Pearl oysters are farmed in oligotrophic tropical atoll lagoons where planktonic communities are dominated by production from cyanobacteria smaller than 2 pm. Paradoxically, the pearl oyster Pinctada margaritifera only retains particles larger than 2 Pm. In this study, we assess the relative contribution of hetero/mixotrophic microbiota to the available planktonic resource. In Takapoto Atoll, picocyanobacteria are the dominant biomass (20 mug C l(-1)). The carbon biomass of ciliates and dinoflagellates ranges from 1 to 24 and 0.5 to 5 mug C l(-1) respectively, with a mean of 6 mug C l(-1) for ciliates and 2 mug C l(-1) for dinoflagellates. The possible retention by P. margaritifera on a natural protist suspension was investigated. Due to its high clearance rates (ca 20 l h(-1) g(-1)) the pearl oyster retained 85 mug C h(-1) g(-1) from ciliates and 55 mug C h(-1) g(-1) from dinoflagellates. Conversely, cyanobacteria were not efficiently retained by the bivalve and did not efficiently contribute to its diet. From our experiments, we concluded that hetero/mixotrophic protists rapidly and efficiently process the picoplanktonic resource towards filter-feeders, particularly pearl oysters.