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International audience; Significant mortality of Crassostrea gigas juveniles is observed systematically every year worldwide. Pacific Oyster Mortality Syndrome (POMS) is caused by Ostreid Herpesvirus 1 (OsHV-1) infection leading to immune suppression, followed by bacteraemia caused by a consortium of opportunistic bacteria. Using an in-situ approach and pelagic chambers, our aim in this study was to identify pathogen dynamics in oyster flesh and in the water column during the course of a mortality episode in the Mediterranean Thau lagoon (France). OsHV-1 concentrations in oyster flesh increased before the first clinical symptoms of the disease appeared, reached maximum concentrations during the moribund phase and the mortality peak. The structure of the bacterial community associated with oyster flesh changed in favour of bacterial genera previously associated with oyster mortality including Vibrio, Arcobacter, Psychrobium, and Psychrilyobacter. During the oyster mortality episode, releases of OsHV-1 and opportunistic bacteria were observed, in succession, in the water surrounding the oyster lanterns. These releases may favour the spread of disease within oyster farms and potentially impact other marine species, thereby reducing marine biodiversity in shellfish farming areas

International audience; The Thau lagoon, located at the South of France, provides 10% of the French pacific oyster Crassostrea gigas production. Despite this intensive shellfish production, the larval recruitment of this species within the Thau lagoon is still misunderstood and spat collection remains highly variable; as a consequence, shellfish farmers rely mostly on hatchery”s spat. The present study aimed to describe reproduction features of C. gigas within the Mediterranean, as well as to explore spawning triggers of this species. To achieve this goal, the reproduction cycle from gametogenesis to spawning was followed during two years in several locations within the Mediterranean Thau lagoon. The spawning behavior of C. gigas within the Thau lagoon appeared to be slightly different from the one of the Atlantic Coast, showing several spawning events from June to October. The minimal temperature for massive spawning was 23 °C, which is much higher than temperatures mentioned in the literature for this species. A strong relationship was found between phytoplankton communities and gametogenesis, the latter being improved by a higher diatom/dinoflagellate ratio. Temperature was insufficient alone to explain spawning, highlighting other potential triggers such as the moon or lightning strikes. We made the hypothesis of a hierarchy of events triggering spawning, encompassing locking, synchrony and intensifying triggers. Statement of relevance This paper is filling in a gap in the literature regarding to reproduction of C. gigas in the Thau lagoon, which is always associated to temperature. However, temperature may not be the only factor involved in oyster reproduction, that is what we tried to show in this paper.

Japanese oysters spatfields were recently discovered in the French Mediterranean Thau lagoon farmed for shellfish. This discovery led to interesting issues concerning research for a highly seashell exploited ecosystem. The analyses of various environmental parameters characterized favorable sites and periods for spat collection. Spat collection in Thau lagoon presented high temporal and spatial heterogeneities related to hydrodynamic, predation, competition and trophic resource variability. Here, the results highlight various explicative factors of the Japanese oysters recruitment within Thau lagoon and underline the high ecological variability in the studied system under oligotrophication.

WOS:000561438700036; International audience; Shellfish aquaculture has the potential to alter benthic assemblage composition with subsequent modifications of ecosystem functioning. While the impacts of aquaculture on the taxonomic structure of macro- and — to a lesser extent — meiofauna, have been widely studied, the functional changes of these communities remain relatively unknown. The recent development of biological trait analysis (BTA) has made it possible to produce information about how ecosystem functioning could change across specific terrestrial or aquatic system. In the present study, we used a BTA in parallel with standard taxonomic analysis to evaluate how well the two approaches detected the potential influence of juvenile oyster culture on the benthic community in the French Mediterranean Thau Lagoon. Two sites were sampled under farm structures and compared with two reference sites beyond the influence of farming. This study is the first detailed parallel description of macro- and meiofauna (nematodes). A total of 118 and 41 taxa were determined for macrofauna and nematodes respectively. Some taxa were more abundant or exclusively observed under farm structures such as Chaetozone gibber and Neanthes acuminata for macrofauna and Daptonema fallax and Anticoma eberthi for nematodes. Overall, our results indicate that biological traits and functional indices both detected the impact of shellfish culture on benthic assemblages, whereas macrofauna taxonomic indices revealed no difference. Our results thus suggest that trophic and ecological groups are particularly good indicators of the effects of shellfish culture. This study confirms the relevance of the functional approach, and more generally of multi-index approaches, to detect the influence of aquaculture on benthic communities. Further work is required to test multiple traits in different regions and under different systems, but this work paves the way for environmental impact assessment using a trait based approach.

