Date Awarded


Document Type


Degree Name

Doctor of Philosophy (Ph.D.)


Virginia Institute of Marine Science


Emily B. Rivest

Committee Member

Marjorie A.M. Friedrichs

Committee Member

Roger L. Mann

Committee Member

Richard A. Snyder

Committee Member

Cheryl A. Logan


This dissertation evaluates population-level variability in physiological tolerances to environmental stressors in the eastern oyster, Crassostrea virginica, and the hard clam, Mercenaria mercenaria, two ecologically and economically important bivalve species that inhabit the eastern coast of the United States. In Chapter 2, an assessment of the acidification tolerance of larval eastern oysters spawned from two reefs in Chesapeake Bay revealed physiological differences under control and acidified conditions. Differences observed at control conditions indicate potential variations in basal metabolic processes, while the variations in acidification tolerance illustrate the potential for some oyster populations to possess greater resilience to ongoing acidification. More population-level assessments are needed as variability among other populations across larger spatial scales is likely present. In Chapter 3, stress tolerances to elevated temperatures and low salinities were evaluated for juvenile clams from five populations along the US East Coast. Differences in nonlethal temperature stress responses were minimal among populations, although the two most southernly distributed populations had the highest survival under an extreme high temperature. Further population-level variability was observed in response to low salinity stress; however, all populations showed a similar low salinity tolerance limit below 15. More multifaceted assessments are needed to better capture population-level differences in stress response mechanisms. Following Chapter 3, three population crosses were conducted to assess if physiological tolerances could be modified (Chapter 4). Nonlethal temperature tolerances were similar among juveniles from all crosses. Juveniles from outcrosses of Pocomoke Sound, VA with Wachapreague, VA and Bogue Sound, NC with Cape Cod, MA showed higher survival compared to a Wachapreague, VA self-cross under an extreme high temperature. Interestingly, all three crosses showed marked declines in oxygen consumption below a salinity of 20, which was not seen in any parent population until below a salinity of 15. This variability between experiments indicates that genetic variations in salinity tolerance may exist within study populations. Elevated temperature and low salinity tolerances of larvae produced from the same three crosses were also evaluated (Chapter 5). Larvae from the Wachapreague, VA self-cross showed the largest decline in survival under elevated temperature stress, as seen in juvenile cross assessments. When exposed to low salinity stress, larvae from the Bogue Sound, NC and Cape Cod, MA cross showed the highest survival; however, larvae from all crosses showed a steep drop in cellular energy reserves. Alongside this physiological assessment, larval microbiomes were sequenced to provide insight into microbial community structures and to explore the impacts of environmental stress on these communities. Larval microbiomes from the Bogue Sound, NC and Cape Cod, MA cross were clearly different from those of the other crosses, demonstrating the influence parental microbiomes can have on their offspring. Lastly, in all crosses, low salinity stress resulted in the greatest shifts in microbial community composition observed here. Overall, this dissertation provides deeper insights into physiological stress mechanisms in early life stages of C. virginica and M. mercenaria. Population-level variability will likely play an important role in the long-term persistence of eastern oysters and hard clams as climate changes continues.




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