Synergistic Impacts of Hypoxia and High Temperature on the Bay Scallop, Argopecten irradians: Mortality, Gene Expression, and Hemocyte Responses

Abstract

Hypoxia coupled with high water temperatures during summer poses a significant threat to aquatic animals. This study investigated the combined effect of these stressors on the bay scallop, Argopecten irradians, to understand potential causes of mass mortality events. Scallops were exposed to control (19℃, 6.76 mg O2/L), hypoxia (19℃, 1.01 mg O2/L), and combined hypoxia and high temperature (28℃, 1.02 mg O2/L) conditions for 48 hours. Quantitative PCR (qPCR) assessed mRNA expression of hypoxia-inducible factor 1-alpha (HIF1-α) and heat shock protein 90 (HSP90) in the digestive gland. Flow cytometry evaluated cellular responses, including phagocytosis capacity, nitric oxide (NO) production, and reactive oxygen species (ROS) production of hemocytes. Scallops exposed to combined stressors showed 50% mortality within 48 h, starting at 30 h, while hypoxia alone caused only 18% mortality. Gene expression remained unchanged under hypoxia, but HIF1-α and HSP90 were significantly upregulated under combined stressors. Hemocyte parameters did not show statistically significant differences across groups; however, there was a trend of decreased phagocytic capacity and increased ROS and NO production under combined stressors. These results suggest that the synergistic effect of hypoxia and high temperature during summer can exert sublethal to lethal impacts on bay scallops, potentially explaining mortality events in this species. Further research is needed to explore the underlying mechanisms and potential adaptive strategies in scallops facing such environmental challenges.