International audienceWe developed a full life-cycle bioenergetic model for the great scallop Pecten maximus relying on the concepts of the Dynamic Energy Budget (DEB) theory. The covariation method was implemented to estimate the parameters of a standard DEB model. Such models are able to predict various metabolic processes from a food availability marker and temperature in the environment. However, suspension-feeders are likely to feed on various trophic sources, from microalgae cells to detritus. They are also able to sort and select food particles very efficiently, depending on their size, energetic value or quality. The present model includes a mechanistic description of the feeding processes, based on Kooijman's Synthesizing Unit principle which allows to deal with several food sources. Moreover we tested the hypothesis of a differential selectivity between two potential substrates (phytoplankton cell and the remaining particulate organic matter). Simulations of shell length, daily shell growth rate, dry weight and gonado-somatic index (GSI) variations were realized and compared to field data from a monitoring conducted in the Bay of Brest (Brittany, France) for six years. The model shows its capacity to efficiently reproduce all life history traits of the wild great scallops. Predicted length data were estimated to the nearest millimeter. The fit of simulated weights to observed data was very satisfactory. GSI predictions were also in accordance with observations but improvements are required to better capture the sharp increase of gametogenesis at the beginning of the year. Finally, results bring evidences that P. maximus is actually preferentially feed on living algae cells rather than on the rest of organic particles

Brown Ring Disease (BRD) is a bacterial disease caused by the pathogen, Vibrio tapetis. The disease induces formation of a brown deposit on inner shell of the Manila clam, Ruditapes philippinarum. Development of this disease is correlated with a decrease in the condition index of infected clams. Experiments were conduced in order to assess the effect of the development of BRD on two parameters affecting the energy balance of the clams: the clearance and the respiration rates. Experiments were performed in a physiological measurement system that allowed simultaneous measures of clearance and respiration rates. During both acclimation and measurements clams were fed with cultured T-iso and temperature was close to seasonal field temperature (10 • C). Our results showed that severely diseased clams (conchiolin deposit stage, CDS ≥ 4) are subject to weight loss in comparison to uninfected ones, indicating that BRD induces a desequilibrium in the energy balance. We demonstrated a reduction of the clearance rate of severely diseased clams which led to a decrease in energy acquisition. Respiration rate showed asignificant decrease with BRD symptoms, but evidence in the literature allowed us to hypothesize that energy mobilised for an immune response and lesion repair increases overall organism maintenance costs. Both factors should thus contribute to the degradation of the energy balance of diseased clams. Because effects of BRD on naturally infected clams only appears significant for CDS ≥ 4, when brown ring assumes a significant place on the inner shell, we consider that the Manila clam is tolerant of low disease levels.

This study was designed to assess the contribution of feeding behavior to inter-individual variability of paralytic shellfish toxin (PST) accumulation in the Pacific oyster Crassostrea gigas. For this purpose 42 oysters were exposed for 2 days to non-toxic algae and then for 2 other days to the PST producer Alexandrium minutum. Individual clearance rate (CR) of oysters was continuously monitored over the 4 days using an ecophysiological measurement system. Comparison of CR values when exposed to toxic and non toxic algae allowed to estimate a clearance rate inhibition index (CRII). Toxin concentration of oysters was quantified at the end of the experiment. These data allowed to estimate the toxin accumulation efficiency (TAE) as the ratio of toxin accumulated on toxin consumed. Changes of clearance rate during the experiment indicated that all individuals stopped feeding immediately after being exposed to A. minutum for at least 7 h. This fast response likely corresponded to a behavioral mechanism of avoidance rather to a toxin-induced response. Individuals also showed high inter-variability in their recovery of filtration after this period. Most of the inter-individual variability (78%) in PST accumulation in C. gigas could be explained by the consumption of A. minutum cells, thus emphasizing the importance of the feeding behavior in accumulation. Based on the toxin concentration in their tissues, oysters were clustered in 3 groups showing contrasted patterns of PST accumulation: the high accumulation group was characterized by high feeding rates both on non-toxic and toxic diet and subsequently a low CRII and high TAE. Inversely, the low accumulation group was characterized by low filtration rates, high CRII and low TAE. Both filtration capacity and sensitivity of oysters to toxins may account for the differences in their accumulation. The contribution of TAE in PST accumulation is discussed and might result from differences in assimilation and detoxification abilities among individuals.

RNA-Seq transcriptome data were generated from mantle tissue of the great scallop, Pecten maximus. The consensus data were produced from a time course series of animals subjected to a 56-day thermal challenge at 3 different temperatures. A total of 26,064 contigs were assembled de novo, providing a useful resource for both the aquaculture community and researchers with an interest in mollusc shell production.

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We combined the use of pigments, fatty acids and sterols as biomarkers of the seasonal variation in food sources of the great scallop Pecten maximus. From March to October 2011, on a biweekly to twice-weekly basis, scallops and seawater from the water column and the water-sediment interface were collected in the Bay of Brest (Brittany, France). Pigment compositions in the seawater and in the stomach and rectum content of the scallops were analyzed by HPLC. Fatty acids and sterols from digestive gland (DG) tissue were analyzed by gas chromatography. Potential relationships between the temporal proportion of each marker in the environment and in the digestive tract were tested using multivariate analysis. Proportions of diatoms and Dinophyceae biomarkers alternated in the digestive tract of P. maximus. This switch of feeding was confirmed by the polar lipid composition of the DG. Peridinin, characterizing Dinophyceae, occurred in high proportions in the digestive tract compared to the low ambient concentration, suggesting a selection of this microalgae group by the scallop. Chlorophyceae and green macroalgae tracers were found in low proportions, suggesting they were not actually ingested. Markers of Prymnesiophyceae were also observed at significant levels. Cyanobacteria tracers showed that this microalgae class was not ingested by the scallops during monitoring but may be of higher importance during winter. Switching from one food source to another as well as selectivity in feeding are discussed relative to the season.

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A field monitoring of hemocyte parameters of the Manila clam Ruditapes philippinarum was conducted from July 2004 to September 2005 in Gulf of Morbihan (France), in order to assess (1) the factors controlling the hemocyte parameters of the Manila clam and (2) their relative contribution to the overall variability of these parameters. Monthly, sixty clams were sampled and Total Hemocyte Count (THC), granulocyte and hyalinocyte counts, phagocytosis, phenoloxidase specific activity, length, flesh dry weight, and condition index were measured individually. Perkinsus sp. infection and Brown Ring Disease symptoms were also monitored.Temperature and trophic resource were also monitored.Results indicate that temperature controls granulocyte counts and subsequently THC. Other environmental factors had no direct influence on the measured hemocyte parameters. Almost all measured parameters were significantly affected by size/age and condition index. There were poor relationships between both pathologies and hemocyte parameters presumably because of low infection intensities. Nevertheless, high Perkinsus sp. infection intensity significantly increased total and granulocyte counts and decreased phagocytosis.An interesting result of this study is that the measured biotic and abiotic factors poorly contribute to the explanation of the total variability of hemocyte parameters. Granulocyte concentration was the best explained parameter. However, only 16.4% of its variance was explained by cumulating temperature, length, condition index and Perkinsus sp. infection effects. This study emphasizes the need for a better understanding of hemocyte functions and the factors modulating these functions.