CRABS
HOW CRABS GROW
If you hunt about in the pools among the rocks when the tide goes out,
and look behind the masses of sea-weeds which cover them, you are quite
sure to find a good many crabs of several different kinds. Before I
tell you about these, however, I think you would like to know something
about the way in which these curious creatures grow.
Remember, then, in the first place, that what we always call the
“shell” of a crab is not really a shell at all. That is, it is not in
the least like the shell of an oyster, or a periwinkle, or a cowry, or
a whelk. In these creatures the shell grows together with the animal
inside it, and is never thrown off all through their lives. But the
“shell” of a crab never grows at all. It is really a kind of crust
of lime on the outside of the skin, which will not even stretch in
the very least degree. So the only way in which crabs can grow is by
throwing off their “shells,” in order that the soft bodies underneath
may increase in size.
So once in every year, until it reaches its full size, every crab has
to cast off its shelly covering and get a new one in its place. A few
days before the change takes place it always goes and hides away in
some dark crevice among the rocks, or behind an overhanging mass of
sea-weed, where none of its many enemies are likely to find it. It
knows perfectly well, you see, that while it is without its coat of
mail it will be quite helpless; for its claws will be so soft that it
will not be able to use them, while its body will be quite unprotected.
Then a very strange thing indeed takes place. Something like a third
part of its flesh turns into water! If you were to catch the animal at
this time and to shake it, you would be able to hear the water swishing
about inside its shell! Then it gets very restless indeed, and begins
to wriggle about a good deal, turning and twisting from side to side,
and rubbing its legs against one another, till it is quite tired out.
It then rests for a little while, and begins to wriggle and twist
about again. The fact is that it is trying to get loose, as it were,
inside its “shell.” After a time it succeeds in doing this, so that the
“shell” is no longer fastened to its body at all. Then, quite suddenly,
a rent opens right across its back, and the crab gathers itself
together and leaps, with a mighty effort, right out of its old coat!
And as soon as it has done so the rent closes up again, so that unless
you look very carefully indeed you cannot see it. You might really
think that two crabs were lying side by side together.
For about a couple of hours the crab now lies perfectly still; and if
you were to feel it you would find that its body was hard and knotted
all over. That is because its muscles are cramped after the violent
efforts which it has been making. After a time, however, the cramp
passes off. Then the animal begins to grow. It grows very fast indeed.
In fact it grows so fast that you can almost see it growing, and in
less than twenty-four hours it is sometimes nearly half as big again as
it was before. A new “shell” then begins to form upon the skin, and in
about a couple of days more the animal is able to leave its retreat,
clothed once more in a suit of good stout armour.
That is the way in which crabs, and lobsters, and shrimps, and prawns
all grow. Once in every year at least they get new “shells”; and every
time that they do so they increase in size. But after they reach a
certain age they grow no more; and the coats of mail which they are
wearing then are kept to the end of their lives.
HOW CRABS SEE
Perhaps, too, you would like to know something about the eyes of crabs;
for these creatures see in a very odd way. On each side of the head is
a kind of stalk, something like those which you may see on the heads
of slugs and snails, only very much smaller. And at the tip of each
stalk is a small black spot. Now if you were to put one of these little
stalks under the microscope, and to look at the black spot, you would
find that it was made up of hundreds and hundreds of very tiny eyes,
very much like those of insects, except that instead of being six-sided
they are square. So that altogether, perhaps, a crab may have three or
four thousand eyes, or even more!
That sounds a very large number, doesn’t it? But then, you see, a crab
cannot move its eyes up and down, and from side to side, as we can.
They are fixed, and cannot be moved at all. Each eye, however, looks in
rather a different direction from all the rest. Some eyes look upwards,
some look downwards, some look forwards, some look backwards, and some
look out on either side. So without moving its head at all the crab is
able to see all round it.
Think of it in this way.
Suppose that you take a telescope and look through it. You can only
see the objects at which the telescope is pointed, not the objects
above it, or below it, or on each side. But if you had four thousand
telescopes, fastened together in two bundles of a couple of thousand
telescopes each, all pointing in different directions, _and if your
eyes were made in such a way that you could look through all the
telescopes at once_: then you would be able to see all round you,
though you would only be able to look in any special direction through
just one or two of the telescopes.
Now that is very much like the way in which the eyes of crabs are made.
Each of these four thousand eyes is really a kind of telescope. And as
they all point in different directions, the crab is able to see above
it and below it and on all sides, though it only looks at any special
object through one or two eyes.
HOW CRABS HEAR AND SMELL
The way in which crabs hear and smell is almost as curious as the way
in which they see, for they have very odd little ears and noses in very
odd places.
