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Tires can release a large number of chemical compounds that are potentially hazardous for aquatic organisms. An ecophysiological system was used to do high-frequency monitoring of individual clearance, respiration rates, and absorption efficiency of juvenile oysters (8 months old) gradually exposed to four concentrations of tire leachates (equivalent masses: 0, 1, 10, and 100 μg tire mL−1). Leachates significantly reduced clearance (52 %) and respiration (16 %) rates from 1 μg mL−1, while no effect was observed on the absorption efficiency. These results suggest that tire leachates affect oyster gills, which are the organ of respiration and food retention as well as the first barrier against contaminants. Calculations of scope for growth suggested a disruption of the energy balance with a significant reduction of 57 %. Because energy balance directs whole-organism functions (e.g., growth, reproductive outputs), the present study calls for an investigation of the long-term consequences of chemicals released by tires.

WOS:000616240600001; International audience; Early life stages (ELS) of numerous marine invertebrates mustcope with man-made contaminants, including plastic debris, during their pelagic phase. Among the diversity of plastic particles, nano-sized debris, known as nanoplastics, can induce effects with severe outcomes in ELS of various biological models, including the Pacific oyster Crassostrea gigas. Here, we investigated the effects of a sub-lethal dose (0.1 mu g mL(-1)) of 50 nm polystyrene nanobeads (nano-PS) with amine functions on oyster embryos (24 h exposure) and we assessed consequences on larval and adult performances over two generations of oysters. Only a few effects were observed. Lipid analyses revealed that first-generation (G1) embryos exposed to nano-PS displayed a relative increase in cardiolipin content (+9.7%), suggesting a potential modification of mitochondrial functioning. G1-larvae issued from exposed embryos showed decreases in larval growth (-9%) and lipid storage (-20%). No effect was observed at the G1 adult stage in terms of growth, ecophysiological parameters (clearance and respiration rates, absorption efficiency), or reproductive outputs (gonadic development, gamete quality). Second generation (G2) larvae issued from control G1 displayed a significant growth reduction after G2 embryonic exposure to nano-PS (-24%) compared to control (as observed at the first generation), while no intergenerational effect was detected on G2 larvae issued from G1 exposed embryos. Overall, the present experimental study suggests a low incidence of a short embryonic exposure to nano-PS on oyster phenotypes along the entire life cycle until the next larval generation.

International audience; The Pacific oyster Crassostrea gigas is currently being impacted by a polymicrobial disease that involves early viral infection by ostreid herpesvirus-1 (OsHV-1) followed by a secondary bacterial infection leading to death. A widely used method of inducing infection consists of placing specific pathogen-free oysters ('recipients') in cohabitation in the laboratory with diseased oysters that were naturally infected in the field ('donors'). With this method, we evaluated the temporal dynamics of pathogen release in seawater and the cohabitation time necessary for disease transmission and expression. We showed that OsHV-1 and Vibrio spp. in the seawater peaked concomitantly during the first 48 h and decreased thereafter. We found that 1.5 h of cohabitation with donors was enough time to transmit pathogens to recipients and to induce mortality later, reflecting the highly contagious nature of the disease. Finally, mortality of recipients was associated with increasing cohabitation time with donors until reaching a plateau at 20%. This reflects the cumulative effect of exposure to pathogens. The optimal cohabitation time was 5-6 d, the mortality of recipients occurring 1-2 d earlier.

International audience; Harmful algal blooms are a threat to aquatic organisms and coastal ecosystems. Among harmful species, the widespread distributed genus Alexandrium is of global importance. This genus is well-known for the synthesis of paralytic shellfish toxins which are toxic for humans through the consumption of contaminated shellfish. While the effects of Alexandrium species upon the physiology of bivalves are now well documented, consequences on reproduction remain poorly studied. In France, Alexandrium minutum blooms have been recurrent for the last decades, generally appearing during the reproduction season of most bivalves including the oyster Crassostrea gigas. These blooms could not only affect gametogenesis but also spawning, larval development or juvenile recruitment. This study assesses the effect of toxic A. minutum blooms on C gigas reproduction. Adult oysters were experimentally exposed to A. minutum, at environmentally realistic concentrations (10(2) to 10(3) cells mL(-1)) for two months during their gametogenesis and a control group, not exposed to A. minutum was fed with a non-toxic dinoflagellate. To determine both consequences to next generation and direct effects of A. minutum exposure on larvae, the embryo-larval development of subsequent offspring was conducted with and without A. minutum exposure at 10(2) cells mL(-1). Effects at each stage of the reproduction were investigated on ecophysiological parameters, cellular responses, and offspring development. Broodstock exposed to A. minutum produced spermatozoa with decreased motility and larvae of smaller size which showed higher mortalities during settlement. Embryo-larval exposure to A. minutum significantly reduced growth and settlement of larvae compared to non-exposed offspring. This detrimental consequence on larval growth was stronger in larvae derived from control parents compared to offspring from exposed parents. This study provides evidence that A. minutum blooms, whether they occur during gametogenesis, spawning or larval development, can either affect gamete quality and/or larval development of C gigas, thus potentially impacting oyster recruitment.

International audience; As other filter-feeders, Crassostrea gigas can concentrate paralytic shellfish toxins (PST) by consuming dino-flagellate phytoplankton species like Alexandrium minutum. Intake of PST in oyster tissues mainly results from feeding processes, i.e. clearance rate, pre-ingestive sorting and ingestion that are directly influenced by environmental conditions (trophic sources, temperature). This study aimed to develop a mechanistic model coupling the kinetics of PST accumulation and bioenergetics in C. gigas based on Dynamic Energy Budget (DEB) theory. For the first time, the Synthesizing Units (SU) concept was applied to formalize the feeding preference of oysters between non-toxic and toxic microalgae. Toxin intake and accumulation were both dependent on the physiological status of oysters. The accumulation was modelled through the dynamics of two toxin compartments: (1) a compartment of ingested but non-assimilated toxins, with labile toxins within the digestive gland eliminated via faeces production; (2) a compartment of assimilated toxins with a rapid detoxification rate (within a few days). Firstly, the DEB-PST model was calibrated using data from two laboratory experiments where oysters have been exposed to A. minutum. Secondly, it was validated using data from another laboratory experiment and from three field surveys carried out in the Bay of Brest (France) from 2012 to 2014. To account for the variability in PST content of A. minutum cells, the saxitoxin (STX) amount per energy units in a toxic algae (ppsT) was adjusted for each dataset. Additionally, the effects of PST on the oyster bioenergetics were calibrated during the first laboratory experiment. However, these effects were shown to depend on the strain of A. minutum. Results of this study could be of great importance for monitoring agencies and decision makers to identify risky conditions (e.g. production areas, seawater temperature), to properly assess detoxification step (e.g. duration, modalities) before any commercialization or to improve predictions regarding closing of shellfish areas.

International audience; 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.

Mortalities of oyster Crassostrea gig-as seed associated with ostreid herpesvirus OsHV-1 mu Var have been observed in many oyster-producing countries since 2008. The present study, comprised of 4 complementary experiments, aimed to identify factors associated with disease-induced oyster mortality in order to propose mitigation strategies. Our first experiment compared survival of oysters from natural spatfall with others sampled from nurseries, after thermal elevation in the laboratory from

International audience; Vibrios are frequently associated with oyster mortality; however whether they are the primary causative agent or secondary opportunistic colonizers is not well understood. Here we combine analysis of natural infection dynamics, population genomics and molecular genetics to ask (i) to what extent oysters are passively colonized by Vibrio population present in the surrounding water, (ii) how populations turn over during pathogenicity events and (iii) what genetic factors are responsible for pathogenicity. We identified several populations of Vibrio preferentially associated with oyster tissues. Among these, Vibrio crassostreae is particularly abundant in diseased animals while nearly absent in the surrounding water, and its pathogenicity is correlated with the presence of a large mobilizable plasmid. We further demonstrate that the plasmid is essential for killing but not necessary for survival in tissues of oysters. Our results suggest that V. crassostreae first differentiated into a benign oyster colonizer that was secondarily turned into a pathogen by introgression of a virulence plasmid into the population, possibly facilitated by elevated host density in farming areas.

