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.
[Illustration: _PLATE XXX_
THE "NORWAY LOBSTER," _Nephrops norvegicus_, ABOUT ONE-THIRD NATURAL
SIZE
(_From Brit. Mus. Guide_)]
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
(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 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 "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.
[Illustration: 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.
[Illustration: 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.
[Illustration: 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
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 _Penaeus_ are of
considerable importance. Thus, in the Mediterranean countries, _Penaeus
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 _Palaemon_, 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 Atyidae, 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.
[Illustration: _PLATE XXXI_
THE COMMON EDIBLE CRAB, _Cancer pagurus_. BRITISH. (MUCH REDUCED)]
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 maenas_ (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 (Portunidae). 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 Portunidae, 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.
[Illustration: FIG. 80--THE GRIBBLE (_Limnoria lignorum_). MUCH ENLARGED
(From British Museum Guide, after Sars.)]
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.
[Illustration: _PLATE XXXII_
PIECE OF TIMBER FROM RYDE PIER SHOWING DAMAGE CAUSED BY _Limnoria_
AND _Chelura_
(_From Brit. Mus. Guide_)]