On its head, as perhaps you know, a crab has two pairs of feelers. We
call them the “lesser feelers” and the “greater feelers.” Now if you
were to look at the first joint of the lesser feelers through a good
microscope, you would find on each a little gland, or bag, containing a
very tiny drop of salt and water. These are the crab’s ears. Of course
they are not nearly so good as our ears are. Indeed, I do not think
that a crab can hear sounds in the air at all. But water carries sounds
much more readily than air does, so that if you were to dive into a
lake, or into the sea, on a calm, still day you could easily hear the
beat of the oars in a boat half a mile away. And the ears of the crab
are made in such a way that they can hear sounds in the water quite
well, even though they may be deaf to sounds in the air.
Then if you look at the first joint of the greater feelers through the
microscope, you will see two other tiny glands. These are the crab’s
noses, by which it can smell odours in the water just as we can smell
odours in the air. It always seems to find its food by scent, and if
one of those basket-like traps which we call crab-pots is baited with
a few pieces of decaying fish and lowered into the sea, crabs will
smell the bait from quite a long distance away, and come hurrying up
to obtain a share in the banquet. And they seem to do so by means of
those odd little noses on the lower joints of their greater feelers.
PLATE XXI
THE EDIBLE CRAB
Now let me tell you something about the different kinds of crabs which
you may find on the shore.
First of all, of course, there is the Edible Crab. This is the crab
which is so largely used for food, and which you may see in any
fishmonger’s shop. Sometimes it grows to a very great size, and has
claws so big and strong that if it were to seize a man by the wrist he
would find it very difficult indeed to set himself free. You will not
find crabs as big as this among the rocks, for these giant creatures
always live in rather deep water. But one often discovers a crab four
or five inches across hiding in a rock-pool, and even he is quite big
and strong enough to give one a very sharp nip.
[Illustration: PLATE XXI
THE EDIBLE CRAB.]
It is rather amusing to get one of these crabs out on to the open sand,
and then to stand just in front of him. He will at once raise both his
great claws and hold them in readiness to strike at you if you attempt
to seize him. Then if you walk slowly round and round him he will
turn round and round too, so as to keep facing you, over and over
and over again. And if you put your hand anywhere near him he will snap
at it so quickly that it is really not at all easy to avoid his stroke.
Edible crabs often have their shells covered with barnacles and the
tubes of some of the sea-worms. Old crabs, indeed, which no longer
change their coats of mail every year, are often so covered with these
creatures that one can hardly see their shells at all.
PLATE XXII
THE SHORE CRAB (1)
This is sometimes known as the Green Crab, because it is generally
more or less green in colour. But you may often find examples, which
are deep brown all over, while others are bright yellow, with black
markings upon their backs. It does not grow to nearly such a great size
as the edible crab, and although its flesh is quite good to eat there
is so little of it that the animal is hardly ever used for food. But it
is wonderfully strong, and if you find a green crab hiding beneath a
big stone or behind a mass of sea-weed, you must be very careful not to
get a nip from its claws.
The green crab spends a great part of its life out of the water, for
its gills are made in such a manner that they will keep moist for a
very long time. And as long as its gills are damp a crab can breathe
quite as easily on land as if it were in the sea. It is very active,
and if you go down near the water’s edge while the tide is coming in
you may often see it hunting sandhoppers and even flies, creeping up
to them very carefully until it is only a few inches away, and then
pouncing upon them so suddenly that they have no time to escape. And it
is often very troublesome to fishermen, for it will seize their bait
with its strong nippers, and pull it off the hooks before a fish is
able to take it.
This crab is very easily kept in confinement, and will soon become
quite tame, so that it will even come and take food from your fingers
just like a dog. But you must be careful to pile up a few stones in the
water in which you keep it, so that it may sit upon them and take an
airing whenever it feels inclined. And it will even enjoy an occasional
run about the room.
[Illustration: PLATE XXII
1. THE SHORE OR GREEN CRAB.
2. THE FIDDLER CRAB.]
PLATE XXII
THE FIDDLER CRAB (2)
The crabs about which I have been telling you live in the sea, though
they often leave it for some little time and run about on the shore.
But none of them can swim, and if they are thrown into deep water they
just sink to the bottom with their legs sprawling, feeling about for
some object to which they can cling. Sometimes, however, if you look
into one of the pools which are left among the rocks when the tide
goes down, you may see a small crab swimming through the water with
some little speed. This is quite sure to be a Fiddler Crab, and if you
catch it and examine its hinder legs, you will find that instead of
being quite slender, with hooked claws at the tips, as they are in most
crabs, they are flattened out into broad, oval plates. And you will
also find that these plates have a fringe of rather long hairs growing
all round them.