International audience; The spread of non-indigenous species in marine ecosystems world-wide is one of today's most serious environmental concerns. Using mechanistic modelling, we investigated how global change relates to the invasion of European coasts by a non-native marine invertebrate, the Pacific oyster Crassostrea gigas.Location: Bourgneuf Bay on the French Atlantic coast was considered as the northern boundary of C.gigas expansion at the time of its introduction to Europe in the 1970s. From this latitudinal reference, variations in the spatial distribution of the C.gigas reproductive niche were analysed along the north-western European coast from Gibraltar to Norway.Methods: The effects of environmental variations on C.gigas physiology and phenology were studied using a bioenergetics model based on Dynamic Energy Budget theory. The model was forced with environmental time series including insitu phytoplankton data, and satellite data of sea surface temperature and suspended particulate matter concentration.Results: Simulation outputs were successfully validated against insitu oyster growth data. In Bourgneuf Bay, the rise in seawater temperature and phytoplankton concentration has increased C.gigas reproductive effort and led to precocious spawning periods since the 1960s. At the European scale, seawater temperature increase caused a drastic northward shift (1400km within 30years) in the C.gigas reproductive niche and optimal thermal conditions for early life stage development.Main conclusions: We demonstrated that the poleward expansion of the invasive species C.gigas is related to global warming and increase in phytoplankton abundance. The combination of mechanistic bioenergetics modelling with insitu and satellite environmental data is a valuable framework for ecosystem studies. It offers a generic approach to analyse historical geographical shifts and to predict the biogeographical changes expected to occur in a climate-changing world.

Depuis 2008, des mortalités massives d’huîtres creuses âgées de moins d’un an sont relevées sur le littoral français dès que la température de l’eau de mer atteint le seuil de 16°C. Ces mortalités de naissains sont associées à la détection du virus OsHV-1 μVar. Depuis 2010, des travaux de qualification zoo-sanitaire ont montré qu’environ un lot de naissains de captage sur deux était infecté par le virus avant l’apparition des mortalités dans le milieu naturel. En 2014, dans le cadre de la première action QUALIF, les résultats de la qualification zoo-sanitaire des naissains du captage de l’année 2013 à l’échelle nationale ont mis en évidence le caractère infectieux des maladies développées par ces naissains porteurs latents d’OsHV-1 μVar. L’objectif 2015 du second volet QUALIF consistait à reproduire le travail de qualification zoo-sanitaire OsHV-1 μVar pour les naissains du captage de l’année 2014. Les résultats (2014 et 2015) permettront d’aborder la variation interannuelle du statut zoo-sanitaire des naissains pour chacun des sites étudiés. La méthodologie de l’épreuve de qualification zoo-sanitaire consiste en une période de 1 mois d’isolement des naissains en conditions contrôlées de laboratoire. Lors de cette épreuve, la température de l’eau de mer est maintenue constante à 21°C et la mortalité est relevée tous les 10 jours. La survie finale et les analyses qPCR permettent de qualifier le statut sanitaire en terme de portage OsHV-1 μVar de l’échantillon (infecté ou non infecté). De début janvier à fin mars 2015, 39 échantillons de naissains âgés de 5 à 7 mois ont été prélevés dans 6 sites du littoral français (étang de Thau, bassin d’Arcachon, bassin de Marennes Oléron, baie de Bourgneuf, embouchure de La Vilaine et rade de Brest) pour être testés en épreuve thermique de laboratoire (ETL) dans l’outil expérimental Ifremer à Argenton. Les résultats montrent que 25 des 39 (soit 64 %) lots de naissains de captage testés ont développé des maladies en ETL associés à des mortalités cumulées variant de 6 à 72 %. Cette valeur de 64 % de lots détectés est plus élevée que celle précédemment observée en 2014 (55 %). Les résultats de cette seconde campagne de qualification zoo-sanitaire sont favorables pour les naissains de l’étang de Thau (absence de mortalité), moins favorables pour ceux d’Arcachon, Marennes Oléron, et ceux de la baie de Bourgneuf (réponses contrastées en terme de mortalité intra-site) et très défavorables pour les naissains de La Vilaine et de la rade de Brest (mortalités observées sur tous les échantillons testés). La principale conclusion de cette seconde étude de qualification zoo-sanitaire est identique à celle de 2014, à savoir qu’il est existe dans le milieu naturel en période hivernale des lots de naissains de captage infectés par OsHV-1 μVar sans développement apparent des maladies. Ces lots de naissains porteurs latents d’OsHV-1 μVar sont détectables en ETL et la présence d’OsHV-1 μVar peut alors être confirmée par qPCR. Dans le contexte actuel d’épizooties chroniques, nos résultats confirment à nouveau qu’il est possible d’identifier précocement les lots de naissains infectés ou non par OsHV-1 μVar. Ces animaux infectés sont un réservoir du virus en période hivernale. Dès que la température de l’eau de mer franchit le seuil de 16°C, ils participeront à la réémergence des maladies en milieu naturel. Par ailleurs, cette seconde étude de qualification zoo-sanitaire confirme la possibilité d’apprécier le risque sanitaire pour chaque zone de captage en fonction de la détection précoce des lots de naissains infectés par OsHV-1 μVar.

Seasonal variation in blue mussel, Mytilus edulis, retention efficiency (RE) responses to natural seston concentrations were investigated in the Lysefjord, Norway. The smallest particles shown to be effectively retained (100%) were approximately 7 μm and this was observed on only one occasion. The most common response was for RE to progressively increase from small to large particles with maximum RE at 30 to 35 μm (8 of 12 experiments). Temporal changes in the seston size distribution towards a dominance of smaller particles coincided with a decrease in the RE maxima to particles between 7 and 15 μm diameter. The RE of 1 μm and 4 μm diameter particles ranged between 14–64% and 12–86%, respectively, showing that small particles occasionally serve as a significant dietary component. The observed temporal variations in particle capture suggest a capacity of M. edulis to control particle retention mechanisms based on exogenous dietary cues that are directly or indirectly related to the ambient particle size distribution. Indirect clearance rate (CR) measurements require that all particle sizes under study are effectively retained and this has widely been assumed for particles larger than 4 μm. CR was calculated for particles ranging in size from 5 to 35 μm and the percentage reduction in CR, relative to particle sizes shown to be effectively retained, ranged from 11% to 87%. CR calculations that included all particle counts between 5 and 35 μm diameter were underestimated by a mean of 26% (0–48% range). Similar errors in diet characterization may be expected to affect particle selection and absorption efficiency measurements and the compounding of errors may become extreme when a number of physiological rates and efficiencies are integrated (e.g. scope for growth). Highlights ► Particle retention efficiency (RE) of natural seston was investigated in M. edulis. ► The smallest particles shown to be effectively retained (100%) were close to 7 μm. ► RE most often increased progressively from small to large particles. ► Maximum RE was reported at 30 to 35 μm in 8 of 12 experiments. ► Temporal changes in particle size distribution coincided with changes in RE maxima.

