Amongst the first living creatures to greet the visitor as he lands on the beach of a coral island in the Pacific, is a small species of Hermit-Crab, belonging to the genus Coenobita, which frequents the beach in great numbers. The crab withdraws itself just within the mouth of the shell, where it forms a perfect operculum, by means mainly of the large flattened chelæ of the left great claw which is arched over by the left leg of the third pair, whilst the right claw and the right leg of the second pair serve to complete the shield The most plucky and pugnacious of these little crabs are those which occupy cast-off Nerita shells, a character which probably arises from their consciousness of the solid strength of the home they have chosen: and, strange to say, the tiny bosses on the surfaces of the large pincers, which are outermost in the improvised operculum, resemble similar markings on the outer side of the operculum of the Nerita (N. marmorata, Hombr and Jacq), whose shell they often inhabit. Mr. Darwin[474] observed that the different species of hermit-crabs, which he found on the Keeling Islands in the Indian Ocean, used always certain kind of shells; but I could not satisfy myself that such was the case in the instance of the Solomon Island hermit-crabs. In the case of the common beach species of Coenobita, I found, after carefully examining a number of individuals to satisfy myself of their being of the same species, that shells of the genera Turbo, Nerita, Strombus, Natica, Distorsio, Truncatella, Terebra, Melania, &c., &c., contained the same species of Coenobita, whether the individual was large enough to occupy a Turbo shell of the size of a walnut or sufficiently small to select the tiny shell of the Truncatella for its home. Another species of the same genus prefers usually the vicinity of the beach; but it may occur at heights up to 200 feet above the sea. It is rather larger than the beach species, and differs amongst other characters in the more globose form of the large claws and in the greater relative size of the left one. It occupies shells of different kinds, such as those of Nerita, Turbo, &c. A still larger species, which frequents the vicinity of the beach, usually selects Turbo shells, apparently because of their larger size. All the other species of Coenobita, which I met with, used, when I touched them, to withdraw themselves within their shells and close them up at once with their claws; but this kind, when I caught hold of the Turbo shell that it carried, left the shell behind in my fingers with apparent unconcern and crawled leisurely away, displaying, somewhat indecorously, the rudimentary plates on the back of its abdomen. These are the plates that attain their greatest development in the Cocoa-nut Crab (Birgus latro), which is thus able to dispense with a shell altogether. The greatest heights at which I found hermit-crabs were in the island of Faro on the two highest peaks, which are elevated respectively 1600 and 1900 feet above the sea. In both these localities, the crab had reached the very summit and could not have climbed higher. The species was apparently different from, though closely allied to, the common beach species, and frequented the shells of a land-snail (Helix). I was indebted to Lieutenant Heming for directing my attention to the hermit-crab, found 1900 feet above the sea. It appears to me likely that these hermit-crabs will be found at much greater heights in this group, since, in this island, their ambition to rise had carried them up as far as they could go.
[474] “Journal of the Beagle,” p. 457.
Other species of hermit-crabs, that are common in these islands, belong to the genus Pagurus. They are conspicuously distinguished from the species of Coenobita, above described, by their first pair of claws, which are small and weak and ill-adapted for defensive purposes. For this reason, these species are less able to look after themselves; and since they cannot form the operculum-like shield with their claws at the mouth of the shell, they always choose shells which will permit of their retiring well within it, so as to be out of the reach of their enemies. Some species are found in the stream-courses and in the brackish water near their mouths, when they often frequent cast-off Melania shells. Other species (?) prefer the sea-water on the reef-flats. I noticed one individual that displayed its eccentricity of disposition, in selecting, as its abode, the hollow tube of a small water-logged stick, about six inches long, which it dragged about after it during its peregrinations, and into which it retreated when alarmed. On one occasion, I observed a large Dolium shell, moving briskly about in a pool of salt-water, which, on picking up, I found to be tenanted by a Pagurus, so ridiculously small, in comparison with the size of the shell, that when frightened it retreated to the very uppermost whorl, and, notwithstanding the wide mouth of the shell, could not be seen. So light was the weight of the crab, that, on account of the buoyancy of its shell, it floated lightly on the surface of the water, on which I had placed it with the mouth of the shell uppermost, and was blown by a slight breeze across a pool of water, some twenty yards in width. While it was afloat, the shrewd little occupant retired to the innermost recess of its home; but as soon as the shell had grounded, it protruded its head and pincers and endeavoured to overturn the shell, which it finally succeeded in accomplishing.
