Physiological responses to salinity change and diel-cycling hypoxia in gills of Hong Kong oyster Crassostrea hongkongensis

Global climate change is a frequent cause of salinity fluctuation in seawater, especially in aquaculture sites. Moreover, anthropologic activities often cause seawater eutrophication with the consequence that hypoxia appears often during nighttime. The Hong Kong oyster Crassostrea hongkongensis, as a species that inhabits estuarine and coastal waters, is faced with such challenges. In this study, oyster physiological changes were considered to be closely related to hypoxia and salinity changes. Physiological indices were examined in Hong Kong oysters by employing six treatments to shed light into the effects of diel-cycling hypoxia (periodical hypoxia) and salinity change. Three salinities (10‰, low salinity; 25‰, normal salinity; and 35‰, high salinity) and two types of dissolved oxygen (normoxia, 6 mg/L throughout the day) and periodical hypoxic condition (6 mg/L at daytime for 12 h and 2 mg/L at nighttime for 12 h) were set. After 14- and 28-day exposures, gill tissues were sampled to detect changes in gill ATP production, mitochondrial membrane potential (MMP), mitochondrial reactive oxygen species production (ROS), and gill respiratory metabolic enzymes. Results indicated that periodical hypoxia and salinity change led to increased hexokinase (HK) and pyruvate kinase (PK) (p < 0.05). By contrast, they had no significant effect on mitochondrial number (MN). Adenosine-triphosphate (ATP) production only increased in the early exposure. In addition, low salinity with periodical hypoxia resulted in decreased MMP, lactate dehydrogenase (LDH), and succinate dehydrogenase (SDH, p < 0.05). On the contrary, periodical hypoxia with high salinity led to increases in ATP and ROS and decreases in SDH, MMP, and LDH (p < 0.05). These results revealed that when diel-cycling hypoxia occurs with salinity change, the gill metabolism of Hong Kong oysters are gradually dominated by glycolysis while aerobic respiration decreases. Moreover, gill functions could be affected although energy accumulation exists during early exposure. Therefore, long-term exposure to periodical hypoxia with salinity change poses risk to the health and growth of Hong Kong oysters, impairing oyster aquaculture and coastal ecosystem health.