Many studies based on bioenergetics growth models have investigated the effects of environmental factors on oyster (Crassostrea gigas) growth and physiology. However, most of these models are site-specific and cannot be app lied to other culture sites without the re-estimation of parameters or re-formulation of some processes. We aimed to develop a gen eric growth model suitable for application in contrasting environments, with a constant set of parameters. We tested the oyster-DEB model (Bourles et al. 2009) for the stimulation of C. gigas growth in different cohorts (spats and adults) at major shellfish culture sites in France, in several years: Arcachon (1993-1994); Marennes-Oleron (2007); Quiberon (1999, 2000, 2001); Rade de B rest (2008); Baie du Mont-Saint-Michel (2003); Baie-des-Veys (2002). These different ecosystems offer a w ide range of values for the two forcing variables of the model: water temperature (range: 6-24 °C) and phytoplankton concentration (annual average: 110-700 x 10 3 cell.l -1 ). The validation data (dry flesh mass of C. gigas) were obtained from various growth surveys carried out by IFREMER. The oyster-DEB model simulated the oyster growth dynamics of both spat and adult stages of C. gigas accurately over time at the various culture sites. The model captures: i) the active spring growth; ii) the timing and amplitude of spawning events; and iii) the lean periods (i.e. loss of dry flesh mass) in autumn and winter. The half-saturation coefficient Xk is the only model parameter that varied between sites and years. This environment-specific coefficient reflects variability in the food of the oysters: quantitative and qualitative effects of the inorganic material and of the phytoplankton species on the feeding response of C. gigas. With a single set of parameters (other than for Xk), this is thus the first bio-energetic growth model for C. gigas robust enough and of a sufficiently generic nature for the accurate simulation of oyster growth in different Atlantic ecosystems. Highlights ► A generic growth model of the oyster Crassostrea gigas that is suitable for a generic application, i.e. with a constant set of parameters, in contrasting Atlantic environments and for different cohorts (spats and adults) was developed. ► The resulting oyster-DEB model simulated the oyster growth dynamics of both spat and adult stages of C. gigas accurately over time at the various culture sites. The model captures: i) the active spring growth; ii) the timing and amplitude of spawning events; and iii) the loss of dry flesh mass in autumn and winter. ► The half-saturation coefficient Xk is the only model parameter that varied between sites and years. This environment-specific coefficient reflects variability in the food of the oysters.

We compared growth simulations by dynamic energy budget (DEB) and scope for growth (SFG) models of the Pacific oyster Crassostrea gigas, cultivated in Bourgneuf Bay on the French Atlantic coast. This bay is located at a latitude in the middle of the European range of the species, and is characterized by high concentrations of suspended particulate matter (SPM) and a marked gradient between high-turbidity sites in the north (daily SPM > 500 mg L -1 ) and intermediate-turbidity sites in the south. The models use two forcing variables: seawater temperature and food density. We tested two indices of food availability: chlorophyll a and microalgal concentrations. In the SFG model, food intake is simulated by a type-II Holling functional response, as in the DEB formulation, and the effect of turbidity in both models is therefore taken into account principally through the half-saturation coefficient for this functional response. Chlorophyll a concentrations were three to four times higher at the high-turbidity site, but oyster growth rates were significantly lower at this site than at the intermediate-turbidity site. A comparison of observed and simulated values showed that the DEB model performed better than the SFG model if microalgal concentration was used as an index of food availability, with the SFG model underestimating oyster growth in summer and autumn. However, the SFG model was much more efficient if chlorophyll a concentrations were used, with the DEB model systematically overestimating summer and autumn growth. This comparison suggests that both SFG and DEB simulations could be improved, to give a more accurate description of oyster growth in a turbid environment, and that the pre-ingestive selection mechanisms used by suspension feeders in turbid environments should probably be included in the formulation of feeding processes. Highlights ► The growth of Pacific oyster was studied in a high-turbidity ecosystem. ► At the highest turbidity site, the growth was lower compared to an intermediate turbidity site, in spite of a high availability of food resources. ► Growth was simulated by two models (Scope For Growth vs. Dynamic Energy Budget) showing different simulations according to the food quantifier tested (chlorophyll a vs. cell concentration). ► Both models could be improved by including pre-ingestive feeding responses, playing a significant role for suspension-feeders inhabiting turbid environments.

A Dynamic Energy Budget (DEB) model for simulation of growth and bioenergetics of blue mussels (Mytilus edulis) has been tested in three low seston sites in southern Norway. The observations comprise four datasets from laboratory experiments (physiological and biometrical mussel data) and three datasets from in situ growth experiments (biometrical mussel data). Additional in situ data from commercial farms in southern Norway were used for estimation of biometricalrelationships in the mussels. Three DEB parameters (shape coefficient, half saturation coefficient, and somatic maintenance rate coefficient) were estimated from experimental data, and the estimated parameters were complemented with parameter values from literature to establish a basic parameter set. Model simulations based on the basic parameter set and site specific environmental forcing matched fairly well with observations, but the model was not successful in simulating growth at the extreme low seston regimes in the laboratory experiments in which the long period of negative growth caused negative reproductive mass. Sensitivity analysis indicated that the model was moderately sensitive to changes in the parameter and initial conditions. The results show the robust properties of the DEB model as it manages to simulate mussel growth in several independent datasets from a common basic parameter set. However, the results also demonstrate limitations of Chl a as a food proxy for blue mussels and limitations of the DEB model to simulate long term starvation. Future work should aim at establishing better food proxies and improving the model formulations of the processes involved in food ingestion and assimilation. The current DEB model should also be elaborated to allow shrinking in the structural tissue in order to produce more realistic growth simulations during long periods of starvation.

International audience; Successive disease outbreaks in oyster (Crassostrea gigas) beds in France have resulted in dramatic losses in production, and subsequent decline in the oyster-farming industry. Deaths of juvenile oysters have been associated with the presence of a herpes virus (OsHV-1 μvar) and bacterial populations of the genus Vibrio. Although the pathogenicity of OsHV-1 μvar, as well as several strains of Vibrio has been demonstrated by experimental infections, our understanding of the complexity of infections occurring in the natural environment remains limited. In the present study, we use specific-pathogen-free (SPF) oysters infected in an estuarine environment to study the diversity and dynamics of cultured microbial populations during disease expression. We observe that rapid Vibrio colonization followed by viral replication precedes oyster death. No correlation was found between the vibrio concentration and viral load in co-infected animals. We show that the quantity of viral DNA is a predictor of mortality, however, in the absence of bacteria, a high load of herpes virus is not sufficient to induce the full expression of the disease. In addition, we demonstrate that juvenile mortalities can occur in the absence of herpes virus, indicating that the herpes virus appears neither essential nor sufficient to cause juvenile deaths; whereas bacteria are necessary for the disease. Finally, we demonstrate that oysters are a reservoir of putative pathogens, and that the geographic origin, age, and cultivation method of oysters influence disease expression.

International audience; The controlled upwelling of nutrient-rich deep water in oligotrophic coastal regions has been proposed as a means of increasing phytoplankton and, subsequently, bivalve aqua culture production. This was tested as part of a large-scale upwelling experiment in an oligo trophic environment (Lysefjord, Norway). The mean chlorophyll a concentration in the upwelling area was consistently higher than at the control site (mean +/- SD: 3.3 +/- 1.9 and 1.5 +/- 0.6 mg chl a m(-3), respectively) during the 4 mo of controlled upwelling. After 2 mo with upwelling, the dry flesh weight of 1 yr old and 2 yr old mussels was 95% and 24% higher, respectively, than that of the mussels at the control site. The 1 yr old mussels (Mytilus edulis) at the upwelling site achieved up to 2.4-fold higher dry flesh weight compared to the control. Reproductive output was also higher at the upwelling site and only there, spawning of 1 yr old mussels was detected. Standardized clearance and respiration rates showed maximum values during the most intense period of tissue growth. Average ingestion rates were 40% higher at the upwelling than at the control site. Tissue growth and clearance rates were not correlated with the measured seston parameters, suggesting that food acquisition was responsive to other exogenous parameters and/or to increased endogenous energy demands. It was concluded that the sustained upwelling of nutrient-rich deep water in an oligotrophic fjord can increase phytoplankton biomass, resulting in improved mussel growth performance and increased aquaculture production carrying capacity. Thus, controlled upwelling represents a simple but effective ecosystem engineering approach for enhancing human food production.