Now these are the paddles with which the crab rows itself through the
water, and it is called the “Fiddler Crab” because the movements which
it makes with them are rather like those of a man who is playing the
violin. You can easily keep it in an aquarium, and a very interesting
little pet it makes. But you must remember that it is a very savage
little animal, and will certainly do its best to kill any other
creatures that you may put into the same vessel. Even if you put two
fiddlers together they are almost sure to fight; and the one which wins
the battle will kill and eat the one which loses it.
When the Fiddler Crab is alive it is really a very handsome little
creature, for its blackish shell is covered all over with soft, short
down, looking rather like velvet, while its legs are striped with blue,
and its claws are partly blue and partly scarlet.
PLATE XXIII
THE MASKED CRAB (1)
The broad shelly shield which covers the back of a crab is called the
“carapace,” and there are certain markings upon it which are rather
like the features of a human face. But there is one crab in which
these markings are so deep and strong that it looks just as if it were
wearing a mask. So it is always known as the “Masked Crab.” It is found
on the southern and western shores of England and Wales, and you may
always know it if you meet with it, not only because of the face-like
markings upon its back, but also because its carapace is a good deal
longer than it is broad, whereas in other crabs it is nearly always
broader than it is long. Besides this, the great claws are not really
“great” at all, for they are very long indeed and very slender, with
quite small nippers at the tips, while the greater feelers are quite as
long as the claws. So altogether the masked crab is a very odd-looking
crab indeed. But if you want to find it you will have to look for it
very carefully, for it has an odd way of burying itself in the sand,
and only leaving just its feelers and its eyes above the surface.
PLATE XXIII
THE THORNBACK CRAB (2)
This is perhaps the very oddest of all our British crabs.
In the first place, it looks much more like a big spider than a crab;
for its body is very small, while its legs are very long and very
slender. Indeed, the group of crabs to which it belongs is often called
“spider crabs” in consequence. In the second place, its carapace is
covered all over with rather long sharp spikes, which project in all
directions, so that it strongly reminds one of a tipsy-cake! And, in
the third place, the crab nearly always has a number of tufts of
sea-weed or sponge growing upon its back.
Perhaps you might think that these come there by accident. But they
do not. The crab himself plants them there! If you keep him in an
aquarium you may often see him doing so. First of all he turns one of
his long claws over his back and scratches away at the carapace, so as
to roughen the surface. Then he pulls up a little sprig of sea-weed
or sponge and actually plants it on his shell, pressing the rootlets
firmly down. And besides the spikes upon the shell there are numbers
of tiny hooks, which help to hold it in position. Then the crab plants
another piece of weed or sponge in just the same way, and so he goes on
planting piece after piece until his back is completely covered.
Now why do you think he takes all this trouble?
Well, the reason is that he does not want to be seen; for he has a
great many enemies, and he knows perfectly well that if he were to lie
among the sea-weeds or sponges at the bottom of the sea they would be
quite sure to notice him as they passed by, and then he would almost
certainly be killed and eaten. So he clothes himself with either
sea-weeds or sponges, as the case may be, and then feels that he is
perfectly safe, and that as long as he keeps quite still even the
sharpest eye will fail to notice him. And if you catch one of these
crabs which is covered with sea-weeds and put it into an aquarium in
which sponges are growing, it will very soon strip the weeds off its
back and cover itself with sponges instead; while if you catch one that
is covered with sponges, and put it into a tank in which sea-weeds are
growing, it will strip off the sponges and cover itself with sea-weeds!
[Illustration: PLATE XXIII
1. THE MASKED CRAB.
2. THE THORNBACK CRAB.]
The thornback crab often grows to a rather large size. Indeed, next to
the edible crab, it is the largest of all the crabs which are found
in our British seas, for its carapace is sometimes as much as eight
inches long and six inches wide, while its great claws may be fourteen
or fifteen inches in length. On some parts of the coast it is used for
food, but its flesh is rather coarse and of poor quality.
PLATE XXIV
THE LONG-BEAKED SPIDER CRAB (1)
This crab has an even smaller body in proportion to its size than the
thornback, and its legs are so very long and so very slender that they
remind one of those of a daddy-long-legs. Its carapace is drawn out
in front into a kind of beak, which is quite as long as the carapace
itself, and while the crab is alive it is of a most beautiful pink and
puce colour. It is not a very common creature, but is sometimes to be
found in the rocky pools near low-water mark on our southern coasts,
and is covered, very often, with sea-weeds or sponges, just like the
thornback.
