The combined effects of temperature and exogenous bacterial sources on mortality in Crassostrea virginica under severe hypoxia

Abstract

Abstract In aquatic environments, low dissolved oxygen concentrations can result in depressed bivalve defense systems while promoting anaerobic bacterial growth, ultimately leading to increased bivalve mortality rate. There are discrepancies between laboratory and field studies examining bivalve mortality under low oxygen conditions, possibly leading to an underestimation of the impact of hypoxic events. Indeed, laboratory studies typically exclude potentially influential factors that may affect survival, e.g., exogenous bacteria. In this study, adult oyster (Crassostrea virginica, 60 ± 5 mm shell length) survivability was investigated during severe hypoxia (< 0.1 mgO2L− 1) in combination with high temperature (20˚C vs. 28˚C), and the introduction of a secondary bacteria source (anoxic marine sediment). In addition, an experiment tested if the conventional methodological approach in these types of experiments, i.e., removing dead bivalves from the population, impacted survivability. Results demonstrate that at the highest tested temperature (28˚C) the effect of a secondary bacterial source did not significantly impact survival rates (time taken for half the population to die (LT50) (LT50: 9.7 ± 0.5 vs. 10.9 ± 0.4 days secondary bacterial source vs. no secondary bacterial source, respectively). However, at the lower temperature (20˚C) the presence of a secondary bacterial source did decrease survival rates (LT50: 9.8 ± 0.4 vs. 13.7 days bacterial source vs. no secondary bacterial source, respectively). Additionally, dead oyster removal increased oyster survivability in all treatments relative to when they were not removed. This study highlights the mechanisms by which mortality rates are underestimated in laboratory compared to field studies.