WOS:000355602000023; Feeding strategies and digestive capacities can have important implications for variation in energetic pathways associated with ecological and economically important traits, such as growth or reproduction in bivalve species. Here, we investigated the role of amylase in the digestive processes of Crassostrea gigas, using in vivo RNA interference. This approach also allowed us to investigate the relationship between energy intake by feeding and gametogenesis in oysters. Double-stranded (ds) RNA designed to target the two alpha-amylase genes A and B was injected in vivo into the visceral mass of oysters at two doses. These treatments caused significant reductions in mean mRNA levels of the amylase genes: -50.7% and -59% mRNA A, and -71.9% and -70.6% mRNA B in 15 and 75 mu g dsRNA-injected oysters, respectively, relative to controls. Interestingly, reproductive knock-down phenotypes were observed for both sexes at 48 days post-injection, with a significant reduction of the gonad area (-22.5% relative to controls) and germ cell under-proliferation revealed by histology. In response to the higher dose of dsRNA, we also observed reductions in amylase activity (-53%) and absorption efficiency (-5%). Based on these data, dynamic energy budget modeling showed that the limitation of energy intake by feeding that was induced by injection of amylase dsRNA was insufficient to affect gonadic development at the level observed in the present study. This finding suggests that other driving mechanisms, such as endogenous hormonal modulation, might significantly change energy allocation to reproduction, and increase the maintenance rate in oysters in response to dsRNA injection.

The objective of the DEBIB project (Dynamic Energy Budgets in Bivalves) was to combine the French expertise of bivalve ecophysiology and modelling with the Dutch expertise in ecology and modelling to achieve the development of a general framework based on Dynamic Energy Budgets [Kooijman, S.A.L.M., 2000. Dynamic Energy and Mass Budgets in Biological Systems. Cambridge University Press, Cambridge] for the analysis of the functioning of bivalve species (growth, reproduction) in relation to the environmental conditions (water temperature, food conditions). The collaboration has yielded promising insights for studying life strategies of different bivalve species sharing the same resources and new perspectives (i) for coupling ecophysiological models with population dynamics and ecosystem approaches and (ii) for addressing important issues such as carrying capacity, trophic interactions between bivalves and sustainability of shellfish resources. Fields for future research include improvement of parameter estimation procedures and analysis of intraspecies variability in parameters.

In competition experiments, decisions are made not only about experimental conditions such as initial population densities, of course, but also about population size structure, for instance. Here we use an individual-based simulation model to study the effect of size-grading of mussels. With low individual variability, predicted yield was lower and less variable, there was no density-dependent mortality, and optimal stocking density for aquaculture was lower than with high individual variability, whereby self-thinning occurred and yield was quite variable. Thus, individual variability was a critical factor for estimating survival effects of overstocking, at the expense of precision of growth estimates. Therefore, competition experiments are inherently incomplete. We argue that in practice, incompleteness cannot be overcome by using genetic information as a covariate because evidence from the literature shows that the effect of genetic makeup in competition situations is frequency-dependent. Apparently, the only approach presently available to obtain unbiased estimates is to use a size structure similar to that of the population under study. This contrasts with a literature review of bivalve stocking experiments published in Aquaculture through the last 30 years which clearly shows that the issue of size structure of test populations has been largely overlooked. The same principles hold for competition studies in natural settings.

We examined the influence of food availability and population density on the morphometry and shell length body mass relationship of Mytilus edulis. Mussels were reared in the laboratory for 22 mo at 8 different density levels in 11 chambers supplied with natural seston at 2 different concentrations. This allowed us to assess separately the effects of food availability and mussel density. The shell length/width and shell height/width ratios were affected by food, density and time. Mussels tended to be narrower (flatter) at high density and at low food level. Therefore, narrow shells could result from reduced food concentration in high density situations without implying physical interference. Shell mass was also influenced by both food and density levels, but to a lesser extent than tissue dry mass. In contrast with soft tissue mass, shell mass increased significantly for all food and density levels between October 1995 and October 1996, The elevation of the shell length-body mass-population density relationship was lower at low food availability, The slope of the tridimensional relationship, however, increased with decreasing food level, indicating apparent asymmetric competition for all food-density treatments pooled together, This pattern, however, is misleading because mussels obviously cannot interact among chambers. Since the slopes of length mass relationships are used in predicting self-thinning exponents in space-regulated situations, it follows that self-thinning exponents in mussels should be sensitive to background food level, thus limiting the use of self-thinning relationships for resolving factors regulating growth.

We grew mussels (Mytilus edulis) under two different food regimes and eight population density levels to estimate the joint effects of density and biomass on their growth and survival and to determine the shape of the biomass-density (B-IV) relationship. Mussels were reared for 22 months, between December 1994 and October 1996, in 1-L experimental chambers supplied with natural seston. Growth in shell length, individual wet mass and ash free dry mass (m) decreased with decreasing food availability and increasing population density. Survival was negatively correlated with density but did not differ significantly between food regimes during the first year. Variations in concentration of available food did not alter the effects of crowding on mussels, as judged from the slopes of the body size-density curves. The general patterns exhibited by B-N curves were not consistent with expectations since we found 1) no classical competition-density (C-D) effect as reported in plants at intermediate competition levels, and 2) a slope of -0.648 for m-N curves in both food regimes, which was higher than expected from self-thinning (ST) theory. This value does not support present food-driven ST theory. This study introduces an unusual m-N region which combines properties of both ST and C-D effect.

Growth and maturation of female snow crab (Chionoecetes opilio) in the northwestern Gulf of St. Lawrence were elucidated using carapace size-frequency distributions, indices of molting activity, ovarian mass, and laboratory and field measures of growth per molt. Females develop in three major stages: immature, with narrow abdomen and no detectable ovaries; prepubescent, with narrow abdomen and previtellogenic followed by vitellogenic ovaries; and adult, with broad abdomen and reproductive capability. Additionally, there is an ephemeral pubescent stage represented by females temporally close to the maturity molt. A reduction in the rate of carapace and abdomen growth occurs at the passage from immature to prepubescent, because energy is diverted into germinal growth, and adulthood is reached at a terminal molt to maturity. The pattern of abdomen growth relative to carapace is complex, consisting in successive phases of low, high, and again low positive allometry with increasing size of immature-prepubescent females. Over the period 1989-1996, a few females became adult at instar VIII at 4.5 years postlarval age, but more commonly, maturity occurred at instars IX and X at ages of 5.5 and 6.5 years, respectively. Average size at maturity may be temperature dependent, and within cohorts, larger females may mature earlier than smaller females.

