A GOODLY COMPANY OF CRABS
“The slimy bottom of the Deep.”--_King Richard III._
In all the wealth of Ocean-life, perhaps no creature flourishes more
abundantly than do Crabs and their relatives.
We still talk of Jelly-fishes, Starfishes, Craw-fishes, Cuttlefishes,
but no longer of Crab-fishes.
Once upon a time that name was equally common. Everything that had its
home in the sea was popularly supposed to be a fish. We know now that
not one of the above belongs to any Fish tribe; yet in Conservative
style we keep the old titles going.
Crabs and Lobsters, Shrimps and Prawns, are members of the vast
CRUSTACEAN division or family. They are so called on account of the
hard protecting _crust_, or armour, by which their soft bodies are
guarded. Chief and foremost among the numberless kinds included
under this head is the busy vigorous sidling Crab.
And very important animals are crabs, in the economy of the Ocean. Not
only from their enormous numbers; not only from the wide extents of
land and sea which own to their presence; not only from the fact that
they form a principal item of food to fishes and even to whales; but
also because at least one species is reckoned excellent eating for man.
There are land-crabs as well as sea-crabs. There are deep-sea crabs, as
well as shallow-water and shore crabs.
[Illustration: A DEEP-SEA CRAB
The holes in front are for the admission of water containing nutritious
animal food
_Face page 220_
]
There are crabs much less than half-an-inch across; and there are giant
Japanese crabs, carrying heavy back-armour or “carapace,” about a foot
in length and breadth, while the sprawling limbs extend to a yard and a
half or more on either side.
Those of ordinary sizes are found everywhere; in numbers beyond
reckoning. Not only in tropic waters, but in moderate climes, and in
frigid zones; alike near the poles and under the equator. Different
species, of course, in different parts, but all connected.
Moreover, they are quite as much at home in the depths of the sea, with
miles of water piled over their heads, as on shelving beaches near to
land.
During the _Challenger_ Expedition crabs were fished up from all
depths--from one mile, two miles, three miles, even between three and
four miles. In those dark and icy regions the cold must be great, the
pressure tremendous; still crabs innumerable are there.
Some sub-ocean kinds have displayed what may be looked upon as an
inquiring disposition.
A ship passing near the Azores brought up in a trap many large ones,
belonging to a then new species. And the curious part of the matter was
that not all of them need have come. Some were entrapped, and could
not get away; but others rose to the surface of their own free will,
clinging to the outside of the trap, and not attempting to escape, even
when drawn out of the water, and on deck.
Perhaps they were dazzled by the unaccustomed brightness of light,
after the few and feeble glimmers below. But the puzzle remains--why,
in the beginning, they should have chosen to leave their home, and
to journey upward into unknown regions? Were they of an aspiring
character? Had they friends caught in the net, from whom they would not
be parted? Or was the attraction merely that of the appetising food
which those friends were enjoying?
An interesting haul was one day made by the _Challenger_ while crossing
the Bay of Biscay; not indeed from a great depth, since the Bay is in
no part very profound, but from the sea-bed. More than five thousand
small sharp-clawed crabs came up in company; and they took quite kindly
to the new conditions of life on board. They cheerfully explored the
whole vessel, turning up in every direction, during the next few days.
Something in the nature of a love for home has been shown by crabs, at
least in one instance. Many years ago a singular story to this effect
was told in a scientific paper.[8]
[8] _Nature_, April 3rd, 1873.
The fishermen of Falmouth, capturing crabs off the rocks at Lizard
Point, brought them by boat to Falmouth Harbour, and kept them there
till needed, in a box sunk under water, some four miles out. Each crab
had been branded on the back with its owner’s mark.
By accident one of these boxes was broken; and all the prisoners made
good their escape. Naturally, no one expected to see them again.
But only two or three days later those very same crabs--with branded
carapaces--were caught anew by the fishermen; not near Falmouth, but as
before, off the Lizard. How they had managed to get there, how they had
first found their way to Falmouth Harbour, a distance of four miles,
and then for miles more along the coast, are questions more easily put
than answered. The word “instinct” fails to meet the difficulty.
One thing only is clear, that they preferred the neighbourhood to which
they were accustomed.
Very fearless creatures are small crabs, generally; brave, no doubt,
partly because of their numbers.
In nets and trawls, where animals of many kinds are together for long
hours of captivity, the weak are often destroyed by the strong, before
they can be drawn up.
