|The Australian Wonder Book of Knowledge - [Home]|
Strange behavior of reef crabs
The crabs of the Barrier Reef, vying with other curious forms of life, present us with some extraordinary peculiarities.
Soldier Crabs. The small blue and white soldier crabs march along the beaches in great armies while the tide is out, and immediately someone approaches they quickly dig into the soft sand, corkscrew fashion.
Sand Crabs. Sand crabs, the color of the sand and fleet of foot, dig deep and scatter pellets of sand in all directions.
Fiddler Crabs. Where there is mud,
as on some of the coral islands in the
Hermit Crabs. The hermit crab abounds everywhere. It has a soft abdomen, which is very vulnerable, and so to protect it the crab seeks a snail-like shell and backs into it, guarding the entrance with its formidable claw. As the crab crawls about, it drags the shell with it and leaves it only when it becomes necessary to moult. When these "growing pains" develop, the crab's main object in life is to find a larger shell. It tests shell after shell with its nippers and, having found a size to its liking, it leaves its old home and backs into the new one. Perhaps a suitable shell will be hard to find, and then the crab will proceed to attack another crab whose home it covets. The fight may be long and hard, and it frequently ends with one or the other' being maimed, and occasionally the crab attacked finds itself ." forcibly evicted. Some hermit crabs, not satisfied with the protection of a hard molluscan shell, plant anemones on it. These, armed with stinging darts, are avoided by other animals, and thereby provide yet a further security to the crab.
Spider Crabs. Perhaps the most interesting of the crabs of the Barrier Reef are the spider crabs. They have long, spindly legs which would automatically attract the unwelcome attention of their enemies if they roamed about without taking necessary precautions to camouflage themselves. For this their bodies are well adapted. Their nippers are very effective cutting tools; their backs and legs are covered with stout hooked hairs capable of gripping securely such objects as may be placed on them; and the last pair of legs, or in some the last two pairs, are turned up over the back. As the crab wanders about the reef, it nips off pieces of sea-weed, sponges, etc., which it secures to the hooks on its back and legs ., and in no time it is completely concealed beneath a cultivated garden. But then the day comes when the crab has to moult —it cannot grow . otherwise—so slipping away from beneath its old shell, it immediately begins replanting the garden on the one newly formed.
Considerable discrimination is shown by the spider crab in the choice of material used to decorate itself, for if this did not conform with the type of marine life amongst which it was wandering it would naturally lose its camouflage value. Thus, where necessary, it will divest itself of its old garden and redecorate itself with a new one.
Gall Crab. Another crab of outstanding interest is the gall crab. When young, it takes up its position in the depression of a coral about to branch, and in some mysterious way controls the growth of the coral so that it forms a gall or pocket around it, openings being left to allow the water to circulate about the crab and to bring to it the food necessary ' for its existence. The galls are inhabited by females only; and the males, which are invariably much smaller, are free to roam about at will.
There is a crab, too, that feigns death. If frightened, it will tuck its legs away beneath its body and remain perfectly motionless; like this, it closely resembles an eroded piece of coral and is unlikely to . attract attention.
|Wonder Book of Knowledge|