Published online by Cambridge University Press: 11 May 2009
The adults ofNereis fucata (Savigny) are commensal with hermit crabs and live within the upper whorls of the shells of their hosts. The mechanism of host adoption has been examined in juveniles reared in the laboratory. There are two distinct phases in their response to their host. The first is a searching phase during which the anterior two-thirds of the worm is extended from the tube and makes wide sweeping movements in its immediate vicinity. This phase can be initiated in the laboratory solely by vibrations in the substrate; the presence of a hermit crab is not required. The second phase begins as soon as a suitable gastropod shell is touched, when the worm immediately leaves its tube and enters the shell. In the laboratory this phase can be achieved without the presence of a hermit crab, so that it is presumably primarily dependent upon the tactile stimuli received from the surface of the shell. It is argued that in nature only hermit-crab shells would commonly give this combination of stimuli to the worm and that the association betweenNereis fucata and hermit crabs is not accidental but commensal.