PARTICLE SELECTION IN THE OYSTER CRASSOSTREA-GIGAS (THUNBERG) STUDIEDBY PIGMENT HPLC ANALYSIS UNDER NATURAL FOOD CONDITIONS

Selective feeding in the adult oyster Crassostrea gigas was studied us ing natural particle assemblages in the estuarine environment of the B assin de Marennes-Oleron (France). Oysters were exposed to natural var iations of suspended particulate matter by direct pumping of sea water from the bay during various tidal cycles. Experiments were conducted during the spring over both spring tides and neap tides, so as to obta in a maximum range of variation in seston characteristics, spring tide being characterized by very strong currents with high loads of resusp ended materials (sediment, microphytobenthos and detritus). Particles were analysed for their chloropigment content by HPLC. The native pigm ents consisted exclusively of chlorophyll a and c, from both benthic a nd pelagic diatoms. Phaeophorbides were the main degradation products both in oyster faeces and in particles present in resuspended material , mainly composed of biodeposits. Comparisons between the composition of water-column particulate matter and pseudofaeces showed a negative selection against these detritus types by the oyster. This selective c apacity was either not effective or was undetected at very high seston loads. Implications for the nutrition of Crassostrea gigas living in estuarine environments are discussed.