Studies in the physiology of commensalism. I. The polynoid genus arctonoe.

Significant work on the role of specific chemical factors in controlling hostparasite or host-commensal relationships has been published by Thorpe and Jones (1937) and by Welsh (1930, 1931). The former authors have demonstrated the significance of olfactory conditioning in host-selection by parasitic insects. Welsh has shown that the normally positively phototactic sign of the mite Unionicola is reversed by a substance released from its host, the fresh-water clam Anodonta, and also that the sign of three species of Unionicola occurring in clam hosts of different genera can be reversed by substances from their hosts alone. To the writer's knowledge, however, no experimental work has been done on any of the legion of hostparasite or host-commensal associations to be found in the sea, nor has it been demonstrated whether or not in any of these a chemical bond exists between host and partner. The members of the polynoid genus Arctonoe present an interesting commensal complex. These worms can readily be collected in Puget Sound both in the intertidal zone and by dredging; along with their hosts they are in many ways admirably suited to experimental work. In Table I the host genera of the three described species of Arctonoe are listed, as given by Pettibone (1947) and the Berkeleys (1948). It is to be noted that A. fragilis (Baird) is listed as commensal with certain starfish only, while A. pulchra (Johnson) and A. vittata (Grube) are commensal with certain starfish, holothurians, gastropods, chitons and terebellids. The records would seem to show that some hosts (see arrows) may harbor two species of commensals, though there are no records at hand of an individual host with more than one species of commensal. The cucumber Stichopus, although not listed as a host of vittata by these authors, is included on the basis of commensals identified for the writer by Dr. Olga Hartman, according to whom the commensals of Stichopus may show some characteristics of pulchra and some of vittata. While some commensal populations of Arctonoe appear to be relatively distinct on anatomical grounds (markings, condition of neuropodial setae and margins of elytra), others are variable and appear to include intergrades. By dealing with the complex as made up of populations on certain specific hosts it seemed that it might be possible to cast some light on the basic physiology of the partnerships and thereby, incidently, on the taxonomic problems involved. The primary question arose: "How are these worms bound to specific hosts, existing as they may in environments