Intraspecific patterns of mortality and cardiac response to hypoxia in the eastern oyster, Crassostrea virginica

Hypoxia is a naturally occurring phenomenon in coastal and estuarine waters that is increasing in frequency and extent due largely to human activities. Elucidating how organisms differentially tolerate hypoxia as a conse-quence of prior exposure can yield insights into their capacity for adaptation or acclimatization to changing oxygen availability. The eastern oyster, Crassostrea virginica, is an ecologically significant bivalve that inhabits estuarine environments with highly variable oxygen conditions. This study aims to increase knowledge on intraspecific variability in oyster hypoxia tolerance and how prior exposure affects this tolerance. Survival and heart rate in response to hypoxia exposure were measured in oysters sourced from reefs with differing frequency of exposure to low oxygen. Regardless of the source site, oysters showed significantly greater mortality, particularly for large oysters, and significantly reduced heart rate upon hypoxic exposure. Additionally, oysters from normoxic versus hypoxic reefs showed no significant differences in survival upon experimental hypoxia exposure. In contrast, heart rate reduction upon hypoxia exposure differed between oysters from hypoxic versus normoxic sites, with significantly less reduction in heart rate observed for oysters from one hypoxic site, and a trend towards smaller reduction in a second site. Results indicate that oysters with prior hypoxia exposure show no difference in survival in hypoxia but show some physiological difference in tolerance, particularly for oysters with greater frequency of prior hypoxia exposure. Overall, these results suggest that hypoxia has negative effects on oyster physiology, and underscore the importance of continued research into oyster's capacity to tolerate future intensification of coastal hypoxia.