La conchyliculture, et principalement l’élevage de l’huître creuse, Crassostrea gigas, constitue la principale activité aquacole française. Cette activité repose, en grande partie, sur le recrutement naturel de l’espèce qui assure 70% des besoins en jeunes huîtres (naissain) : cette activité de collecte s’appelle le captage. Les deux principaux centres de captage en France sont les bassins d’Arcachon et de Marennes-Oléron. Or, depuis une dizaine d'années, sur le Bassin d'Arcachon, le captage devient très variable: à des années de captage nul (par exemple les années 2002, 2005, 2007) ou faible (2009, 2010, 2011) succèdent des années excellentes voire pléthoriques (les années 2003, 2006, 2008, 2012). A Marennes-Oléron, cette variabilité existe, mais s’avère beaucoup moins marquée. En outre, à la faveur du lent réchauffement des eaux, le captage peut désormais se pratiquer de plus en plus vers le nord. Ainsi, la Baie de Bourgneuf, mais aussi la Rade de Brest sont devenues, depuis quelques années, des secteurs où un nombre croissant d’ostréiculteurs pratiquent avec succès le captage, mais avec, là aussi, des irrégularités dans le recrutement qu’il convient de comprendre. Enfin, depuis la crise des mortalités de 2008, il se développe aussi sur l’Etang de Thau une volonté de pratiquer le captage. Afin de mieux comprendre les facteurs de variations du captage, l’Ifremer a mis en place, à la demande du Comité National de la Conchyliculture, un réseau national de suivi de la reproduction : le Réseau Velyger. Créé en 2008 sur fonds européens et financé désormais par la Direction des Pêches Maritimes et de l’Aquaculture, ce réseau apporte, chaque année, sur les écosystèmes cités précédemment, une série d’indicateurs biologiques (maturation, fécondité, date de ponte, abondance et survie larvaire, intensité du recrutement, survie du naissain) dont l’analyse croisée avec des indicateurs hydrologiques et climatiques permet progressivement de mieux appréhender les causes de variabilité du recrutement de l’huître creuse en France, modèle biologique et espèce clé de la conchyliculture française. Ce rapport présente donc les résultats 2012 de ce réseau d’observation et fait appel, pour la partie hydro-climatique, à des observations acquises par d’autres réseaux. Il détaille toutes les caractéristiques par secteur du cycle de reproduction de l’huitre creuse : maturation des adultes, période de ponte, abondance et survie des larves, intensité du captage et mortalités précoces. Il fournit ensuite une interprétation et une synthèse des résultats 2012 par secteur et à la lueur des résultats des années antérieures. Ainsi, pour l’année 2012, on retiendra les faits majeurs suivants : • Après une année 2011 particulièrement sèche, l’année 2012 s’avère plutôt humide à partir du printemps avec un gradient Nord-Sud marqué. Ce gradient se retrouve dans les concentrations phytoplanctoniques : elles sont élevées en Rade de Brest et Baie de Bourgneuf, normales à faibles du Bassin de Marennes Oléron à l’Etang de Thau. En outre, jusqu’au mois de juillet, les températures de l’air et de l’eau sont tout justes dans les normes voire déficitaires pour les sites les plus au nord. • Ces conditions hydro-climatiques ont imprimé un gradient dans la maturation et la fécondité des huitres adultes avec un indice de condition très élevé en Rade de Brest, élevé en Baie de Bourgneuf, normal à faible du Bassin de Marennes Oléron à l’Etang de Thau. En outre, le manque de chaleur jusqu’en juillet s’est traduit par une ponte plutôt tardive sur l’ensemble des écosystèmes de la côte atlantique, à l’exception de la ponte en Baie de Bourgneuf qui a été très précoce compte tenu des conditions. A contrario, le Bassin d’Arcachon enregistre la ponte la plus tardive jamais observée sur ce secteur (au-delà du 15 août). A ce titre, le mécanisme du déclenchement de la ponte mériterait une attention particulière dans les années à venir. • Cela dit, grâce à un mois d’août favorable sur le plan thermique, les cohortes larvaires présentes à cette période ont eu une survie normale à bonne (e.g. 0.3 à 0.8 %), ce qui s’est traduit par un captage modéré à bon sur l’ensemble des secteurs de la façade atlantique. Il convient cependant de noter que la ponte trop précoce en Baie de Bourgneuf, qui s’est déroulée dans des eaux fraîches, n’a pas permis une bonne évolution de la cohorte larvaire (survie de 0.03 %) et le captage reste, sur ce secteur, faible à modéré en 2012. • En conséquence, l’année 2012, très différente de l’année 2011, se caractérise par un captage qualifié de « bon » dans le Bassin d’Arcachon (~230 naissains/coupelle), modéré dans le Bassin de Marennes-Oléron (~ 180 naissains/coupelle) et en Rade de Brest (~ 100 naissains/coupelle), modéré à faible en Baie de Bourgneuf (15 naissains/coupelle) et faible à nul dans l’étang de Thau (~ 5 naissains/coupelle). Ce rapport montre aussi que l’étang de Thau joue un rôle d’exception. Malgré une ponte d’intensité normale, des abondances de larves élevées et une bonne survie des cohortes, le captage 2012 y est très faible ce qui positionne l’étang de Thau en dehors du modèle de fonctionnement admis pour le cycle de reproduction de l’huître creuse sur la façade atlantique. Il semble que, sur ce site, la métamorphose constitue un verrou biologique qu’il convient d’étudier.

Environmental conditions experienced by aquatic organisms are archived in biogenic carbonates such as fish otoliths, bivalve shells and coral skeletons. These calcified structures present an accretionary growth and variations in optical properties (color or opacity) that are used to reconstruct time. However, full and reliable exploitation of the information extracted from these structures is often limited as the metabolic processes that control their growth and optical properties are poorly understood. Here, we propose a new modeling framework that couples both the growth of a biogenic carbonate and its optical properties with the metabolism of the organism. The model relies on well-tested properties of the Dynamic Energy Budget (DEB) theory. It is applied to otoliths of the Bay of Biscay anchovy Engraulis encrasicolus, for which a DEB model has been previously developed. The model reproduces well-known otolith patterns and thus provides us with mechanisms for the metabolic control of otolith size and opacity at the scale of an individual life span. Two original contributions using this framework are demonstrated. (1) The model can be used to reconstruct the temporal variations in the food assimilated by an individual fish. Reconstructing food conditions of past and present aquatic species in their natural environment provides key ecological information that can be used to better understand population dynamics. (2) We show that non-seasonal checks can be discriminated from seasonal checks, which is a well-recognized problem when interpreting fish otoliths. We suggest further developments of the model and outline the experimental settings required to test this new promising framework.

In order to assess the potential of various marine ecosystems for shellfish aquaculture and to evaluate their carrying capacities, there is a need to clarify the response of exploited species to environmental variations using robust ecophysiological models and available environmental data. For a large range of applications and comparison purposes, a non-specific approach based on ‘generic’ individual growth models offers many advantages. In this context, we simulated the response of blue mussel (Mytilus edulis L.) to the spatio-temporal fluctuations of the environment in Mont Saint-Michel Bay (North Brittany) by forcing a generic growth model based on Dynamic Energy Budgets with satellite-derived environmental data (i.e. temperature and food). After a calibration step based on data from mussel growth surveys, the model was applied over nine years on a large area covering the entire bay. These simulations provide an evaluation of the spatio-temporal variability in mussel growth and also show the ability of the DEB model to integrate satellite-derived data and to predict spatial and temporal growth variability of mussels. Observed seasonal, inter-annual and spatial growth variations are well simulated. The large-scale application highlights the strong link between food and mussel growth. The methodology described in this study may be considered as a suitable approach to account for environmental effects (food and temperature variations) on physiological responses (growth, reproduction) of filter feeders in varying environments. Such physiological responses may then be useful for evaluating the suitability of coastal ecosystems for shellfish aquaculture. Research highlights ► We used satellite data (temperature, Chl-a) with a dynamic energy budget model. ► The model correctly describes the observed spatio-temporal variability in mussel growth. ► A large-scale application highlights the strong link between food and mussel growth. ► The methodology developed may be useful for evaluating the suitability of coastal ecosystems for shellfish aquaculture.

