| Résumé | Oysters accumulate relatively low levels of domoic acid (DA) compared to other bivalves. Mafra et al. (2009, in this Theme Section) identified feeding mechanisms of oysters that may lead to low DA accumulation during monospecific blooms of Pseudo-nitzschia multiseries. However, several different species of Pseudo-nitzschia, as well as other diatoms and flagellates, may co-occur during a bloom. Therefore, the present study investigates pre-ingestive feeding processes that operate when oysters Crassostrea virginica are exposed to mixed phytoplankton assemblages containing P. multiseries of varying cell length. Guided by video-endoscopy, material transported along the ventral and dorsal gill tracts was sampled and analyzed to determine the site for sorting of microalgae on the pallial organs. There was no preferential rejection of P. multiseries in pseudofeces when oysters were exposed to the alga in a mixed suspension with other diatom species (Thalassiosira weissflogii or Chaetoceros muelleri). In contrast, P. multiseries was preferentially rejected when mixed with the flagellates Isochrysis galbana or Rhodomonas lens, suggesting a qualitative mechanism for particle sorting. This occurred on the gills, followed by further selection on the palps. Oysters also preferentially rejected larger P. multiseries cells (82 to 90 µm) relative to smaller ones (24 to 28 µm) on the gills, while no further selection based on size occurred on the palps. This effect is attributed to the fact that P. multiseries cells with a length that exceeds the width of the principal filament aperture (ca. 68 µm) are more likely directed to the ventral tract and rejected in pseudofeces. These findings offer an additional explanation for the relatively low DA levels found in oysters during natural Pseudo-nitzschia spp. blooms. |
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