PHYSIOLOGICAL AND ECOLOGICAL STUDIES ON THE BIVALVE MOLLUSK ARTHRITICA BIFURCA (WEBSTER, 1908) LIVING COMMENSALLY WITH THE TUBICOLOUS POLYCHAETE PECTINARIA AUSTRALIS EHLERS, 1905

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Abstract
1. The bivalve mollusk Arthritica bifurca (Webster), previously considered to be free-living, has been found in large numbers living commensally with the tubicolous polychaete Pectinaria australis Ehlers. 2. Arthritica bifurca is attached by several fine byssus threads to the outer surface of the head-end of the tube of Pectinaria australis, orientated vertically with its anterior end directed downwards. Biological reasons for this commensal relationship are obscure. 3. Ecological observations on the Peclinaria australis population at Petone Beach, Wellington Harbour, suggest that the polychaete lives for only 12 or 18 months, and is subject to an extremely high mortality rate during growth and maturation and also after breeding in the spring. There is evidence for a fairly extended breeding period, and the population is probably replenished mainly during late summer by settlement of larvae at or near the high tide level. 4. Larvae of Arthritica bifurca are incubated by the parent at least to the shelled prodissoconch stage. A short planktonic larval life supported by pelagic or postpelagic host-finding behavior is the probable means by which the commensal bivalve is dispersed. 5. Physiological experiments with live material established that A. bifurca is able to distinguish between sea water that has passed over the host and sea water which has not done so. The bivalves are attracted chemotactically to Pectinaria australis by way of exhalant water currents from the tube worm, and there is a direct orthokinetic response as the bivalves move closer to the host.

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