Genetic differentiation of the flat oyster (Ostrea edulis) was studied along the European coast, from Norway to the Black Sea, by means of variation at five microsatellite loci. The results show a mild but significant isolation-by-distance profile, a noticeable between-sample variance in expected heterozygosity, and a tendency for Atlantic populations to be less variable than Mediterranean ones. This does not provide support for the existence of a single large panmictic population for this larvae-broadcasting species, but rather for the relative independence of local stocks. Comparison with data on allozyme variation from the literature confirms this view. It also leads us to suggest that the behavior of some sampled protein loci may depart from the average, so caution should be used when inferring neutral gene flow.

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Since 2014, France's blue mussel industry has been facing heavy mortality outbreaks (90-100%) affecting both juveniles and adults. This report presents evidence of heavy genomic abnormalities associated with mortality outbreaks in blue mussels, Mytilus edulis-galloprovincialis, from the Atlantic coast of France. In this study, ploidy characteristics of hemic cells were investigated using Flow CytoMetry (FCM), revealing an unusual, broad continuum of ploidy distribution from hypodiploidy to tetraploidy. FCM was additionally used to evaluate, at individual and populations levels, different thresholds of genomic abnormality (GA%) using the percentage of non-diploid nuclei. Individual mussels were considered to be abnormal when more than 10% of hemocytes in S-G2/M phase were present. At the population level, a threshold of 6% for the mean intensity of the abnormality is proposed, which means in the population, more than 6% of individual mussels have to present with more than 10% of their hemocytes in S-G2/M phase. GA% was found to be significantly predictive of the final mortality. Based on the established thresholds, only two mussel stocks analyzed in this study were considered to have good cytogenetic quality, while all other stocks appeared to be affected. FCM offers a very powerful tool to help manage current blue mussel mortality in France. We also believe that annual and extensive determination of cytogenetic quality of wild and cultivated mussel beds along with exclusive use of FCM-qualified mussel seeds should be a priority.

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