Résumé
Oyster farming is one of the major aquaculture productions worldwide. Among pathogens hampering its development, some Haplosoporidium species are associated with oyster mortality events. Initially characterized in the Eastern oyster Crassostrea virginica in Virginia, USA, in the context of seasonal outbreaks, the cercozoan parasite Haplosporidium costale, has subsequently been detected in the Pacific cupped oyster Magallana gigas in the USA and China. Recently, it has been detected in farmed M. gigas in France, mostly outside mortality events and at low infection intensity. In order to clarify the geographic distribution of the parasite and assess its potential diversity, natural oyster populations sampled in various contexts in France in 2018 and 2020 were tested by Real-Time PCR and positive oysters were selected for sequencing. Complementary, the dynamics of the infection were investigated in oysters produced in an experimental hatchery and deployed in sites monitored as part of a national network. These analyses allowed to assess the potential correlation between the parasite dynamics and environmental factors and the growth and mortality of oysters. Obtained results demonstrate that Haplosporidium costale is present along the French coastline, with apparent northern and southern distribution limits. Its dynamics appear to be significantly influenced by both temperature and salinity. Low genetic diversity was observed among the tested samples, suggesting that introduction of the parasite into France is not recent.These results not only improve knowledge of bivalve protozoan parasite interactions but also provide information of interest to better monitor and control oyster diseases.