International audience; We studied the dynamics of stable isotopes δ13C and δ15N of an opportunistic suspension feeder the Pacific oyster (Crassostrea gigas) to better understand the factors that influence the trophic enrichment (trophic-shift, Δ) between primary producers and consumers. Most of the previous studies on this topic do not quantify mass fluxes or isotopic discrimination phenomena in the organism, which are two pillars in isotope ecology. We used a dynamic energy budget (DEB) approach (Kooijman, 2010) to quantify i) the fluxes of elements and isotopes in C. gigas soft tissues and ii) the impact of the scaled feeding level, the organism mass and the isotopic ratio of food on the "trophic-shift" Δ, and isotope turnover in tissues. Calibration and parametrization modelling were based on data from the literature. We showed that a five-fold increase in scaled feeding level leads to a decrease of the trophic-shift value of 35% for carbon and 43% for nitrogen. This can be explained by the molecule selection for the anabolic and/or catabolic way. When f increases due to the reserve dynamic formulation in the standard DEB model, the half-life of the isotopic ratio tδ 1/2 in tissues also decreases from 13.1 to 7.9 d for δ13C and from 22.1 to 10.3 d for δ15N. Organism mass also affects the trophicshift value: an increase of the individual initial mass from 0.025 g to 0.6 g leads to an enrichment of 22% for δ13C and 21% for δ15N. For a large individual, these patterns show that a high structural volume has to be maintained. Another consequence of the mass effect is an increase of the half-life for δ13C from 6.6 to 12.0 d, and an increase of the half life for δ15N from 8.3 to 19.4 d. In a dynamic environment, the difference in the isotopic ratios between the individual tissues and the food (δ13CW − δ13CX) exhibits a range of variation of 2.02‰ for carbon and 3.03‰ for nitrogen. These results highlight the potential errors in estimating the contributions of the food sources without considering the selective incorporation of isotopes. We conclude that the dynamic energy budget model is a powerful tool to investigate the fate of isotopes in organisms.

This second special issue of the Journal of Sea Research on development and applications of Dynamic Energy Budget (DEB) theory concludes the European Research Project AquaDEB (2007-2011). In this introductory paper we summarise the progress made during the running time of this 5. years' project, present context for the papers in this volume and discuss future directions. The main scientific objectives in AquaDEB were (i) to study and compare the sensitivity of aquatic species (mainly molluscs and fish) to environmental variability within the context of DEB theory for metabolic organisation, and (ii) to evaluate the inter-relationships between different biological levels (individual, population, ecosystem) and temporal scales (life cycle, population dynamics, evolution). AquaDEB phase I focussed on quantifying bio-energetic processes of various aquatic species (e.g. molluscs, fish, crustaceans, algae) and phase II on: (i) comparing of energetic and physiological strategies among species through the DEB parameter values and identifying the factors responsible for any differences in bioenergetics and physiology; (ii) considering different scenarios of environmental disruption (excess of nutrients, diffuse or massive pollution, exploitation by man, climate change) to forecast effects on growth, reproduction and survival of key species; (iii) scaling up the models for a few species from the individual level up to the level of evolutionary processes. Apart from the three special issues in the Journal of Sea Research - including the DEBIB collaboration (see vol. 65 issue 2), a theme issue on DEB theory appeared in the Philosophical Transactions of the Royal Society B (vol 365, 2010); a large number of publications were produced; the third edition of the DEB book appeared (2010); open-source software was substantially expanded (over 1000 functions); a large open-source systematic collection of ecophysiological data and DEB parameters has been set up; and a series of DEB tele-courses and symposia have been further developed and expanded, bringing together people from a wide variety of backgrounds (experimental and theoretical biologists, mathematicians, engineers, physicists, chemists, environmental sciences, computer scientists) and training levels in DEB theory. Some 15 PhD students graduated during the running time of AquaDEB with a strong DEB component in their projects and over 15 will complete their thesis within a few years. Five post-doctoral projects were also part of the training network. Several universities (Brest, Marseille, Lisbon, Bergen) included DEB courses in their standard curriculum for biology students. © 2011 Elsevier B.V.

The European Research Project AquaDEB (2007–2011, http://www.ifremer.fr/aquadeb/ ) is joining skills and expertise of some French and Dutch research institutes and universities to analyse the physiological flexibility of aquatic organisms and to link it to ecological and evolutionary processes within a common theoretical framework for quantitative bioenergetics [Kooijman, S.A.L.M., 2000. Dynamic energy and mass budgets in biological systems. Cambridge University Press, Cambridge]. The main scientific objectives in AquaDEB are i) to study and compare the sensitivity of aquatic species (mainly molluscs and fish) to environmental variability of natural or human origin, and ii) to evaluate the related consequences at different biological levels (individual, population, ecosystem) and temporal scales (life cycle, population dynamics, evolution). At mid-term life, the AquaDEB collaboration has already yielded interesting results by quantifying bio-energetic processes of various aquatic species (e.g. molluscs, fish, crustaceans, algae) with a single mathematical framework. It has also allowed to federate scientists with different backgrounds, e.g. mathematics, microbiology, ecology, chemistry, and working in different fields, e.g. aquaculture, fisheries, ecology, agronomy, ecotoxicology, climate change. For the two coming years, the focus of the AquaDEB collaboration will be in priority: (i) to compare energetic and physiological strategies among species through the DEB parameter values and to identify the factors responsible for any differences in bioenergetics and physiology; and to compare dynamic (DEB) versus static (SEB) energy models to study the physiological performance of aquatic species; (ii) to consider different scenarios of environmental disruption (excess of nutrients, diffuse or massive pollution, exploitation by man, climate change) to forecast effects on growth, reproduction and survival of key species; (iii) to scale up the models for a few species from the individual level up to the level of evolutionary processes.

A bio-energetic model, based on the DEB theory exists for the Pacific oyster Crassostrea gigas . Pouvreau et al. [Pouvreau, S., Bourles, Y., Lefebvre, S., Gangnery, A., Alunno-Bruscia, M., 2006. Application of a dynamic energy budget model to the Pacific oyster, C. gigas , reared under various environmental conditions. J. Sea Res. 56, 156–167.] successfully applied this model to oysters reared in three environments with no tide and low turbidity, using chlorophyll a concentration as food quantifier. However, the robustness of the oyster-DEB model needs to be validated in varying environments where different food quantifiers reflect the food available for oysters, as is the case in estuaries and most coastal ecosystems. We therefore tested the oyster-DEB model on C. gigas reared in an Atlantic coastal pond from January 2006 to January 2007. The model relies on two forcing variables: seawater temperature and food density monitored through various food quantifiers. Based on the high temperature range measured in this oyster pond (3–30 °C), new boundary values of the temperature tolerance range were estimated both for ingestion and respiration rates. Several food quantifiers were then tested to select the most suitable for explaining the observed growth and reproduction of C. gigas reared in an oyster pond. These were: particulate organic matter and carbon, chlorophyll a concentration and phytoplankton enumeration (expressed in cell number per litre or in cumulative cell biovolume). We conclude that when phytoplankton enumeration was used as food quantifier, the new version of oyster-DEB model presented here reproduced the growth and reproduction of C. gigas very accurately. The next step will be to validate the model under contrasting coastal environmental conditions so as to confirm the accuracy of phytoplankton enumeration as a way of representing the available food that sustains oyster growth.

