Retention of ciliates and flagellates by the oyster Crassostrea gigas in French Atlantic coastal ponds: Protists as a trophic link between bacterioplankton and benthic suspension-feeders

International audience; In French Atlantic coastal ponds of Charente, oysters can grow under conditions where phytoplankton production is limited by nutrients exhaustion. Such ponds typically show a high concentration of ciliates and flagellates during the growing season (1 x 104 to 3 x 105 cells l-1 in June 1997). In order to evaluate the importance of the "protozoan trophic link " for energy transfer from the " microbial food web" to large benthic suspension feeders, we offered a coastal pond community of ciliates and flagellates as potential prey to the oyster, Crassostrea gigas. Clearance rate, filtered particles and relative retention efficiency were evaluated. In the grazing experiment, 94 % of ciliates and 86 % of flagellates (size between 4 and 72 μm), were retained by the oyster. Whatever their size, protists were similarly retained by the oyster gills. In terms of carbon, oyster retain on average 126 μg carbon (C) h-1 g-1 dry weight, a value over 4 times higher than reported for phytoplankton. These results indicate that a field community of protists can contribute in coastal oyster rearing ponds to the energy requirements of the oyster Crassostrea gigas. We report here the first experimental evidence of a significant retention of a protist community by oysters, supporting the role of protists as a trophic link between picoplankton and benthic filter-feeding bivalves.