EXPERIMENTAL FEEDING OF THE OYSTER CRASSO STREA-GIGAS (THUNBERG) WITH3 POPULATIONS OF DIFFERENT-SIZED MODES OF THE DIATOM HASLEA-OSTREARIA(SIMONSEN)

The influence on the feeding physiology (clearance, filtration, ingest ion, absorption) of the Japanese oyster Crassostrea gigas of low conce ntrations of suspended particulate matter (SPM ranging from 2 to 20 mg .1(-1)) and high food quality (mean organic fraction = 54 %) was studi ed in the laboratory. Experimental diets were made up with three strai ns of the ''blue diatom'' Haslea ostrearia, which is responsible of th e greening of oyster-ponds. The strains were characterized by differen t sizes (48, 65 and 98 mu m respectively) but possessed the same densi ty and biochemical composition. Clearance rate was found constant at 2 .2 1.h(-1).g(-1) dry weight and therefore was not influenced by increa sing seston load. Pseudofaeces production was observed for SPM concent ration adjusted at 2.1 and 2.3 mg.1(-1), which suggests a decrease of the threshold of pseudofaeces production for diets having a high food quality. The effect of particle size on particle selection during pseu dofaeces production was not demonstrated. Digestibility fluctuated fro m 60 to 90 %, but no differences between the three sizes were recorded . Total ingestion rate, which was regulated by pseudofaeces production , levelled off at 6.51 mg.h(-1).g(-1). Organic ingestion and organic f aeces production models suggest that physiological processes in the Ja panese oyster lead to a constant organic absorption estimated at 1.22 mg.h(-1).g(-1) in this study.