Mussel aquaculture is well developed in various ecosystems of temperate waters. In the aim of developing predictive tools for management of mussel aquaculture, a better understanding of relationships between the environmental conditions and mussel growth is necessary. For this purpose, development of bioenergetic models, linking environmental variables (especially food resource and temperature) and mussel growth and reproduction, are of a particular interest. Dynamic Energy Budget (DEB) theory offers a general framework to study energy flows in organisms, from assimilation to use for maintenance, growth and reproduction. This theory is appropriate for growth modelling of marine organisms and have first been applied in 1993 to model mussel growth. Nevertheless, the different studies that applied DEB theory on the mussel Mytilus edulis lead to different parameter values though based on the same equations. In this context, our work aims to refine parameters estimation of the DEB model for M. edulis in order to obtain a generic parameter set, i.e. which allows modelling mussel growth and reproduction in various environments. For this purpose, methods which have been developed recently for parameter estimation have been applied using data from the literature. New parameter estimates are compared to previously obtained values and are validated by simulating mussel growth and reproduction in various environments.

Reared sea bass (Dicentrarchus labrax) represent 13% of the economic weight of the Mediterranean aquaculture production (FAO, 2005) and is a subject of selection program. However, the traditional goal of selection that prevailed in the last century, i.e. increasing productivity, have to be modulated by new objectives such as better feed management or increasing fish welfare. The aim of our study is to evaluate the influence of domestication and selection, and their interactions with stress, on the energy balance in sea bass (Dicentrarchus labrax). Therefore, we used two experimental datasets and analysed their results through a mathematical model. Four strains of sea bass were studied: i) Wild strain, ii) Domesticated strain (first generation obtained by reproduction in rearing condition without selection), iii) Selected strain 1 (same first generation obtained but by siring the 5% longest fish) and iv) Selected strain 2 (first generation obtained by Prosper selection (Chevassus et al., 2004)). In the first experiment (Vandeputte et al., 2009), growth was monitored while the four strains were raised separately with ad libitum feeding. In the second one, the fish were exposed to recurrent stress events, while their food consumption and their weight were monitored throughout the experiment (the food was delivered by self-feeder). Since the goal of the present study is to quantify the fish energy balance, we used a bio-energetic model to quantify energy fluxes and to compare between strains and stress conditions.

The Dynamic Energy Budget (DEB) model (Kooijman, S.A.L.M., 1986. Energy budgets can explain body size relations. J. Theor. Biol. 121, 269-282; Kooijman, S.A.L.M., 2000. Dynamic Energy and Mass Budgets in Biological Systems. Cambridge University Press, Cambridge, 424 pp.) has been adapted to describe the dynamics of growth and reproduction of the Pacific oyster (Crassostrea gigas) reared in different areas under conditions ranging from controlled to natural. The values of the model parameters were estimated from available physiological data and from published information. The sets of data used to validate the model came from three long-term growth experiments (> 5 months) performed on Pacific oysters reared under different conditions of food and environment. The forcing variables were temperature and phytoplankton densities, the latter being assessed from in vivo fluorescence and chlorophyll-a concentration measurement. The successful validation of the model on the three data sets demonstrated its ability to capture the dynamics of the energy budget in the Pacific oyster in various environments with the same set of parameters. The only parameter that varied between simulations was the half-saturation coefficient (XK), because of a different diet composition between the three environments under test. The model successfully reproduced quantitatively the growth and reproduction and the timing of spawning. These first simulation data led us to propose several promising perspectives of application for this model in shellfish ecosystems.

We present a cathodoluminescence (CL) study of growth layer deposition in the shell of the oyster Crassostrea gigas. CL is based on the physical properties of lattice-bound manganese (Mn2+), which is the main activator in calcium carbonate. Our study involved chemical marking by immersing individuals in seawater to which manganese chloride had been added, and subsequent reading of the shell with CL microscopy coupled with numeric treatment of microphotographs; CL emission was analyzed using a scanning electron microscope coupled to a spectrometer. Since the marking did not harm the oysters, repeated markings were possible, allowing validation of the inferences made from analysis of the shell rings. Oysters reared in a French Mediterranean lagoon (Thau lagoon) were marked in June and October 1999, i.e. at the beginning and end of the main shell growth period, respectively. In a few specimens sacrificed 24 h immediately after marking, we observed a bright marking line, suggesting rapid coprecipitation of Mn2+. In oysters collected from March to July 2000, we found 3 luminescent lines (with an emission peak at 620 nm peak) that corresponded to the 3 successive markings. The shells also exhibited weak background Mn2+-induced luminescence, which contrasted with the marking lines both in intensity and color. This background signal varied in intensity over time at 3 distinct scales, i.e. seasonal and daily scales plus an intermediate scale that might reflect tidal patterns. The luminescence intensity was clearly positively correlated with seawater temperature. We postulate that the temperature of the lagoon, combined with abiotic and biotic factors, may control both Mn2+ uptake by the bivalve through a stimulation of biomineralization processes, and Mn2+ bioavailability in water through biogeochemical processes. Our study confirmed the usefulness of Mn marking coupled with CL analysis to reveal (1) the complex functioning of the mantle epithelium with respect to carbonate deposition on a microscale, and (2) the relationship between the bivalve and its environment.

A Dynamic Energy Budget(DEB) model was used to analyse growth and toxin elimination processes in musselsthat had been exposed to Diarrhetic Shellfish Toxins (DST). After DST exposure mussels were placed inan experimentalin-situ setup, expected to have low concentrations of toxic phytoplankton, and DST concentrations in mussel tissueswere regularly monitored along with temperature and seston concentration and composition in the water. A one-compartment model for DST concentrations and elimination rate was integrated with the DEB model, assuming that storage and structural compartments absorb DST. The model was used to simulate growth and toxin elimination rates from mussel tissues. Toxin elimination rate was obtained by a calibration between observed and simulated DST concentrations in mussel tissues over the experimental period. The results demonstratethat tissue concentrations of DST decrease temporally due to the dilution effects from growth, but dilution alone wasnot sufficient to explain the observed elimination rates. Thus there seems to be an active elimination processof toxins in mussels, which in conjunction with dilution through growth contributes to the decreasing DST concentrations in mussels over time.

Oyster farming is the main aquaculture activity in France, where Pacific oyster (Crassostrea gigas) is spatially cultured from the English Channel to the Mediterranean coasts. Oyster growth performance monitored along French coasts are widely heterogeneous among culture sites and over years. Many studies have been carried out to understand the effects of environmental factors on oyster growth and physiology, by using bioenergetics growth models. However, most of these studies were site-specific. As an example, the model developed by Barillé et al. (1997), which was extensively specified with more than 50 parameters, was validated in the Marennes-Oléron Bay, but cannot be successfully applied to other culture sites without re-estimating parameters or re-formulating some processes. In that context, our study aims to develop a generic growth model, i.e. a model than can be applied in various contrasted environments with a constant set of parameters. We used the DEB model designed for the Pacific oyster C. gigas (Pouvreau et al., 2006; Bourlès et al., 2009) based on the DEB theory (Kooijman, 2000). The resulting oyster-DEB model is built on 10 main DEB parameters with some extra parameters to consider specific bioenergetics of C. gigas. Only one DEB parameter, i.e. the half-saturation coefficient Xk which depends on food quality and therefore on ecosystems, was calibrated for each site in order to evaluate the generality and the limits of the model. The results of simulating oyster growth are presented herein by using the oyster-DEB model in several French ecosystems: Arcachon, Marennes-Oléron and Quiberon bays along the Atlantic coastline, Thau Lagoon in Méditerranée, and the Baie du Mont-Saint-Michel and the Baie des Veys in the English Channel. Applying DEB model in contrasted environments allows to explain oyster growth variability among rearing areas according to food quantity (e.g. chlorophyll a, phytoplankton abundances) and quality (phytoplankton species), as well as seawater temperature. The variability of spawning events among sites is also successfully simulated according to the spawning processes implemented in the oyster-DEB model.