International audience; Since 2008, juvenile Crassostrea gigas oysters have suffered from massive mortalities in European fanning areas. This disease of complex etiology is still incompletely understood. Triggered by an elevated seawater temperature, it has been associated to infections by a herpes virus named OsHV-1 as well as pathogenic vibrios of the Splendidus Glade. Ruling out the complexity of the disease, most of our current knowledge has been acquired in controlled experiments. Among the many unsolved questions, it is still ignored what role immunity plays in the capacity oysters have to survive an infectious episode. Here we show that juvenile oysters susceptible to the disease mount an inefficient immune response associated with microbial permissiveness and death. We found that, in contrast to resistant adult oysters having survived an earlier episode of mortality, susceptible juvenile oysters never exposed to infectious episodes died by more than 90% in a field experiment. Susceptible oysters were heavily colonized by OsHV-1 herpes virus as well as bacteria including vibrios potentially pathogenic for oysters, which proliferated in oyster flesh and body fluids during the mortality event. Nonetheless, susceptible oysters were found to sense microbes as indicated by an overexpression of immune receptors and immune signaling pathways. However, they did not express important immune effectors involved in antimicrobial immunity and apoptosis and showed repressed expression of genes involved in ROS and metal homeostasis. This contrasted with resistant oysters, which expressed those important effectors, controlled bacterial and viral colonization and showed 100% survival to the mortality event. Altogether, our results demonstrate that the immune response mounted by susceptible oysters lacks some important immune functions and fails in controlling microbial proliferation. This study opens the way to more holistic studies on the "mass mortality syndrome", which are now required to decipher the sequence of events leading to oyster mortalities and determine the relative weight of pathogens, oyster genetics and oyster-associated microbiota in the disease.

International audience; The accumulation of EHMC, OCT and OD-PABA, three common UV filter compounds, was investigated in marine mussels. Wild Mytilus edulis and Mytilus galloprovincialis were sampled in ten sites along the French Atlantic and Mediterranean coasts from June to November. In mussel tissues, 100% of the samples had quantifiable EHMC concentrations ranging from 3 to 256 ng g− 1 dry weight, while 55% of the samples had detectable OCT concentrations ranging from under 2 to 7 112 ng g− 1 dry weight. These concentrations significantly increased with the rising air temperature in summer, the recreational pressure and the geomorphological structure of the sampling sites (its lack of openness to the wide). This is the first study to report bioaccumulation of UV filters in marine mussels, thus highlighting the need for further monitoring and assessment.

We developed a growth model for the oyster Crassostrea gigas cultured in Thau Lagoon. The oyster standing stock in the lagoon ranged between 10,000 and 15,000 tons a year. Two culture methods are presently in use in Thau Lagoon which are used in about the same proportions. At seeding, initial size of oysters is different among methods. The model was calibrated on (1) growth data accounted for both culture methods and (2) hydrobiological data (temperature, salinity, suspended particulate matter and chlorophyll a ), both recorded in several sites in the lagoon between March 2000 and October 2001. The lagoon is slightly eutrophic: total chlorophyll a and total particulate matter averaged 1.2 μg l −1 and 2.2 mg l −1 , respectively. Organic content accounted for ca. 40–50% of particulate matter. There was no seasonal trend in seston, whereas temperature and salinity were minimal in winter. Oyster growth varied among sites in response to spatial variations in seston. Growth was maximal in summer and minimal in winter because of temperature seasonality. For each location, we modelled growth as a function of particulate organic matter and temperature. Chlorophyll a was left out of the model because of a weaker fit with growth. Growth was modelled as G = a POM b T c Y d , where G is the growth rate in shell length (mm day −1 ) or in mass (g day −1 ), POM is particulate organic matter (mg l −1 ), T is temperature (°C) and Y is either shell length (mm) or mass (total individual mass or dry flesh mass in g). Allometry ( Y d ) allowed us to use the same model for both culture methods. The model yielded a good fit with actual size, either as measured by shell length ( R 2 =0.96) or total individual mass ( R 2 =0.93).

