CLEARANCE AND RETENTION EFFICIENCY OF NATURAL SUSPENDED PARTICLES BY THE ROCK-POOL BIVALVE VENERUPIS-CORRUGATUS IN RELATION TO TIDAL AVAILABILITY

The natural spectrum of food particles available to, and used by, the burrowing clam Venerupis corrugatus (Gm.) was studied under contrasting weather conditions: prevailing SE wind in summer and a NE wind in winter. Seawater samples, collected hourly during a 13 h spring tidal cycle from a rock/sand pool, were analysed for particle size range (including bacteria), particulate quantity (counts, total and organic dry wt) and quality (organic content). Particle numbers increased on the flood tide, especially those <4-mu-m in diameter which rose 15-fold. Furthermore maximum particle size increased from 12-mu-m at low tide to 23 gm at high tide, while total particulate dry weight increased 3- to 4-fold to a maximum of 23 mg l-1. However total particulate dry weight never differed by more than 1.5-fold between sampling occasions. At low tide V. corrugatus filtered out particles of 5 to 9-mu-m at maximum rates of 2.3 to 3.1 l h-1 g-1; particle retention efficiencies were 70 to 100 %. At high tide particles of 8 to 13-mu-m were cleared at the fastest rates (3.2 to 4.4 1 h-1 g-1) and retained with efficiencies between 75 and 100 %. Possible mechanisms for these reponses are discussed. Such feeding behaviour enabled V corrugatus to increase gross ingestion of organic material by 65 % at high tide; even when pseudofaeces production was considered, net organic intake at high tide was still 39 % above that at low tide. V corrugatus appears to make maximum use of a natural resource whose supply is predictable with respect to tidal cycles, but highly variable in terms of particle size spectra, quantity and quality.