The response of the sea anemoneCalliactis parasiticato shells of the hermit crabPagurus bernhardus

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Abstract
The five stages in the behaviour pattern by which Calliactis parasitica transfers to shells of Buccinum, normally occupied by Pagurus bernhardus, are described. The first of these, the clinging of tentacles to shell, is a trigger for the pattern as a whole; it occurs in Calliactis already settled on glass or plastic, and more consistently, in Calliactis whose pedal disks are unattached. Tests on the frequency and speed of the clinging response under various conditions gave this information: (a) individual shells used by Pagurus vary greatly in potency; (b) Calliactis cling to shells of Pecten and Mytilus, but only erratically and after long delays; (c) Buccinum shells that have never been occupied by crabs, evoke the clinging response most consistently and rapidly; (d) removal of periostracum from Buccinum shells reduces, but does not abolish, clinging, though Calliactis responds to isolated strips of periostracum as to a shell; (e) Calliactis does not cling to dummy shells, to shells boiled in alkali, or to shells coated with a thin plastic layer; (f) Calliactis does not cling to inactivated shells when these are re-occupied by crabs. It is concluded that the clinging response depends entirely on a general molluscan shell-factor. In the settlement of the foot on the shell (the fourth stage of the behaviour pattern), a specific response to shell-factor is also involved. Vertical incisions, even complete bisection, do not abolish clinging but merely slow down the orderly pattern of the transfer to a shell. Horizontal incisions impair the pattern more. Complete horizontal section, including removal of the pedal disk alone, abolishes the clinging response and the whole behaviour pattern. Interposed stimuli show features consistent with a 5-stage programme in which the completion of each stage is a trigger for the next. But each stage may be greatly prolonged, and, if interrupted, starts again. The neuromuscular links required to co-ordinate this behaviour pattern are listed and discussed.

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