In situ suspension feeding behaviour of the pearl oyster, Pinctada margaritifera: combined effects of body size and weather-related seston composition

In situ clearance rate (CR) and biodeposition of the black pearl oyster, Pinctada margaritifera, were followed during several field experiments from 1996 to 1998 in the lagoon of Takapoto. Serial measurements of total particulate matter (TPM, mg l(-1)), particulate inorganic matter (PLM, mg l(-1)), and particulate organic matter (POM, mg l(-1)) were related to meteorological conditions, especially wind speed. As a general case, POM and PLM increased with wind speed. Nevertheless, PLM increased faster than POM so that the organic content (OC, %) of the TPM decreased progressively when wind speed increased. These TPM variations induced direct changes in feeding processes of P. margaritifera. CR (l h(-1)) averaged 22 l h(-1) for a pearl oyster of 1 g dry tissue weight and varied with POM, PLM and dry tissue weight (W, g) according to the following equation: CR = 26.96 (PIM-0.42POMW0.61)-W-0.96. This clearance activity appeared to be the highest of those mentioned for bivalve species in their natural habitats. Pseudofaecal (PF, mg h(-1)) production started for very low PIM load (i.e., 0.17 mg l(-1))and POM load (i.e., 0.28 mg l(-1))in water and followed the equation: PF = 32.6(POM - 0.28)(PIM - 0.17)W-0.77. In other bivalve species, PF are generally observed for higher PIM or POM levels. These PF were mainly constituted of mineral matter (more than 80% in weight). Faecal production (F, mg h(-1)) increased with seston load and reached progressively a plateau (i.e., maximal intestinal transit time) as shown by the equation describing the faecal biodeposition F = 20 W-0.49(1 - e(-0.66) (TPM)). The quantity and the composition of the faecal biodeposit were in a range commonly found in literature. This work confirms, by in situ experiments, previous results obtained in laboratory and especially that P. margaritifera has developed a trophic strategy which consists of processing large amounts of water to gain sufficient energy in poor waters. But this work also showed that meteorological conditions have indirect influences on feeding processes of pearl oyster by modifying significantly the concentration and the composition of seston.