DISEASE TRANSMISSION AND ASSOCIATED MORTALITIES OF THE PACIFIC OYSTER Crassostrea gigas: DOES SALINITY MATTER?

In France, since 2008, young Pacific oysters, Crassostrea gigas (< one year) have exhibited mass mortality due to OsHV-1 µVar when seawater temperature is between 16 and 24°C. Since then, these mortality events have been observed in other European countries, New Zealand and Australia. Although temperature is a major trigger for this disease, salinity may play a role on disease dynamics, as it does for other bivalves diseases. The aim of this study was to explore the effect of salinity on disease transmission and related mortality of Pacific oyster, Crassostrea gigas (Experiment A) and to assess the effect of low salinity on pathogens involved in this mortality (Experiment B).

Experiment A consisted in testing the effect of 4 salinities, namely, 10, 15, 25 and 35 ppt on host-pathogen interaction. Oysters were acclimated to the different salinity treatments to understand how salinity could modulate host-pathogens interaction, acting on the physiology of animals prior to infection. For infection, naïve oysters were infected at 21°C with a source of infection consisting in water coming from tanks where field-infected animals were maintained. Experiment B consisted in exposing pathogens from source of infection to salinities of 10 ppt (low salinity) and 25 ppt (control salinity) prior to challenging naïve oysters acclimated at 25 ppt.

For Experiment A, results showed that mortality rate was significantly reduced at 10 ppt in constrast with that observed in other salinities. In addition, levels of OsHV-1 DNA were lower in oysters maintained at 10 ppt compared to that of animals held at 15-35 ppt. In experiment B, results showed that salinity had a direct effect on pathogens, with no mortality induced after an exposure to 10 ppt in contrast with 25 ppt.