The Life of Crustacea Part 12

A, Nauplius, just hatched; B, young female taken from gills of Flounder; C, free-swimming stage of female, after leaving Flounder; D, free-swimming male; E, female just after settling on gills of Whiting; F, fully-developed female.]

On the gills of Cod, Haddock, and other common fish, we often find a red worm-like parasite, _Lernaea branchialis_ (Fig. 73, F), which at first sight seems to bear no sort of resemblance to a Crustacean. The soft body is curiously doubled up, and is attached to the host by a narrow neck; while dissection will reveal a small head buried in the flesh of the fish's gills, and having three branched outgrowths, which penetrate into the surrounding tissues and make the attachment of the parasite more secure. Near the hinder end of the body are two coiled threads, which are the egg-ma.s.ses. The reduced mouth parts and the microscopic vestiges of the swimming feet may be detected on and near the head, but apart from these it would be hard to find any characters to show that the animal is a Crustacean.

The life-history of _Lernaea_ is very remarkable. The young are hatched in the nauplius stage (Fig. 73, A), and after pa.s.sing through some further free-swimming stages they become parasitic on a fish. Curiously enough, however, they choose a very different host from that on which the adults are found, for at this stage (Fig. 73, B) they attach themselves to the gills of one of the Flat-fishes (Pleuronectidae), such as the Flounder, Plaice, etc., attachment being effected by a frontal cement gland similar to that of the larval Caligidae, already mentioned.

The animal is now without the power of swimming, its appendages becoming reduced to stumps and losing their setae. After pa.s.sing some time in this condition, the larva again acquires the power of swimming, and leaves its host. Both s.e.xes become mature in this free-swimming stage (Fig. 73, C, D), and impregnation is effected. The males die without developing further, but the females seek a second host, a fish of the family Gadidae, such as the Cod, Haddock, etc., and, settling on the gills, become metamorphosed (Fig. 73, E) into the adult form described above.

Within the gill cavities of the strange-looking fish known as the Angler or Fis.h.i.+ng-frog (_Lophius piscatorius_) there may often be found specimens of another parasitic Copepod, _Chondracanthus gibbosus_. It has a soft, unsegmented body about half an inch long, provided with numerous blunt lobes which give it a very irregular shape. On the under-side, near the front, are forked lobes representing two pairs of the swimming feet. At the hinder end are usually attached a pair of long thread-like egg-ma.s.ses. Just at the point where the egg-ma.s.ses are attached, close inspection of the under-side of the body will reveal a very minute maggot-like object. This is a male individual, which is attached, like a secondary parasite, to the body of the enormously larger female.

[Ill.u.s.tration: FIG. 74--STAGES IN THE LIFE-HISTORY OF _Haemocera danae_, ONE OF THE MONSTRILLIDae. (From Lankester's "Treatise on Zoology," after Malaquin.)

A, Free-swimming nauplius larva; B, embryo after penetrating into the body of the worm _Salmacina_; C, D, E, successive stages in the body of the host; F, free-swimming adult female. (All greatly enlarged, not to same scale.) _a'_, Antennule; _br_, brain; _e_, nauplius eye; _f_, swimming feet; _g.s._, hairs on which the eggs are carried; _m_, position of mouth; _md_, hooked mandible of nauplius; _n_, nerve cord; _ov_, ma.s.s of eggs carried by female; _ovy_, ovary; _pr_, absorptive processes.]

In all the cases mentioned, the animal is parasitic in the final state of its existence--at least in the female s.e.x--but there are a few Copepoda which are free-swimming, both when young and when adult, but parasitic in the intermediate stages. Among the Copepoda taken by the tow-net in British seas, there may sometimes be found species of the family Monstrillidae (Fig. 74, F), which are remarkable for having no appendages between the antennules and the first pair of swimming feet.

