The effect of low salinity on phenoloxidase activity in the Sydney rock oyster, Saccostrea glomerata

This study tested the effects of low salinity on phenoloxidase activity in Sydney rock oysters (Saccostrea glomerata). Phenoloxidase is a key component of the immune system in S. glomerata. Previously, we have shown that decreased phenoloxidase activity is associated with susceptibility to fatal QX disease. In the current study, laboratory-based experiments were used to identify factors that contribute to decreased phenoloxidase activity. We found that exposing oysters to water collected from a QX-prone oyster growing area after heavy rain significantly inhibited their phenoloxidase activities. Similar inhibition was evident when oysters were held in oceanic water with artificially lowered salinities. These results suggest that exposure to low salinity decreases phenoloxidase activity. Field trials that exploited a natural salinity gradient in the Georges River, Sydney, supported this conclusion. The phenoloxidase activities of oysters transplanted to up-river sites that had low salinities were significantly lower than those of oysters held at seaward sites with higher salinities. All of these data implicate low salinity as a key environmental stressor that is associated with inhibition of the S. glomerata immune system.