The Ways of Crabs

Crabs that wear clothes, others that carry arms, and still others that march like regiments of soldiers are among the curiosities of Australia’s Great Barrier coral reef.

One crab forces the coral polyp to build a limestone palace for its abode. The female of this species lodges on the polyp when it is in the larval state and causes an irritation which forces the host animal to build up the walls. The resulting house is just big enough for the crab to move about in comfortably. There always is a door through which she obtains her food.

Another species merely sits on the end of a sprouting coral which, growing outward, makes a long, circular burrow for the crustacean. Through this it can move backwards and forwards at will. The forward part of its body is enclosed in a hard shell the exact color of the coral so that when the crab sits at the door of its burrow it cannot be distinguished from the coral.

Still another crab carries two sea anemones, one in each “hand”, wherever it goes. In its first few months of life it seizes these plant-flowers—living animals with stalks and petals like flowers which ordinarily are attached to rocks under the water—about the centers of the stalks. Thenceforth it moves about like a person carrying two umbrellas.

The most logical explanation of this behavior is that the anemones serve as weapons, killing or paralyzing small sea animals which come in contact with them. This species of anemone has stinging cells in its disk. These curious weapons are carried by the crab continuously and seem essential to its life. When one of them is taken away, the crustacean moves automatically to grasp it again. When a crab is killed slowly in alcohol it clings to its weapons even in its death struggles.

There are spider crabs which cut and wear clothes. They cut off pieces of living sponges and place them on their backs. These sponges become entangled in tiny hairs which protrude through the animal’s shell, and continue to grow until they protrude several inches over the back. Thin layers also cover the under part of the body and the legs. Every time a crab sheds its shell, it must make itself a new suit The practice probably is beneficial to both animals. The crab, living in a forest of sponges, looks like a sponge itself and is thus concealed from its enemies. The sponge benefits by being carried to new food sources. When the shell is shed the sponge simply attaches itself to a rock and continues to grow.

One of the most remarkable cases of commensalism in nature has been found by Dr. Melbourne Ward, Australian zoologist in a degenerate type of barnacle which makes its way through the thin shell of one of the Barrier Reef crabs. It wanders through the blood stream of the crab and finally comes to the surface where it forms a little sac for itself. Here it metamorphoses into another form and sends long, thread-like filaments into every part of its host’s body. In some respects it is like a cancer among higher animals, except that in this case the malignant growth is that of an individual animal of another species. It lives off the food eaten by the crab but never kills nor apparently seriously injures its host. The one notable effect, for which there is no adequate explanation, is that it changes a male crab into a female.

The soldier crabs are beachdwellers, about two inches long. They march across the hard sand in perfect order, as if they were under the control of leaders. No “officers”, however, have been observed. When approached, they burrow rapidly in waves, like a regiment of infantry. First the front rank disappears in the sand, followed in order by those behind. The regiment disappears completely in a very short time.

The soldier crabs can hardly be driven into the water. When Dr. Ward succeeded in pushing a few of them off the shore they were set upon by ferocious small fish which rapidly devoured them. Realization of this danger apparently is instinctive in the animals.

Some of the land-dwelling crabs of the mud flats dig very intricate burrows with labyrinthine cross and side galleries. Some species live in a communal life. Each crab has its own burrow, but from each there is a passage into a large central hall which seems to be a community gathering place. Other species are intensely individualistic. Each excavates an elaborate labyrinth in the mud, considers this its own home, and vigorously defends it.

During courtship some of these mud crabs perform dances like the courtship dances of birds. The male of one variety, after attracting a mate by his dancing, picks her up bodily in one of his nippers and carries her away. Another variety of sand crab seems to have perfected an engineering technique which still evades human skill—that of building a burrow in soft, dry sand. These burrows are about two inches in diameter. The crab is able in some mysterious fashion to compress the soft sand into a solid substance with its nippers.

In precision of instinctive behavior, Dr. Ward found, these Great Barrier crabs come quite close to the spiders, their distant relatives.