They have no trace of jaws, and only a minute vestige of a mouth-opening; while internally there is no food-ca.n.a.l, so that the animals are incapable of taking nourishment. Their development was for long a mystery, but it is now known that the greater part of their life is pa.s.sed as internal parasites in certain bristle-footed worms (Polychaeta). The young are hatched as nauplius larvae (Fig. 74, A) without mouth or food-ca.n.a.l, but capable of swimming, and having the third pair of appendages (mandibles) furnished with strong hooks, by means of which they fasten on to the worm which is to serve as their host. The nauplius bores through the skin of the worm, casting its cuticle and losing all its appendages in the process, and making its way into one of the bloodvessels in the form of a little oval ma.s.s of cells (Fig. 74, B), within which no organs except the degenerating nauplius eye can be detected. It later becomes enclosed in a delicate cuticle, and from one end two long finger-like processes grow out, which are believed to have the function of absorbing nourishment from the blood of the host (Fig. 74, C, D). Within the cuticle the organs of the adult animal are gradually differentiated (Fig. 74, E), and when fully formed it bores its way through the tissues of its host by means of rows of hook-like spines surrounding the pointed posterior end of the sac. On reaching the surface the enclosing membrane bursts, and the adult animal is set free.

Of all Crustacean parasites, however, perhaps the most remarkable in their structure and life-history are the Cirripedes of the order Rhizocephala. It is not uncommon on the British coasts to find specimens of the common Sh.o.r.e Crab or other Crabs which carry under the abdomen an oval fleshy body. This is the Rhizocephalan _Sacculina carcini_ (Plate XXIX.), and it would hardly be possible to guess, from its appearance or structure, that it was a Cirripede or a Crustacean at all. It is attached to the under-side of the Crab's abdomen by a short stalk, and in the middle of its opposite surface is a small opening which leads into a cavity separating the outer "mantle" from the body of the animal.

Very often this mantle cavity will be found to be full of eggs enclosed in sausage-shaped packets. At the point where the short stalk enters the abdomen of the Crab, it gives off an immense system of fine branching roots, which penetrate throughout the body of the Crab, and even into its legs and other appendages. By means of these roots the _Sacculina_ absorbs nourishment from the body-fluids of its host. Like most Cirripedes, _Sacculina_ is hermaphrodite, and the body within the mantle cavity contains only the reproductive organs of the two s.e.xes and a small nerve ganglion representing the whole of the nervous system. There is no mouth, no food-ca.n.a.l, and no trace of appendages. Another Rhizocephalan, _Peltogaster_, is not uncommonly found attached to the abdomen of Hermit Crabs. Although the nauplius larva of _Sacculina_ was described, and its resemblance to that of the Cirripedia pointed out, as long ago as 1836, by that acute observer, J. Vaughan Thompson, it is only recently that the full life-history has been made known by the researches of Professor Delage and Mr. Geoffrey Smith. The nauplius larva (Fig. 75, A) resembles that of the normal Cirripedes, especially in the shape of the dorsal s.h.i.+eld, which is drawn out on either side in front into a pair of fronto-lateral horns. It has, however, no mouth, and the food-ca.n.a.l is quite absent. As in the normal Cirripedes, the nauplius is followed by a _cypris_ stage (Fig. 75, B), also mouthless, and it is in this form that the _Sacculina_ seeks the Crab on which it is to become parasitic. It would be almost impossible for the _cypris_ larva to settle on that part of the Crab where the adult _Sacculina_ is afterwards to appear, since the Crab usually has its abdomen closely pressed against the under-side of its thorax. The larva therefore attaches itself on some exposed part of the Crab, often on one of the legs, clinging to a hair by means of its antennules. It bores through the cuticle at the base of the hair, and the contents of its body pa.s.s into the interior of the Crab as a little ma.s.s of cells, the empty _cypris_ sh.e.l.l being cast off. This ma.s.s of cells, which const.i.tutes the embryo _Sacculina_, is carried about by the blood-currents of the Crab till it reaches the under-side of the intestine, where it becomes attached. It now begins to send out roots (Fig. 76), and as it grows the central ma.s.s travels backwards along the intestine of the Crab till it reaches the place where the adult parasite is to emerge. As the ma.s.s increases in size, and the organs of the _Sacculina_ become differentiated within it, its presence causes the living tissues between it and the external cuticle to degenerate, so that when the Crab moults an opening is left through which the body of the parasite protrudes. Owing, no doubt, to the drain on its system due to the presence of the _Sacculina_, the Crab ceases to grow, and it does not moult again as long as the parasite remains alive.

[Ill.u.s.tration: FIG. 75--FREE-SWIMMING STAGES OF _Sacculina carcini_.

