SHELLFISH
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.