The episode in Scottish history called "The Walking Wood of Birnam,"
when the advancing troop masked their approach by cutting down branches
of the trees, has had its counterpart in many countries. But it is also
enacted on the seashore. There are many kinds of crabs that put on
disguise with what looks like deliberateness. The sand-crab takes a
piece of seaweed, nibbles at the end of it, and then rubs it on the back
of the carapace or on the legs so that it fixes to the bristles. As the
seaweed continues to live, the crab soon has a little garden on its back
which masks the crab's real nature. It is most effective camouflaging,
but if the crab continues to grow it has to moult, and that means losing
the disguise. It is then necessary to make a new one. The crab must have
on the shore something corresponding to a reputation; that is to say,
other animals are clearly or dimly aware that the crab is a voracious
and combative creature. How useful to the crab, then, to have its
appearance cloaked by a growth of innocent seaweed, or sponge, or
zoophyte. It will enable the creature to sneak upon its victims or to
escape the attention of its own enemies.
If a narrow-beaked crab is cleaned artificially it will proceed to
clothe itself again, the habit has become instinctive; and it must be
admitted that while a particular crab prefers a particular kind of
seaweed for its dress, it will cover itself with unsuitable and even
conspicuous material, such as pieces of coloured cloth, if nothing
better is available. The disguise differs greatly, for one crab is
masked by a brightly coloured and unpalatable sponge densely packed
with flinty needles; another cuts off the tunic of a sea-squirt and
throws it over its shoulders; another trundles about a bivalve shell.
The facts recall the familiar case of the hermit-crab, which protects
its soft tail by tucking it into the empty shell of a periwinkle or a
whelk or some other sea-snail, and that case leads on to the elaboration
known as commensalism, where the hermit-crab fixes sea-anemones on the
back of its borrowed house. The advantage here is beyond that of
masking, for the sea-anemone can sting, which is a useful quality in a
partner. That this second advantage may become the main one is evident
in several cases where the sea-anemone is borne, just like a weapon, on
each of the crustacean's great claws. Moreover, as the term commensalism
(eating at the same table) suggests, the partnership is _mutually_
beneficial. For the sea-anemone is carried about by the hermit-crab, and
it doubtless gets its share of crumbs from its partner's frequent meals.
There is a very interesting sidelight on the mutual benefit in the case
of a dislodged sea-anemone which sulked for a while and then waited in a
state of preparedness until a hermit-crab passed by and touched it.
Whereupon the sea-anemone gripped and slowly worked itself up on to the
back of the shell.
§ 6