Electronic Theses and Dissertations

Date of Award


Document Type


Degree Name

M.S. in Biological Science

First Advisor

Deborah J. Gochfeld

Second Advisor

Deborah J. Gochfeld

Third Advisor

Marc Slattery


University of Mississippi

Relational Format



Oysters are vital to the health of estuarine ecosystems and provide critical ecosystem services, such as water quality improvement, nursery habitat for commercially important fisheries, and shoreline stabilization. Despite their ecological and economic importance, oyster reefs are exposed to many stressors, and oyster populations in Mississippi have declined dramatically in recent years. The present study focuses on the effects of hypoxia and tributyltin (TBT) on the Eastern oyster, Crassostrea virginica. Hypoxic events are increasingly comin coastal ecosystems worldwide, and TBT is a legacy toxicant once used as an antifoulant. The effects of these stressors were assessed on adult oysters deployed in the Mississippi Sound, where they experienced a natural hypoxic event, and in the laboratory, where oysters were exposed to either hypoxia or TBT alone or to a combination of the stressors (hypoxia+TBT). Effects were quantified through the measurement of mRNA expression of HIF1-α as a marker of hypoxic stress, Tβ-4 to evaluate immunosuppression, and CvPPAR-δ to assess its expression after TBT exposure. Physiological parameters, including hemocyte counts, clearance rates, and oxygen consumption rates, were measured to evaluate how these stressors impact important life sustaining functions. Exposure to a natural hypoxic event for nine days up-regulated mRNA expression of HIF1-α and Tβ-4, while CvPPAR-δ was significantly down-regulated and no effects on physiological parameters were observed. In the lab, oysters exposed to both hypoxia alone and hypoxia+TBT shosignificant up-regulation of HIF1- α and down-regulation of Tβ-4 mRNA expression on days 8 and 12, however, physiological functions were relatively unaffected. CvPPAR-δ mRNA expression was down-regulated under exposure to TBT, hypoxia, and the combined treatments, indicating that this nuclear receptor pathway is responsive to these stressors. Findings from the study indicate potential for immunosuppression as a result of combined stressors, which could increase oyster vulnerability to pathogens. Additionally, a field study was performed during April-September 2019 to assess the effects of an extremely low salinity event due to dual openings of the Bonnet Carré Spillway (BCS) on oyster mortality. Two months following the second closing of the BCS, oyster mortality was 100% at five oyster reefs in the Mississippi Sound.

Included in

Toxicology Commons



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