PLATE XXIV
THE FOUR-HORNED SPIDER CRAB (2)
Perhaps this is the commonest of the British spider crabs. Indeed, it
is so plentiful at Bognor, and at other places on the southern coast
of England, that when a crab pot is taken out of the water as many
as twenty or even thirty of these creatures are sometimes found in
it. They are called by the fishermen “sea-spiders,” and are generally
so clothed with those odd sea-weeds called “corallines” that you can
hardly see any part of their “shells” at all.
In this crab the carapace is drawn out in front into a very long beak
indeed, which has four horns upon it, and the whole upper surface is
covered with short, sharp spikes and stout hairs.
[Illustration: PLATE XXIV
1. THE LONG-BEAKED SPIDER CRAB.
2. THE FOUR-HORNED SPIDER CRAB.]
PLATE XXV
THE PEA CRAB (1)
This is a very odd crab indeed. In the first place it is extremely
small. Even when it reaches its full size it is scarcely ever so much
as half-an-inch across, while its body is so round that it really does
remind one very much of a pea. Only it is not quite the right colour
for a pea, for it is creamy yellow instead of green.
And, in the second place, this crab lives in a very odd place--namely,
inside the shells of living mussels, or pinnas, or even cockles! What
it does there nobody seems quite to know. It does not appear to injure
the animal to whom the shell belongs, although it is very fond of the
flesh of mussels, and if it finds one of those creatures lying dead
will certainly devour it. Perhaps it only creeps inside its shell for
the sake of safety. At any rate, it is a very timid little crab, and
if you open a mussel and find a pea crab lying hidden inside it, it
will tuck up all its legs quite close to its little round body and lie
perfectly still for several minutes in the hope that you will think
that it is dead.
On some parts of the coast pea crabs are so plentiful, that three out
of four mussels are found to have one of these odd little creatures
inside it.
PLATE XXV
CRAB CATERPILLARS (2 and 2 A)
I dare say you did not know that crabs have caterpillars, just as
insects have. We call these crab caterpillars “zoeas,” and they are
not in the least like their parents. There are a great many different
kinds, of course, for every crab has its own zoea, just as every
butterfly and moth has its own caterpillar, and some of them are not
very much like some of the others. But they are always very tiny
indeed--they are scarcely as large, in fact, as the smallest grains of
sand--and they always have a very long curved horn in front of the body
and another one behind, and long waggly tails. And they swim in the
oddest way possible--by turning somersaults in the water, over and over
again!
These zoeas are very useful little creatures, because they feed upon
the tiny scraps of decaying matter which are always floating about
in the sea, and so help to keep the water always pure. They belong,
in fact, to the great army of what I always like to call “nature’s
dustmen”--those little animals whose duty it is to clear away the
rubbish from the world. There are millions and millions of these busy
little workers on the land, and millions and millions of others in
ponds and rivers, as well as in the sea, and so well do they perform
their task that both the air and the water are always kept pure.
Another very interesting fact about zoeas is that they form the chief
food of no less a creature than the Greenland whale. No doubt you know
that whales are of two kinds--those which have teeth, and those which
have none. Those which have teeth feed upon fishes, and giant cuttles,
and could easily swallow a man. But the whales which have no teeth
have throats so small that they would almost certainly be choked if
they tried to swallow a herring! So they have to feed on very small
creatures indeed, and are very fond of zoeas, which often swim about in
such vast shoals that the water of the sea is quite thick with them.
And they catch them in a most curious manner.
You have heard, of course, of the very useful substance which we call
“whalebone;” and no doubt you know that it has nothing to do with the
bones of the whale at all. It is found in the mouths of those whales
which have no teeth, and hangs down in great plates from the gums of
their upper jaws. Very soon these plates split up; and then each part
splits up again; and so on, over and over again, till at their lower
ends they form a kind of thick fringe of close, matted hairs.
Now it is by means of this fringe that the whale catches the zoeas.
When it meets with a shoal of these little creatures it opens its huge
mouth wide, and swims through them. Then it nearly closes its jaws, and
lets down the whalebone plates, so that the hairy fringe forms a kind
of strainer all the way round. It then squirts out the water from its
mouth through this fringe, which allows the water to pass through it,
but keeps back the zoeas; and when it has got rid of all the water it
closes its mouth completely and swallows the zoeas, a few thousand at a
time, after which it opens its jaws again, and swims through the shoal
once more.
Doesn’t it seem strange that the biggest animal on earth should feed on
some of the very smallest?
