PHYSIOLOGICAL AND ECOLOGICAL STUDIES ON THE BIVALVE MOLLUSK ARTHRITICA BIFURCA (WEBSTER, 1908) LIVING COMMENSALLY WITH THE TUBICOLOUS POLYCHAETE PECTINARIA AUSTRALIS EHLERS, 1905 ROBERT G. WEAR r/V/i '//(.' University of IJ'clliui/ton, New Zealand Known commensal bivalves from New Zealand waters are the galeommatid Scintillona zclandica (Odhner) described in association with the synaptid holo-thurian Trochodota doidyi by Morton (1957), Coriarcits neozelanicus Dell (Leptonidae) recorded by Dell (1963) as attached to a new species of the echinoid genus Cyclastcr in deep water, and two species of the leptonid genus Arthritica. A. crassiforniis Powell is found living with the large rock-boring pholad Anchoniasa siutilis (Gray), and Arthritica liiilinci Ponder lives in association with the sea-mouse Ap/irodita aitstralis Baird. The biology and commensal relationships of these two species of Arthritica have been described by Ponder (1965). The third New Zealand species of the genus Arthritica, A. bifnrca (Webster). was considered by Ponder (1965) to be free-living before I noted its association with Pcctinaria aitstralis. I would like to thank Professor J. E. Morton and Mr. W. F. Ponder for co-operation in this work, and Drs. R. K. Dell and R. B. Pike for constructive criticism of the manuscript. SEASONAL VARIATIONS IN PECTINARIA AUSTRALIS AT PETONE BEACH (Fig. 3) The tidal rise and fall at Petone Beach is 4.5 feet (1.37 meters) at spring tides which at low water uncover the sand for a distance of approximately 50 m. Pcctinaria a us trails is found at the high water neap tide level and occurs down to low water spring tides, but may extend below this level. P. aitstralis lives within a conical tube of cemented sand grains in fine muddy sand. The broader anterior end of the tube is directed downwards and the narrow posterior end lies near the surface of the sand (Fig. 1). The location of P. australis on the exposed sand surface is usually indicated by a characteristic small exhalant hole one or two millimeters in diameter. In the aquarium. P. australis showed no evidence of a secondary inhalant hole in the sand near the tube, as illustrated for P. koreni by Watson (1928. p. 31. Fig. 2 ) but draws the inhalant current from around the sides of its tube as shown in Figure 1. Samples were taken at low tide during the third week of each month between April, 1964. and March, 1965. Preliminary sampling showed Pectinaria to have a very patchy distribution. The position of these patches or "beds" on the beach 141
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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