International audience; We present the first large-scale, high-resolution spatial and temporal pattern of disease mortality caused by Ostreid herpesvirus 1 (OsHV-1) that has affected Pacific oyster Crassostrea gigas in France every year since 2008, in relation to local farm use. Mortality was monitored in healthy oyster spat deployed at 106 locations in the Thau Mediterranean lagoon during the epizootic that occurred in spring 2011. We conducted a field survey of farming practices in the areas where the sentinel oysters were placed before the epizootic. We found that the risk of disease mortality in sentinel oysters was higher within the bivalve farming area than outside it, suggesting that the infection pressure is much higher in areas with intensive farming activity. The risk of mortality was higher in farms rearing spat, a developmental stage particularly susceptible to OsHV-1, than in farms rearing adult oysters, which are generally more resistant. Additionally, the mortality risk in farms rearing adult animals was similar to that in empty farms, which suggests that, during the study period, adult oysters did not vector the disease. Interestingly, the mortality risk for sentinel oysters deployed in mussel Mytilus galloprovincialis farms was lower than for those held in oyster farms or empty farms, suggesting that mussels reduced the infection pressure on susceptible oysters.

International audience; Although spatial studies of diseases on land have a long history, far fewer have been made on aquatic diseases. Here, we present the first large-scale, high-resolution spatial and temporal representation of a mass mortality phenomenon cause by the Ostreid herpesvirus (OsHV-1) that has affected oysters (Crassostrea gigas) every year since 2008, in relation to their energetic reserves and the quality of their food. Disease mortality was investigated in healthy oysters deployed at 106 locations in the Thau Mediterranean lagoon before the start of the epizootic in spring 2011. We found that disease mortality of oysters showed strong spatial dependence clearly reflecting the epizootic process of local transmission. Disease initiated inside oyster farms spread rapidly beyond these areas. Local differences in energetic condition of oysters, partly driven by variation in food quality, played a significant role in the spatial and temporal dynamics of disease mortality. In particular, the relative contribution of diatoms to the diet of oysters was positively correlated with their energetic reserves, which in turn decreased the risk of disease mortality.

Experiments were conducted to investi- gate the effects of farming sites, seeding date, age of oysters, ploidy level and rearing structures on mor - tality, pathogens and energetic reserves of oysters Crassostrea gigas. Mortality of oysters coincided with infections involving, primarily, the ostreid herpesvirus OsHV-1 and, secondarily, bacteria of the group Vibrio splendidus, when seawater temperature was between 17 and 24°C. Mortality started in early May and lasted until late September, with an interruption in July and August when seawater temperatures rose above 24°C. Mortality varied with farming site: oysters maintained in the open Mediterranean Sea showed no mortality and OsHV-1 was occasionally detected at a low level only, while oysters maintained in the farming area of the Thau lagoon exhibited mass mortality events concomitantly with outbreaks of OsHV-1. Mortality of oysters maintained in the Thau lagoon but outside the oyster farming area was sporadic and co incided with currents coming from the farming area where mortality was occurring. Mortality of oysters also varied with farming practices. Naive oysters remained susceptible to pathogen-related mortality pressure during their first 2 yr, whereas oysters that had survived a mass mortality event in the past appeared resistant. Mor - talities of diploid (2n) and triploid (3n) oysters were similar during spring but mortalities of 2n oysters were double those of 3n oysters during summer and autumn. Finally, oysters grown in Australian baskets suffered 80% mortality compared with only 30% among oysters cemented onto ropes. The effects of farming practices on mortality are discussed in relation to pathogens and energetic reserves of oysters. OPENEN ACCESSCESS