Fig. 214.—The Long-armed Crab (Corystes Cassivelaunus)
These interesting crabs have been spoken of as the monkeys of the sea, and the comparison will certainly be tolerated by anyone who has watched the creatures as they climb among the corallines and sea firs in an aquarium. Among such growths they are quite at home; and although their movements do not often suggest the extreme agility of the monkey tribe, yet the ease with which they seize the branches of the submarine forest with their long ‘arms’ and pull their bodies from one tree-like structure to another is decidedly monkey-like. Their comparison with the long-legged spiders is also a happy one as far as their general form and movements are concerned, but it must be remembered that they have not the same reputation for cruel, predaceous habits, for they are more truly the scavengers of the deep, subsisting mainly on the decomposing bodies of their dead associates. The movements of most spider crabs are so slow and deliberate that one can hardly imagine them capable of anything of the nature of violent action; yet, when occasion requires it, they will sometimes strike at the object of their wrath with a most vigorous snap of their claws.
Fig. 215.—Spider Crabs at Home
In these crabs, too, we find most interesting instances of protective resemblance to their surroundings. Some of the small, slender-legged species are not to be recognised without a careful search when they are at rest among clusters of sea firs, their thin appendages and small bodies being hardly discernible in the midst of the slender, encrusted branches, and their peculiar forms are still more concealed by their colouring, which generally closely resembles that of the growths among which they live. Further, the carapace of spider crabs is in itself a garden on which thrive low forms of both animal and vegetable life. Minute Algæ, and occasionally some of moderate size, are rooted to the shell, often securely held by the aid of the rough hairs and tubercles that are so characteristic of the exo-skeletons of these creatures; and patches and tufts of animal colonies that have found a convenient settlement on the moving bed still further serve to obscure the nature of the living mass below—a mass that is always in danger of becoming the prey of the fishes which inhabit deep water. It is probable, therefore, that this association is one that is beneficial to both sides as far as the animal life is concerned, the lower species serving to disguise the true nature of the crab, thus protecting it from its numerous enemies, while they in return are conveyed, carriage paid, to the feeding-grounds, where they can freely partake of the fragments that become diffused in the surrounding water.
Our illustration on [p. 288] shows three species of spider crabs, all of which are common on parts of our shores. The Scorpion Spider Crab (Inachus dorsetensis) derives its specific name from the fact that it was first found off the coast of Dorset; but it is abundant off many of our shores, both in the south and north, and may frequently be seen entangled among the fishermen’s nets. It may be distinguished from other and similar species by the four spines arranged in a line across the front portion of the carapace, and the five large, pointed tubercles behind them. This species is undoubtedly a favourite food of the cod, for several specimens may often be taken from the stomach of a single fish.
The next species—The Slender-beaked Spider Crab (Stenorhynchus tenuirostris)—is seldom missing from the dredgings hauled in off the south-west coast, and is fairly common in other parts. Its legs are extremely slender, and bear spines on the inner side, and its body, where free from the incrustations so often covering the carapace of spider crabs, is of a fresh pink colour.
The other one shown in the same illustration is Arctopsis lanata, sometimes known as Gibb’s Crab, the carapace of which is pointed behind, bears a large pointed tubercle on each side, and is completely covered with a thick clothing of stiff hairs. It is also common on many parts of our coasts, more especially the coasts of Devon and Cornwall.