STUDIES IN THE PHYSIOLOGY OF COMMENSALISM

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Abstract
A modified olfactometer was developed which made it possible to determine whether commensal worms of the Polynoid genus Arctonoe are attracted by specific substances to their hosts. Arctonoe fragilis, commensal with the star Evasterias troschelii. are clearly attracted to their host and can distinguish between water containing their host and sea-water alone. Arctonoe (pulehra-vittata ?) commensal with the cucumber Stichopus are likewise attracted to their host. Arctonoe fragilis commensal with Evasterias are not attracted to the non-host star Pisaster which occurs in the immediate environment of their host. Arctonoe (pulchra-vittata?) commensal with Stichopus are not attracted to the non-host Cucumaria Arctonoee (pulchra-fragilis?) commensal with the mud-star Luidia are not attracted, under exptl. conditions at least, to their own host. Arctonoe fragilis commensal with Evasterias are not attracted to the mud-star Luidia, spite the fact that Luidia may be a host of fragilis. These results are perhaps inconclusive, since injury substances may have been released into the apparatus by the mud-stars. Cross expts. (A. fragilis from Evasterias tested against Stichopus and A. pulchra-vittata from Stichopus tested against Evasterias) indicate that attractants released by the 2 hosts are not the same, since neither commensal is attracted to its relative''s host. The possibility that olfactory conditioning may play a part in the establishment of these host-commensal relationships is discussed, also the importance of such conditioning as a mechanism of isolation.
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