Revival capacity of diatoms in bivalve pseudofaeces and faeces

The revival capacity of eight species of diatoms rejected in pseudofaeces a nd faeces of a filter-feeder, the Pacific oyster Crassostrea gigas Thunberg , was assessed by experimental culture tests. Isolated from oyster fattenin g ponds near Bourgneuf Bay on the French Atlantic coast (46-47 degrees N, 1 -2 degrees W), the eight diatom species were: Amphora sp., Chaetoceros sp., Entomoneis alata, Haslea ostrearia, Nitzschia acicularis, Nitzschia closte rium, Nitzschia gandersheimiensis and Skeletonema costatum. The results sho w that none of the diatoms were totally digested and that all underwent sig nificant growth. Cell concentrations with pseudofaecal cultures were higher for all species excepted Haslea ostrearia for which the difference with fa ecal cultures was not significant. This result is consistent with the fact that diatoms rejected in faeces are processed through the bivalve gut, whil e pseudofaeces are simply formed on the gill and the palps. Nevertheless, t he significant biomass obtained with faecal cultures compared to a control, confirms that diatoms have significant survival after extracellular digest ion by the oyster. Survival was probably facilitated by the large amount of inorganic particles added to experimental diets to reproduce conditions ch aracteristic of intertidal oysters cultivated on mudflats. This variable ha s probably more significance for the revival process than the size of the m icroalgae or its ecological type in such environment.