France being the largest consumer of oysters in Europe, oyster farming is deeply rooted in French heritage. The Japanese oyster (Crassostrea gigas) is the oyster species the most exploited in France, and in the world. Due to filter-feeding, these bivalves are sensitive to toxic algal blooms. Although not always lethal, toxic algae can affect oyster physiology and make it unfit for human consumption. Phytoplankton toxins can be classified in several groups: amnesic, neurotoxic, diarrhetic and paralytic. For the latter group, saxitoxins are synthesized by the microalgae of genus Alexandrium that can accumulate bivalve tissues. In recent years, number of studies have been performed on the interactions between C. gigas and saxitoxins. In order to better understand these interactions, mathematical models have been developed but they did not allowed to describe accurately the kinetics of accumulation and detoxification of paralyzing toxins (PSTs). Models on DEB theory (Dynamic Energy Budget) (Kooijman, 2000) have been widely applied to the study of bivalve bio-energetics. This type of model already allowed to quantify growth and reproduction of C. gigas under different environmental forcing. These models have also been applied to study host-pathogen interactions (Flye-Sainte-Marie et al., 2009) and kinetics of accumulation and detoxification of contaminants (Bodiguel et al., 2009; Echinger et al., 2010). The aim of this PhD work is to develop a model based on DEB theory, that describes the interactions between PSTs and oysters. Indeed, different effects of PSTs contamination on oyster physiology have been shown. For example, paralytic toxins alter the immune response (overproduction and phagocytosis of hemocytes), behavior (modification of valve rhythms, production of pseudo-faeces) or organ integrity (myoatrophy, inflamed gills). In this PhD project, we will focus first on describing the accumulation and detoxification kinetics of PSTs in C. gigas and second on quantifying the effects of PSTs on the physiology of C. gigas.

Introduction The blue mussel (Mytilus edulis) culture occurs in temperate waters around the world under a wide range of environmental conditions, e.g. at phytoplankton concentrations below as 1-2 µg L-1 (in Norwegian fjords) up to 9-10 µg L-1 (in French Atlantic coastal sites). Under such contrasted food resources, the aim of our study is to develop a generic bio-energetic model for M. edulis, i.e. that can be applied in various contrasted environments with a constant set of parameters, to simulate growth and reproduction of the blue mussel. Such a model will allow us to assess directly the links between mussel growth performances and environmental parameters in different culture sites.

La conchyliculture représente une activité importante en Normandie et doit faire face à de sérieuses difficultés (mortalités, diminution de la qualité des produits). Un enjeu actuel concerne la réorganisation des zones d’élevage existantes afin d’optimiser les performances. A contrario, la pisciculture marine est très peu présente dans cette région et il existe un réel enjeu de développement économique. Dans ce contexte, les décideurs (services de l’Etat) et les professionnels ont besoin d’identifier des sites propices à la réalisation de ces activités qui soient également compatibles avec les nombreux autres usages du littoral. Au sein du projet SISQUONOR, un démonstrateur sous Système d’Information Géographique, nommé AkvaVis et développé originellement pour l’aquaculture norvégienne, a été transféré et adapté à la conchyliculture normande. L’application française SISAQUA intègre des données spatialisées variées telles que des sorties de modèles biophysiques (courants, vagues…), des données issues des satellites (température de l’eau, concentration en phytoplancton), d’autres données environnementales (bathymétrie, type de substrat…) ainsi que des informations liées au cadre réglementaire (cadastres conchylicoles, plans de protection environnementale…) nécessaires à la gestion et planification des activités conchylicoles. Des indicateurs spatiaux ont ensuite été construits afin d’identifier des sites propices à l’élevage de moules et d’huîtres. L’un de ces indicateurs est basé sur les données satellites et la simulation de la croissance des bivalves à l’aide de modèles d’écophysiologie. Des développements futurs seront conduits dans le cadre du projet H2020 AquaSpace (2015-2018) et incluront notamment la production piscicole.

Biological performances (growth and reproduction) of intertidal bivalves mainly rely on environmental factors such as water temperature and food sources. Both quality and quantity of bivalve food sources, however, are not easy to determine because of complex features of coastal ecosystem functioning, such as high spatial heterogeneity. This is particularly critical for oyster (Crassostrea gigas) culture in France when quantifying and explaining the variability in growth performances of oysters among culture sites at a large spatial scale. In this context, insights for new operational tools to characterise trophic features of coastal ecosystems are needed. The aim of the present study is to couple natural isotope tracers and DEB modeling to describe and quantify bioenergetic processes and isotope dynamics in oyster tissues under varying environmental conditions (temperature, food sources). The model will be tested and validated on carbon and nitrogen isotope data of oyster tissues measured in experimental controlled mesocosms. Once validated, the resulting DEB-isotopes model will be used in inverse method to provide biological indicators (e.g. nutrition level, food sources, availability of reserves, maintenance, growth rate) about oysters cultured in different shellfish areas and to allowcomparison of yields among culture sites.

Since 2008, mass mortalities of 1-yr-old oysters Crassostrea gigas associated with ostreid herpes virus OsHV-1 μVar have occurred along all coasts of France. These mortality events represent the most serious crisis for the French oyster industry since the introduction of C. gigas in the early 1970s. Temperature is commonly one of the major triggering factors of disease epizootics, notably for aquatic species. In France, OsHV-1 is generally detected in dying oysters when seawater temperature is higher than 16° C. The influence of temperature on OsHV-1 detection and expression was also demonstrated for C. gigas larvae and suspected for juveniles. The present study aims to characterize the effect of temperature on disease transmission and survival related with ostreid herpesvirus OsHV-1 µVar in juvenile C. gigas. To examine the effect of seawater temperature on disease transmission and related mortality of oysters, we conducted two sets infection experiments based on a unique biological material, i.e. healthy naive oysters that are free of mortality and negative for OsHV-1 µVar and Vibrio bacteria, which cohabitated with oysters previously exposed (challenged) to natural field conditions in areas where mortalities were occurring. The first set of experiments was carried out at 13.0° C and 20.6° C to investigate whether survival of healthy naive oysters was influenced by cohabitation with presumably infected (challenged) oysters, i.e. that were exposed to field conditions with surrounding mortalities. The second set of experiments aimed to investigate whether disease transmission and related survival of oysters were influenced by seawater temperature. Challenged oysters were placed in contact with naive oysters at 8 temperatures ranging from 13.4° C to 29.0° C. Animals were regularly sampled for OsHV-1 DNA detection by real-time qPCR and for quantification of Vibrio bacteria on CHROMagar media. The optimal temperature range for disease transmission from challenged (field-exposed) to unexposed (naive) cohabiting animals was between 16.2° C and 21.9° C, which corroborated field observations (Pernet et al. 2012). Our results suggested that a long-term period (40 days) at low temperature (13° C) may offer a way to stop or mitigate mortalities in oysters that have been exposed to an infecting environment.

International audience; Oyster diseases caused by pathogenic vibrios pose a major challenge to the sustainability of oyster farming. In France, since 2012 a disease affecting specifically adult oysters has been associated with the presence of Vibrio aestuarianus. Here, by combining genome comparison, phylogenetic analyses and high-throughput infections of strains isolated before or during the recent outbreaks, we show that virulent strains cluster into two V. aestuarianus lineages independently of the sampling dates. The bacterial lethal dose was not different between strains isolated before or after 2012. Hence, the emergence of a new highly virulent clonal strain is unlikely. Each lineage comprises nearly identical strains, the majority of them being virulent, suggesting that within these phylogenetically coherent virulent lineages a few strains have lost their pathogenicity. Comparative genomics allowed the identification of a single frameshift in a non-virulent strain. This mutation affects the varS gene that codes for a signal transduction histidine-protein kinase. Genetic analyses confirmed that varS is necessary for infection of oysters and for a secreted metalloprotease expression. For the first time in a Vibrio species, we show here that VarS is a key factor of pathogenicity.