SHORT-TERM CHANGES IN THE ENERGY-BALANCE OF VENERUPIS-CORRUGATUS (BIVALVIA) IN RELATION TO TIDAL AVAILABILITY OF NATURAL SUSPENDED PARTICLES

This study further investigates the finding that a near doubling in cl earance rates of. Venerupis corrugatus correlated with a significant i ncrease in natural particle concentration and silt load at high tide. At this time full extension of the siphons and open valves indicated i ncreased pumping activity, whilst copious production of pseudofaeces s howed that more filtered material was being rejected at high tide. Aft er considering particle rejection, net organic ingestion (IR, mg h(-1) ) was related to clam size (W, g dry wt) by the allometric equations: IR = 5.821 W-0.649 at low and IR = 7.867 W-0.724 at high tide. This di fference was significant, as was that between quantities of pseudofaec es produced (mg h(-1)) at low (2.498 W-0.447) and high tide (4.686 W-0 .582). Particle sorting efficiencies were size-dependent, ranging from 69 % for large clams to 63 % for juveniles at low tide and from 71 to 54 % at high tide. Thus with an increase in the particle load, sortin g efficiency in adult clams changed little while that in juveniles was depressed. Absorption efficiency declined significantly from 49 % at low tide to 41% on the flood tide. Respiration and excretion rates did not differ significantly between tides; common regressions were R (ml O-2 h(-1)) = 0.785 W-0.721 and U (mu g NH4-N h(-1)) = 2.733 W-1.343. Nonetheless potential for production (P) was significantly greater at high tide, being described by P (J h(-1)) = 15.751 W-0.696 compared wi th P= 12.271 W-0.696 at low tide. Resultant net growth efficiencies we re some 10 percentiles higher (49 to 53 %, depending on size) relative to low tide (37 to 42 %), a difference indicating successful and imme diate exploitation of the tidally induced improvement in food supply. Elevated efficiencies were realised not by changes in absorption effic iency, respiration or excretion, but by an increase in clearance rates which, even after being balanced against a rise in pseudofaeces produ ction, resulted in greater net organic ingestion at high tide. Short-t erm increases in production potential thus correlated with cyclic patt erns of food supply. It is debated whether such production is a result of innate physiological regulation depending on nutritional needs or as a consequence of autonomous processes of water pumping and filtrati on efficiency.