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

In French Atlantic coastal ponds of the Charente, oysters can grow under conditions where phytoplankton production is Limited by nutrient exhaustion. Such ponds typically show a high concentration of ciliates and flagellates during the growing season (1 x 10(4) to 3 x 10(5) cells 1(-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 inn), were retained by the oyster. Whatever their size. protists were similarly retained by the oyster gills. In terms of carbon, oysters retain on average 126 mu g 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 C, 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.