A Year at the Shore

by Philip Henry Gosse

It had often been an interesting speculation with me, in what manner the due relation of size is maintained between the Adamsia and the shell, in the progressive growth of the former. There is a manifest proportion between the two, the young Cloaklet occupying a small shell, such as that of a Littorina or Trochus; the full-grown individual a large one, such as that of a Natica or Buccinum. The Crab is able to shift from a smaller to a larger shell when he needs enlarged accommodation; and since we know that his congener, P. Bernhardus, does this habitually, we naturally conclude that such is the habit of P. Prideauxii. Presuming then that this is the case, what becomes of the Adamsia? If the Crab shifts his quarters and leaves the Adamsia behind, the association is broken, and we should constantly find Paguri without Adamsiæ, and Adamsiæ without Paguri. But we find neither the one nor the other.

On the other hand, if Adamsia is able to shift its quarters also, how does it proceed in its search for a new shell? If it forsakes the old tenement at the same time as the Crab, and together with it takes possession of the new one, by what means is unity of will and action secured? What communication of thought takes place from the one to the other? As the Adamsia does not adhere to the Crab, but to the shell, that is, as they are independent of each other’s movements, who takes the initiative? Who goes to seek the lodging? And at what point of the transaction does the other come in? All these questions I had mused upon with interest; and at length received some light towards their solution.

On the 10th of January, 1859, I obtained, by dredging, in Torbay, a specimen of Adamsia palliata, about half-grown, on a rather small shell of Natica monilifera, tenanted by a Pagurus Prideauxii, which seemed already too big for his habitation. Having put them into a well-established tank of large dimensions, the contents of which were in excellent condition, I succeeded in doing what I had never done before, domiciliating both Crab and Adamsia. Both continued in the highest health, and became quite at home.

After about three months, however, I noticed that the Adamsia was not looking so well. One side or wing had gradually loosed its hold of the shell-lip, so that it hung loosely down beneath the breast of the Crab. Yet in other respects the zoophyte seemed healthy. Latterly, too, the Crab had manifested symptoms of uncomfortable straitness, in the great extrusion of his fore-parts; so great, indeed, as to expose even the front of the soft abdomen. Yet I felt reluctant to present to the Crab a larger shell, fearing that he would, in availing himself of it, desert his zoophyte friend, which would then die, and I should lose the specimen.

At length the desire to solve a problem in science prevailed over this feeling. A fact is better than a specimen. And so (on April 21st) I selected from my cabinet a full-grown Natica shell, and placed it on the tank-floor, not far from the disconsolate trio.

The Pagurus presently found the new shell, and immediately began to overhaul it. He did not do, however, as his brother Bernhard would have done, at once shift into the new house. Having turned it mouth upward, he took hold of the outer and inner lip, each with a claw, and began to drag it about the tank. Occasionally he relinquished the hold of one claw, and probed the interior in the usual manner, and then resumed his march. I watched the proceedings for an hour or more, when, having other work to do, I left him alone.

The thought did occur to me—Can this delay be intended to make the Adamsia cognizant of what is in contemplation, and to prepare it for the change? But I dismissed it as unlikely.

After about an hour’s absence I returned to the examination. The Pagurus was comfortably lodged in his new abode, and the old one, which now looked small indeed, lay deserted at some little distance. I eagerly turned the latter over, to see what was the condition of the Adamsia. Lo! no Adamsia was there; and the Pagurus, presently approaching the front of the tank, I saw, to my great gratification, that the old association was unbroken. There was the Adamsia, with one wing adhering to the lip of the new shell, and apparently the opposite wing also; but, from the position of the group, this I could not be quite certain of. The situation of the zoophyte was quite normal,—the centre immediately below the breast of the Crab, and in contact with the inner lip of the shell, while that wing which I could clearly see was creeping round upon the outer lip.

Examining now more closely the condition of things, with a lens, I saw that the central part of the Adamsia’s base was adherent by a small point of its surface to the under side of the thorax of the Crab, between the bases of the legs.