International audienceSummer mortality of Pacific oysters is known in several countries. However no specific pathogen has been systematically associated with this phenomenon. A complex combination of environmental and biological parameters has been suggested as the cause and is now starting to be identified. A high genetic basis was found for survival in oysters when a first generation (G1) was tested in three sites during summer. This paper presents a synthesis on physiological characteristics of two selected groups (‘R' and ‘S', from families selected for resistance and susceptibility to summer mortality respectively), of the second and third generations. R and S showed improvement or reduction of survival compared with the control in both field and laboratory trials confirming the high heritability of survival of juveniles <1 year old. Interestingly, no correlation was observed between growth and survival. Comparison between the two selected groups showed that S oysters invested more energy in reproduction and stayed a longer time without spawning than R oysters which had high synchronous spawning. This was mainly shown with high rather than low dietary rations (respectively 12% and 4% DW algae/DW oyster) in a controlled experiment. Moreover, early partial spawning was detected in S oysters and not R ones in the high dietary ration. S showed a higher respiration rate and an earlier decrease in absorption efficiency than R during gametogenesis, but they were not significantly different in glycogen or ATP utilisation. Two months before a mortality episode, hemocytes from S oysters had a higher adhesive capacity than R hemocytes and significantly higher reactive oxygen species production capacity. One month before mortality, S oysters had the highest hyalinocyte concentration and their expression of genes coding for glucose metabolism enzymes (Hexokinase, GS, PGM, PEPCK) was significantly lower in the labial palps. After a thermal increase from 13 °C to 19 °C, during 8 days in normoxia, S oysters showed a large HSP70 increase under hypoxia contrary to R oysters, suggesting their high susceptibility to stress. Their catalase activity was lower than in R oysters and showed no further change to subsequent hypoxia and pesticide stresses, in contrast to R oysters. These observations suggest possible links between higher reproductive effort in S oysters, their specific stress response to temperature and hypoxia, ROS production, partial spawning, hyalinocyte increase and the infection process. To compare R and S oysters in a more integrated way, a suppression subtractive hybridisation (SSH) library and a micro-array strategy are being undertaken

Summer mortality has been reported in the Pacific oyster, Crassostrea gigas, for many years in different parts of the world. The causes of this phenomenon are complex. The multidisciplinary program "MOREST", coordinated by IFREMER, was initiated to understand the causes of summer mortality of Crassostrea gigas juveniles in France and to reduce its impact on oyster production. Within this program, three successive groups of bi-parental families were bred in a hatchery in 2001 and placed in the field during summer in three sites (Ronce, Rivière d'Auray, and Baie des Veys). This paper reports the relative importance of family, site and field placement timing for three characters of major importance for oyster production: survival, growth, and yield. At the end of the summer period, significant differences for the three characters were observed among sites and families for each group. Family effect was the largest variance component for survival, representing 46% of the total. Variance component analysis revealed that variation in yield among families depended either on survival or on growth according to the site. Significant family × environment interactions were observed for yield and survival but not for growth. No difference in survival was found among groups in the three sites at the end of the experiment, but a critical period of mortality was identified from late July until early September. The influence of environmental conditions, notably on reproductive allocation and its relationship with the studied traits, is discussed.

Oyster mortality was monitored at multiple sites along the French coastline (5 500 km) between 1993 and 2005. Mortality data for one-and two-year-old oysters were collected over 10-12 years in 39 oyster culture sites bordering 3 different "seas": the English Channel, Bay of Biscaye and Mediterranean. Combining these data with records from environmental monitoring databases, 11 of these sites had consistent chronological data sets including both environmental data and mortality records. Mortality in one-year-old oysters was clearly summer mortality (49% of their annual mortality) whereas mortality of two-year-olds occurred mostly in spring (51%). Analysis of variance revealed that "coastal area" was the main influence on mortality of one-year-olds (77.5%% of the variance) and that "year studied" was the main influence on mortality of two-year-olds (60.6% of the variance). The highest mortalities occurred in Marennes and in several sites in Brittany for both age groups, and in Veys Bay (Normandy) for two-year-old oysters only. Environmental parameters were then analysed to investigate which of these might influence summer mortality. Principal Component Analyses revealed that environmental factors such as chlorophyll a (food resource indicator) and salinity (watershed effect) influence oyster mortality. Chlorophyll a concentration (10% of the variance), water temperature (7% of the variance) and turbidity (5% of the variance) are the main significant factors for the mortality of one-year-olds, while salinity and chlorophyll a have more effect on the mortality of two-year-old oysters (respectively 5% and 4% of the variance).

Mortality and biological performances of halfgrown Crassostrea gigas were studied from spring 2000 to autumn 2001 at six instrumented stations located in two areas (Gefosse and Grandcamp) of the Bay of Veys (Normandy). Shell and meat growth, condition indexes and a macroscopic maturity index were determined on oysters deployed at the six stations in order to assess spatial variability in the influence of environmental conditions. In addition, histological and biochemical analyses were performed in order to determine the sex and establish the reproductive cycle (at all six sites) and the biochemical composition (at four stations). The data set including monthly mean temperatures and data provided by examination of 2,837 oysters were analysed by Principal Component Analysis and a Hierarchical Ascending Clustering which resulted in the formation of four clusters. The highest station on the shoreline belonged to a cluster characterized notably by low total weight due to a short immersion/feeding period, whereas all other stations belonged to another single cluster. Nevertheless, various biological differences were found between these stations, e.g. the reproductive cycle was generally synchronized throughout the bay but some differences relative to spawning occurrence were observed. In 2000, oyster mortality was higher at Gefosse than at Grandcamp, the latter being a more marine area.In 2001, oyster mortalities were significantly higher and all stations were strongly affected. In the Bay of Veys, oyster biological performances and mortality thus showed spatio-temporal variations which were worthy to be discussed.

Summer mortality is an important economic concern for the Pacific oyster, Crassostrea gigas, industry all over the world and particularly in France. Outbreaks appear when oysters are in gonadal maturation. Bivalve defence mechanisms against pathogen invasion are assumed by circulating cells, the hemocytes. A field experiment was conducted in the Marennes-Oleron Basin (Charente-Maritime, France) in order to monitor hemocyte parameters, gametogenesis and mortality rates among C. gigas during 7 months from March to September 2002. Diploid (from natural bed and from hatchery) and triploid oysters were placed at 15 cm or 70 cm above sediment at Marennes-Oleron Basin. Triploid animals were included because they are considered as sterile animals. Mortality rates were monitored and lipid contents were analysed in order to define sexual maturation stages. Hemocyte parameters (cell mortality, enzymatic activities and phagocytosis) for the three oyster groups were monitored by flow cytometry. Results were analysed by pooling data for the whole sampling period and by separating data in three periods related to the gametogenesis process (A, B and C). Mortality outbreak occurred during the ripe gamete period (B). Results showed that triploids presented the highest values for several hemocyte parameters and the lowest mortality rates. Differences between triploids and diploids were highest during the ripe gamete period. Relationships between oyster mortality, gonadal maturation and hemocyte parameters are discussed. This is the first study monitoring hemocyte parameters of wild diploid, hatchery bred diploid and triploid oysters reared in the field.