In the case of these two genera of hermit-crabs, Coenobita and Pagurus, it was interesting to notice the relation existing between the defensive capabilities of the crab, and the relative size of the shell it selected as its home. The Pagurus, with its weak slender pincers, chooses large shells within which it can retire well out of reach when alarmed. The Coenobita, with its stout pincers, prefers shells much smaller, relatively speaking, and ensconces itself snugly in the body whorl, forming an operculum with its claws. As the hermit-crab, Coenobita, crawls along the dry sand of a beach, it leaves behind it characteristic pinnate tracks which may be often traced for several feet. The lateral markings are produced by the claws and legs working on each side of the shell; whilst a central groove is formed by the weight of the shell itself. As shown by the arrow in the [diagram], the lateral markings point in the direction of the course which the hermit-crab has taken. Sometimes only a single row of lateral tracks accompanies the grooves produced by the shell. Such markings were produced by a hermit-crab when frightened by my approach. It turned its front towards me, and crawled backwards, by working most of his claws and legs on one side of the shell. In the case of the larger hermit-crabs, which are much less frequent on the beach, each limb produces a distinct print on the sand; but with the small species of Coenobita which infests the beach, each lateral marking, as shown in the diagram, is produced by a single movement of the claws situated on the same side of the shell. The hermit-crabs only leave their tracks on the dry loose sand. One individual, that I placed on sand, still wet from the retreating tide, crawled along without leaving any impression. I have described these impressions with some care, as they bear on the origin of the surface-markings of rocks of shallow-water formation, a subject recently discussed in the geological world. It is highly probable that some of the larger and heavier forms of the Anomura (and, in fact, of the Decapoda generally) would produce prints such as I have here described, both on mud-flats left dry by the tide, and on the soft bottom in shallow depths. A cast of the impressions thus produced would have an unmistakeable plant-like form.
Whilst examining the island of Simbo, I noticed some singular Medusæ in a small mangrove-swamp, which is inclosed in the low point that forms the south shore of the anchorage. Numbers of these organisms of a large size (8 or 9 inches across the umbrella), and of a dirty-white colour, were lying on the mud with their tentacles, uppermost in depths of from one to three feet of water. I was struck by the handsome mass of arborescent tentacles which they displayed, and by the peculiarity of their lying upside-down. The dark mud which formed the bottom of the swamp was composed of decayed vegetable matter, confervoid growths, diatoms, and a few infusoria: but when I raised up these Medusæ, I found underneath each a patch of white sand corresponding with the outline of the organism, but completely concealed by the umbrella when the Medusa lay in its usual position. The sand was derived from corals, shells, and the volcanic rocks of the island; and the light patches formed a marked contrast with the dark mud around. I was unable to find any satisfactory explanation of these curious patches of sand; and I, therefore, proceeded to interrogate the Medusæ on the subject by watching them, but to no purpose. So I had my revenge by turning them all over on their tentacles, when each one immediately began to contract its umbrella in a most methodical fashion, and, after swimming a short distance, deliberately resumed its former position of tentacles upward. I had an extensive experience of mangrove-swamps after we left Simbo; but these self-willed Medusæ never came under my notice again.[475]
[475] I referred to the habits of these Medusæ in “Nature,” Nov. 9th, 1882.
With regard to these Medusæ, I should remark that they belong to a species of Polyclonia, and are classed amongst the Scypho-Medusæ.[476] Two species of Polyclonia seem to be known, P. frondosa (Agassiz) and P. Mertensii(Brandt), the first found in the Florida seas, and the latter in the Carolines. I am inclined to think that the Solomon Island species is more nearly allied to P. Mertensii. Both species, however, have similar habits, lying on the mud of mangrove-swamps, with their tentacles uppermost.