Walking or crawling is the usual mode of locomotion, and the peculiar sidelong gait familiar to most people in the common shore-crab, is characteristic of most members of the group. The crabs of the family Portunidae, and some others, swim with great dexterity by means of their flattened paddle-shaped feet.
Like many other Crustacea, crabs are often omnivorous and act as the scavengers of the sea, but many are predatory in their habits and some are content with a vegetable diet.
Though no crab, perhaps, is truly parasitic, some live in relations of “commensalism” with other animals. The best known examples of this are the little “mussel-crabs” (Pinnotheridae) which live within the shells of mussels and other bivalve mollusca and probably share the food of their hosts. Some crabs live among corals, and one species at least gives rise to hollow swellings on the branches of a coral like the “galls” which are formed on plants by certain insects. Another crab (Melia tesselata) carries in each of its claws a living sea-anemone which it uses as an animated weapon of defence and an implement for the capture of prey. Many of the sluggish spider-crabs (Maiidae) have their shells covered by a forest of growing sea-weeds, zoophytes and sponges, which are “planted” there by the crab itself, and which afford it a very effective disguise.
Many of the larger crabs are sought for as food by man. The most important and valuable are the edible crab of British and European coasts (Cancer pagurus) and the blue crab of the Atlantic coast of the United States (Callinectes sapidus).
Among the Anomura, the best known are the hermit-crabs, which live in the empty shells of Gasteropod Mollusca, which they carry about with them as portable dwellings. In these, the abdomen is soft-skinned and spirally twisted so as to fit into the shells which they inhabit. The common hermit-crab of the British coasts (Pagurus or Eupagurus Bernhardus) is sometimes called the soldier-crab from its pugnacity. Small specimens are found between tide-marks inhabiting the shells of periwinkles and other small molluscs, but the full-grown specimens live in deeper water and are usually found in the shell of the whelk (Buccinum). As the crab grows it changes its dwelling from time to time, often having to fight with its fellows for the possession of an empty shell. Sometimes an annelid worm lives inside the shell along with the hermit and often the outside is covered with zoophytes. In some species, as in the British Eupagurus prideauxi, a sea-anemone is constantly found attached to the shell, profiting by the active locomotion of the crab and probably sharing the crumbs of its food, while it affords its host protection by its stinging powers.
| Fig. 3.—Gecarcinus ruricola (Violet Land Crab). | Fig. 4.—Portunus puber (Velvet Swimming Crab). |
| Fig. 6.—Eupagurus Bernhardus (Soldier Crab). | Fig. 5. Podophthalmus vigil (Sentinel Spinous Crab). | Fig. 7.—Pinnotheres pisum (Pea Crab). |
| Fig. 8.—Corystes Cassivelaunus (Masked Crab). | Fig. 9.—Eupagurus angulatus (a Hermit Crab). |
In tropical countries the hermit-crabs of the family Coenobitidae live on land, often at considerable distances from the sea, to which, however, they return for the purpose of hatching out their spawn. The large robber-crab or cocoa-nut crab of the Indo-Pacific islands (Birgus latro), which belongs to this family, has given up the habit of carrying a portable dwelling, and the upper surface of its abdomen has become covered by shelly plates. The stories of its climbing palm-trees to get the fruit were long doubted, but it has been seen, and even photographed in the act.