea as yet unsuspected may be added to the list.

At present, however, there is only one case in which a Crustacean has been shown to be concerned in the transmission of a parasite of man. The "Guinea-worm," _Filaria_ (or _Dracunculus_) _medinensis_, is a parasite belonging to the group of "Thread-worms" (Nematoda) which causes dangerous abscesses under the skin of the legs in many parts of tropical Africa. It has been shown that the embryos of the worm, which are discharged in vast numbers on the bursting of the abscess, do not develop unless they fall into water containing certain species of the Copepod _Cyclops_ (see Fig. 14, p. 39). In some way not yet understood, the embryos penetrate into the body cavity of the _Cyclops_, where they undergo a metamorphosis. For their further development it is necessary that the _Cyclops_ should be swallowed by man, as may easily happen in drinking water from a pond. When the _Cyclops_ is digested the larval worms are set free, and they bore their way through the tissues of their human host till they reach the place (generally under the skin of the leg) where they complete their development and produce the innumerable embryos that are set free in the way just described.

A few Crustacea inflict a certain amount of injury on man in more indirect ways. In tropical countries, Land Crabs are often troublesome in gardens, and may cause serious damage to young plants in sugar-cane plantations and rice-fields. In gardens in this country, the Woodlice, as already mentioned, are sometimes destructive to seedlings and delicate plants. The little fresh-water Isopod, _Asellus aquaticus_, is accused of destroying the nets used in fishing for Pollan in Lough Neagh in Ireland.

Probably the most important of all Crustacea, however, as regards their destructive activity, are the species which bore into wood, and sometimes do extensive damage to the submerged timber of piers, jetties, and similar structures. On our own coast the most destructive is a little Isopod known as the "Gribble" (_Limnoria lignorum_--Fig. 80), which is distributed from Norway to the Black Sea, and occurs also on the Atlantic coast of North America. Several species of the same genus having similar habits are found in other parts of the world. The Gribble was first discovered as a British species by Robert Stevenson, the celebrated lighthouse engineer, who found it in 1811 destroying the woodwork employed in the erection of the Bell Rock Lighthouse, and sent specimens to Dr. Leach of the British Museum. The animal is only about one-eighth of an inch in length, and its cylindrical burrow is about one-fifteenth of an inch in diameter, and penetrates for a depth of one or two inches. The excavation of the wood is effected by means of the mandibles, which are unusually strong; and when the animals are numerous the burrows are driven so close together that the surface of the wood is reduced to a spongy mass which is rapidly washed away by the waves (Plate XXXII.). The Gribble is often accompanied by another Crustacean of similar habits, the Amphipod _Chelura terebrans_. The latter is about one-fifth of an inch in length, and differs from most Amphipods in having the body somewhat flattened from above downwards instead of from side to side. The burrows made by _Chelura_ are shallower than those of the Gribble, and generally run more or less parallel to the surface of the wood.