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International audience; Summer mortalities of the adult Pacific oysters Crassostrea gigas are an important economic concern. In 2015 and 2016, the mortality of hatchery origin spat comprising 4 diploid batches and 9 triploid batches as well as 12 batches of wild caught diploids was followed at 3 sites in Normandy. Abnormal mortalities (> 20%) were observed at one site only and were significantly higher in triploid animals (P < .05). Triploid oysters are believed to be partially sterile but a high level of gametogenesis was observed in all monitored triploid batches. In 2015, preliminary results revealed that triploid oysters underwent strong gametogenesis with mature gametes (stage 3) and 50% of them were non-perturbed (i.e. named alpha). In 2016, 42% of triploid oysters were alpha. This study revealed that triploid oyster maturation occurred in September, which corresponded to the time of high mortalities in Normandy.

Since the summer of 2008, the mortality of young Pacific oysters Crassotrea gigas has been greatly increased on the French coast and may be linked to a microvariant of the Ostreid Herpesvirus 1 (OsHV-1), named OsHV-1 μVar. This variant is mainly characterized by a deletion of 12 consecutive nucleotides followed by a deletion of one adenine in a microsatellite area located in the C region. The present study is the continuity of a previous study that was conducted on oysters collected from July 2008 to June 2010 (Martenot et al., 2011). The purpose of the current survey is to determine the genotype of oyster samples collected from July 2010 to August 2011 and samples taken before 2008. The OsHV-1 μVar was detected in 98% of characterized samples and the two new forms of microvariants were found in the remaining 2%. The first genotype has 15 bp successive deletions and the second genotype is similar to the OsHV-1 reference with an insertion of three nucleotides. The OsHV-1 μVar Δ9 previously described in the literature was detected in a sample originating in Brittany. These observations reinforce the possibility of the emergence of different OsHV-1 variants.

French oyster farming has been experiencing severe mortalities since 2008 in Crassostrea gigas spat and juveniles mainly during the summer months. This recent phenomenon can be explained by the involvement of many factors including pathogenic agents such as the Ostreid Herpesvirus 1 (OsHV-1). From July 2008 to September 2010, 424 oyster samples received from French oyster farmers were tested to follow the concentration of OsHV-1 by quantitative real-time PCR. Comparisons of OsHV-1 loads were also made according to oyster age, ploidy, origin (hatchery-bred or wild-collected) and sample status (healthy or dying). The survey revealed that high viral loads detected in spat and juvenile oyster tissues were associated with summer mortality occurrence. No significant differences were observed in pathogen detection in relation to oyster ploidy and origin. Finally, this study allowed us to define a viral load threshold of 8.8 × 103 of OsHV-1 DNA per mg of tissue, which can be useful to oyster farmers in evaluating mortality risk during the different stages of oyster handling.