REVIVAL CAPACITY OF DIATOMS IN BIVALVE PSEUDOFAECES AND FAECES

Abstract

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