AUTHOR=Xie Zhe , Wei Shuaishuai , Dong Haomiao , Chen Hui , Zhang Qianqian , Liu Wei , Peng Jinxia , Sokolova Inna M. , Hu Menghong , Wang Youji 

TITLE=Hemocyte Responses of the Oyster Crassostrea hongkongensis Exposed to Diel-Cycling Hypoxia and Salinity Change

JOURNAL=Frontiers in Marine Science

VOLUME=Volume 8 - 2021

YEAR=2021

URL=https://www.frontiersin.org/journals/marine-science/articles/10.3389/fmars.2021.749623

DOI=10.3389/fmars.2021.749623

ISSN=2296-7745

ABSTRACT=Marine hypoxia caused by nutrient enrichment in coastal waters has become a global problem for decades, especially diel-cycling hypoxia occurs frequently in summer season. On the other hand, sudden rainstorms and freshwater discharge make salinity in estuarine and coastal ecosystems variable, which often occurs with hypoxia. We found mass mortality of the Hong Kong oyster Crassostrea hongkongensis in the field where hypoxia and salinity fluctuation co-occur in summer season during the past several years. To investigate the effects of diel-cycling hypoxia and salinity changes on hemocyte immune function of C. Hongkongensis, oysters were exposed to a combination of two dissolved oxygen (DO) concentrations (24h normal oxygen 6 mg/L,12h normal oxygen 6 mg/L and 12h hypoxia 2 mg/L) and three salinities (10, 25 and 35‰) for 14 days. Subsequently, all treatments were restored to constant normal oxygen (6 mg/L) and salinity under 25‰ for 3 days to study the recovery of hemocyte immune function from the combined stress. Hemocyte parameters were analyzed by flow cytometry, including hemocyte mortality (HM), total hemocyte count (THC), phagocytosis (PHA), esterase activity (EST), reactive oxygen species (ROS), lysosome content (LYSO) and mitochondrial content (MN). The experimental results showed that diel-cycling hypoxia and salinity changes have obvious interactive effects on various immune parameters. In general, diel-cycling hypoxia and decreases in salinity led to increased HM, and low salinity caused heavier impacts. In addition, low salinity and diel-cycling hypoxia also led to decreases in LYSO, EST and THC, while the decrease of PHA only occurs in early stage. On the contrary, the ROS production increased significantly under low salinity and hypoxic conditions. After 3-day recovery, THC, PHA, EST, LYSO and MN basically restored to normal, while HM and ROS were still significantly affected by diel-cycling hypoxia and salinity change, indicating the combined stress of diel-cycling hypoxia and salinity changes had latent effects on the immune function of C. hongkongensis. Our results highlight that diel-cycling hypoxia and salinity change may impair the health and survival of the Hong Kong oyster C. hongkongensis, and may be the key factors for the mass mortality of this oyster in the field.