MUCH ENLARGED. (After Delage.)

A, Nauplius; B, cypris stage.]

[Ill.u.s.tration: FIG. 76--EARLY STAGE OF _Sacculina_ WITHIN THE BODY OF A CRAB. (After G. Smith.)

_i_, Intestine of the Crab; _s_, body of the _Sacculina_, which afterwards emerges on the under-surface of the Crab's abdomen; _r_, roots of the _Sacculina_.]

In addition to this arrest of growth, _Sacculina_ produces in its hosts other changes, which affect chiefly the reproductive organs and the structures a.s.sociated therewith. Crabs of either s.e.x infected with _Sacculina_ are incapable of breeding; the genital gland (ovary or testis) is found on dissection to be shrivelled up, and the external characters indicative of s.e.x become strangely modified. The changes have been most fully studied in the case of a kind of Spider Crab common at Naples--_Inachus mauritanicus_. In this species it is found that females infected with _Sacculina_ show no conspicuous external modification, except that the abdominal appendages, which in the normal females serve for the attachment of the eggs, are greatly reduced in size. Infected males, however, may a.s.sume to a greater or less degree the characters proper to the female s.e.x. Some males show little change, except that the chelipeds remain small and flattened, as in the females and non-breeding males. Other specimens have, in addition, the abdomen much broader than in normal males, and sometimes as broad as in the females. Finally, some males develop on the abdomen, in addition to the rod-like appendages on the first and second somites, characteristic of the male s.e.x, two-branched appendages on the next three somites, as in the females; these individuals are, in fact, so completely intermediate in character between the two s.e.xes that it is only by dissection that it is possible to recognize them as modified males.

An indication of the way in which the degenerate Rhizocephala have been derived from normal Cirripedes is given by a peculiar species of pedunculate Barnacle, _Anelasma squalicola_, which lives attached to Sharks and Dogfish in the North Sea. In _Anelasma_ the peduncle becomes deeply buried in the flesh of the Shark, and its surface is covered with short branching, root-like filaments. As in the case of the Rhizocephala, these roots appear to absorb nutriment from the host, and, although _Anelasma_ possesses a food-ca.n.a.l and mouth, the cirri are reduced in size and devoid of hairs, so that they cannot be used for obtaining food as in ordinary Barnacles.

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 _Bala.n.u.s 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. america.n.u.s_). 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-fis.h.i.+ng 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 fis.h.i.+ng-grounds, but in no case with conspicuous success. A "close time" for the sp.a.w.ning Lobsters has often been tried, but the fact that the female carries the eggs attached to her body for nearly a year after sp.a.w.ning makes it quite impossible to give effective protection by this means. In most Lobster-fis.h.i.+ng 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 sp.a.w.n, or, as it is termed, "in berry," is prohibited.

[Ill.u.s.tration: _PLATE x.x.x_

THE "NORWAY LOBSTER," _Nephrops norvegicus_, ABOUT ONE-THIRD NATURAL SIZE

(_From Brit. Mus. Guide_)]

The so-called "Norway Lobster" (_Nephrops norvegicus_--Plate x.x.x.), 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 quant.i.ties by trawlers fis.h.i.+ng 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 (Palinuridae) 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 Lancas.h.i.+re, 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 Leicesters.h.i.+re, they are said to be used only for garnis.h.i.+ng 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 "ecrevisse a 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 "ecrevisse a 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.

[Ill.u.s.tration: FIG. 77--ROSTRUM AND FORE PART OF CARAPACE, SEEN FROM ABOVE, OF (A) RED-CLAWED CRAYFISH (_Astacus fluviatilis_) AND (B) WHITE-CLAWED OR ENGLISH CRAYFISH (_Astacus pallipes_)]

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.

[Ill.u.s.tration: FIG. 78--THE COMMON SHRIMP (_Crangon vulgaris_). NATURAL SIZE]

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.

[Ill.u.s.tration: FIG. 79--THE NORWEGIAN DEEP-WATER PRAWN (_Pandalus borealis_), FEMALE. (After Sars.)

The second leg of the right side is indicated by dotted lines.]

A large species of Prawn is now imported to this country in considerable quant.i.ties 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 quant.i.ties to the English and other markets.