But sometimes the weak combine together, and prove too much for the
strong. Notably this was the case, when a dog-fish, brought up from a
depth of over fourteen hundred fathoms, was entirely disposed of in
twenty hours by crowds of hungry little crustaceans.
Most of the crabs and lobsters, shrimps and prawns, which live on or
near the bottom, send their young ones up into higher levels for
education. There for a while the juveniles swim and develop, till old
enough in their turn to take to the ocean-bed. Great numbers have
meanwhile been preyed upon by multitudes of fishes in those upper
levels.
Crabs undergo curious changes in the course of their growth from
infancy to adult age. At certain stages they are so utterly unlike in
appearance to what they become later, that for a long while they were
classed by naturalists under separate names, being actually reckoned as
different creatures.
Since the protecting armour of a crab is far too rigid to admit of its
stretching, it becomes from time to time too tight for the growing body
within. When it can no longer be endured, it has to be cast off, and a
new suit, larger in size, has to form in its place.
This armour, like the shells of foraminifers, and the skeletons of
coral-polyps, is unconsciously secreted by the crab; and during the
formation of a new suit of armour the crab has a time of weakness,
of delicate health, and also of dangerous exposure to the attacks of
enemies.
Some kinds of crabs have sunk to the miserable level of parasites,
living on the exertions of others, and refusing to exert themselves.
One prefers to think of the active and independent and intelligent
kinds; and really, crabs have a good deal of intelligence. Quite as
much as an average insect; though perhaps few of them could compete in
an examination with ants or bees.
If any might do so successfully, it would be in all probability some
member of the Spider-crab family.
To this slender-limbed untidy class belongs the unwieldy Giant-crab
of Japan. But it is his smaller relatives who excel in sense. The
spider-crab has a slovenly appearance, because he attaches to himself
stray bits of sea-weed and scraps of sponge or other growths, with the
plain intention of becoming less easily seen.
He keeps to no regular or permanent style of adornment. The nature of
his trimmings depends entirely on the character of his surroundings.
Many years ago my father had been for a short dredging excursion, near
Worthing; and he brought home with him one of these small creatures,
elaborately ornamented with slim strips of bright-red sea-weed.
He placed the crab in a basin of water, which contained a supply of
green sea-weed. And next morning a change was seen. The crab had cast
aside his red ribbons, and had decked himself out with a smart array
of green ribbons instead. Such conduct may, if we choose, be accounted
for by the magic word “instinct;” but it certainly wears the aspect of
deliberate intention, and even of some dim consciousness of cause and
effect, not to speak of a knowledge of colour.
This habit of the spider-crab is now well known and recognised; and he
has been closely watched during the “dressing” process.
It has been noted that he always puts each slip of sea-weed or scrap
of sponge, or aught else that he uses for the purpose, to his mouth,
before fastening it to his body or limbs. A suggestion has been made
that his object in so doing is to lick it and render it sticky.
Perhaps in some cases it could not otherwise be attached; but many of
the spider-crabs have hooked hairs, exactly adapted for holding fast
such objects as they love to adorn themselves with.
Nor are the adornments always merely stuck on. Both sea-weeds and
sponges frequently take root, and flourish as healthy growths upon the
crab’s back. It is an extraordinary fact that the crabs seem to know
perfectly well from which kinds of sea-weed or zoophyte they may snip
off little bits, with a prospect of not killing them.
So unerring, indeed, is their knowledge in this respect, that, as an
authority on the subject has stated, “the keepers of Aquaria have only
to consult the crabs to learn what kinds of sea-animals will bear being
thus transplanted piecemeal.”[9]
[9] _Crustacea_, by STEBBING.
Is this too merely “instinct”?
Many a time baits were lowered in the nets of the _Challenger_ to
attract victims. Oddly enough, it was found that no more effective
bait could be used for crabs and their kindred than--pieces of
looking-glass! May one imagine that some friendly Medusa lent her
little glimmer, far below, to enable the Hermit-crabs, when putting on
fresh ribbons, to see their reflections, and judge of the effect? It
reads rather like a page of _Alice in Wonderland_.
Hermit-crabs or Soldier-crabs, having an insufficient protective armour
of their own, are in the habit of using empty Mollusc shells to shelter
that part of their bodies which is not guarded. One of these crabs,
when young, chooses a shell, fitted to its size; and when grown too
large for it, that shell is discarded in favour of a new one.
But whether the said crab commonly contents himself with an empty
habitation, the occupant of which has died earlier, or whether he first
kills and eats the live Mollusc, and then “commandeers” the shell, does
not seem to be known with certainty.