PLATE XXV
CRAB CHRYSALIDS (3 and 3 A)
When the caterpillar of an insect has reached its full size it throws
off its skin and appears as a chrysalis, or pupa. And the caterpillar,
or zoea, of a crab does exactly the same thing. It casts its skin, and
appears in quite a different form. Only we do not call it a chrysalis,
as a rule. We call it a “Megalopa.”
[Illustration: PLATE XXV
1. PEA CRAB (life-size).
2. CRAB CATERPILLAR (enlarged).
2A. ” ” (life-size).
3. CRAB CHRYSALIDS (enlarged).
3A. ” ” (life-size).]
The word “megalopa” means “a creature with big eyes,” and it is given
to the crab chrysalis because it has eyes which are enormously big
in proportion to the size of the head. They are set on long footstalks,
which project on either side, so that the head looks rather like a
hammer. Then the long curved horns which the zoea had are to be seen no
longer, and the carapace is shaped much more like that of the perfect
animal, while the great claws begin to show, and the legs increase in
length. The tail, however, is still quite free, like that of a lobster,
and the little animal still swims by turning somersaults in the water,
and lives on the same tiny scraps of decaying matter on which it fed as
a zoea. After a few weeks it throws off its skin once more, and appears
in the world as a perfect crab.
PLATE XXVI
HERMIT CRABS (1 and 2)
If you go down among the rocks when the tide is out, and hunt about in
the pools, you may often find the shell of a whelk in which a small
crab is living, with one of his great claws carefully guarding the
entrance. This is a Hermit Crab, and a very curious little creature he
is. For, in the first place, his long tail is quite free, like that of
a lobster, instead of being fastened down to the lower surface of his
body; and in the second place, it is quite soft, without any shelly
covering at all. His body and limbs are covered with armour, just like
those of other crabs, but his tail has none at all.
The consequence is that the hermit crab always has to take the very
greatest care of his tail. He is so dreadfully afraid that one of
his many enemies will come up behind and give it a nip when he isn’t
looking! So he protects it by tucking it away into the empty shell of a
whelk. He never leaves this shell, but drags it about with him wherever
he goes. And if you take hold of him and try to pull him out, you will
find that you cannot do so without injuring him very badly. For at the
end of his tail he has a pair of strong pincer-like organs, with which
he holds on so firmly that it is very difficult indeed to make him let
go.
Indeed, the only way to get a hermit crab out of his dwelling is to put
him, shell and all, into the spreading arms of a big sea anemone. That
frightens him almost out of his wits, for the arms of the anemone at
once come closing in, and he knows quite well that if he stays where
he is he will very soon be swallowed. So he skips out of the shell and
scampers away as fast as he possibly can, leaving the empty shell in
the anemone’s clutches.
[Illustration: PLATE XXVI
1. THE HERMIT CRAB IN WHELK-SHELL.
2. THE HERMIT CRAB OUT OF SHELL.]
The poor little animal is now perfectly miserable. He has no protection
for his tail, you see, and goes hunting about everywhere for some
other shell into which he can tuck it. After a while, perhaps, he finds
that of a periwinkle. It is not of much use, of course, for it is so
small that he can only get just the tip of his tail into it. Still, it
is better than nothing, and he goes crawling about with the periwinkle
shell on the end of his tail, like a thimble on the tip of one’s
finger, in search of a bigger one. By-and-by he discovers one. Then he
whips his tail out of the old shell and into the new one so quickly
that you can hardly see how he does it, and goes off to look for a
bigger shell still. And in this way he will change his dwelling perhaps
half-a-dozen times before he is really satisfied.
Sometimes you may find a hermit crab with a sea anemone fastened to
the edge of the shell in which he is living. That seems strange,
doesn’t it, when you remember how terribly afraid the little animal
is of anemones. But in such a case the anemone never interferes with
the hermit crab, and the crab never interferes with the anemone, while
both of them benefit by the arrangement. The crab benefits, because
no fish will ever touch him so long as an anemone is attached to his
whelk-shell. There are plenty of fishes which would be quite ready to
gobble him up, whelk-shell and all, if it were not for this creature.
But fishes know quite well that sea anemones can sting, and therefore
never think of devouring them, no matter how hungry they may be; so
that so long as an anemone is guarding the whelk-shell in which he
lives, the hermit knows that he is perfectly safe. And the anemone
benefits, because it gets a share of the crab’s meals. When a hermit
crab finds the dead body of some small creature at the bottom of the
sea he pulls it to pieces and devours it; and as he does so a quantity
of tiny scraps are sure to come floating upwards, and are seized by the
outspread arms of the anemone. So the crab gets the big pieces, and the
anemone gets the little ones; and both are perfectly satisfied.