Studies in the Physiology of Commensalism. IV. the Polynoid Genera Polynoë, Lepidasthenia and Harmothoe

The host-response of the polynoid Polynoë scolopendrina Savigny, commensal with the terebellid Polymnia nebulosa (Montagu), is relatively specific within the family Terebellidae. Evidence is presented that an unstable or closely bound attractant is present on the outside of the host and perhaps in tube material, but apparently absent in the mucus secreted by the host. Efforts to demonstrate its presence in ground-up whole host resulted in failure. The polynoid Lepidasthenia argus Hodgson demonstrates a similar specificity of response to its host Amphitrite edwardsi Quatrefages. The polynoid Harmothoë spinifera Ehlers, commensal with Amphitrite gracilis Grube, responds to the alternate host Polycirrus caliendrum Claparede as strongly as to its own host, and more strongly than to other species of Amphitrite. Likewise, Polynoë scolopendrina, commensal with Polymnia, responds with greater intensity to the alternate host, the eunicid Lysidice ninetta Audouin and Milne-Edwards, than to any non-host terebellid. This behaviour may be i terpreted as evidence that the specificity of at least some single commensal species for hosts which are not closely related to each other may depend upon the production of similar attractants by these hosts. The responses of three commensal populations of Harmothoe lunulata (Delle Chiaje), commensal respectively with the ophiuroid Acrocnida brachiata (Montagu), the holothurian Leptosynapta inhaerens (O. F. Miiller), and the terebellid Amphitrite johnstoni Malmgren were tested against seven alternate hosts. Each gave maximum response to its own host. However, the population from Acrocnida responded as strongly to Leptosynapta as to its own host, while giving no responses at all to any of the other five.