Infection with Vibrio aestuarianus limits the utility of increasing resistance of Pacific oyster Crassostrea (Magallana) gigas against OsHV-1 µVar.

Infection with the oyster herpesvirus type 1 microvariant (OsHV-1 μVar) has caused mass mortalities of Pacific oyster larvae and spat in multiple countries. Selective breeding to enhance resilience against that virus had been shown as a promising defence strategy. Mass spat mortalities associated with OsHV-1 μVar affected Pacific oyster farms in Ría de Arousa (Galicia, NW Spain), which led us to explore the potential utility of selective breeding to increase cultured oyster survival. Thus, adult oysters that had survived through culture in that area, heavily affected by OsHV-1 μVar, and oysters collected from a naturalised oyster bed that had never been affected, were used as broodstocks in hatchery facilities to produce spat families from each origin. Spat families derived from each stock were transferred into a culture raft in Ría de Arousa; survival and occurrence of OsHV-1 μVar were monitored through cultivation. Spat mortality associated with OsHV-1 μVar was higher in the families deriving from the naïve stock. Adult oyster mortality was detected close to the end of growing-out, which was not associated with OsHV-1 μVar but putatively caused by Vibrio aestuarianus infection. Adult mortality was higher in the families with the highest V. aestuarianus loads; notably, the oyster families with the lowest spat mortality showed the highest adult mortality. Therefore, a potential increase of spat survival in Ría de Arousa through selective breeding to enhance oyster resilience against OsHV-1 μVar could be counteracted by high adult mortality associated with V. aestuarianus infection.