Semester of Graduation

Summer 2018

Degree

Master of Renewable Natural Resources (SRNR)

Department

Renewable Natural Resources

Document Type

Thesis

Abstract

Increasing prevalence of hypoxia in estuarine waters can pose a serious threat to eastern oysters (Crassostrea virginica). While oysters are considered more tolerant to hypoxia than many other bivalves, their tolerance at temperatures of 28 °C and above, typical in northern Gulf of Mexico estuaries in the summer, is not well characterized. Moreover, it is unknown whether differences in hypoxia tolerance exist among oyster populations or between diploid and triploid oysters. To investigate population differences, wild oysters were collected from four estuaries in the northern and northwestern Gulf of Mexico and spawned. In a series of studies, the progenies were exposed to continuous normoxia (dissolved oxygen > 5.0 mg L-1) and hypoxia (dissolved oxygen < 2.0 mg L-1) at 32 °C. Cumulative mortality was recorded, hemolymph and tissue samples were collected to measure the sub-lethal, physiological response to hypoxia. Oyster valve movement during normoxia and hypoxia was also measured. Significant differences in hypoxia tolerance were found among populations with calculated median lethal times (LT50) differing by population, ranging from 3.9 to 12.5 days. After 3-5 days of hypoxia exposure, the most tolerant population had elevated concentrations of plasma calcium and glutathione compared to the least tolerant population, potentially conferring better protection against hypoxia stress. To investigate ploidy differences, diploid and tetraploid oysters were spawned to produce diploid and triploid progenies. In a series of studies, the adult and seed progenies were exposed to continuous normoxia, hypoxia, and anoxia (dissolved oxygen < 0.5 mg L-1) at 28 °C. Cumulative mortality was recorded, and hemolymph and tissue samples were collected to measure the sub-lethal physiological response to hypoxia. Differences in LT50 were found between ploidies in adult and seed oysters, but not consistently. Adult triploid mortality was delayed by 4.6 days under hypoxia and 1.5 days under anoxia compared to diploids. In seed oysters, diploid mortality was delayed by 2.4 days under hypoxia compared to triploids, and under anoxia triploid mortality was delayed by 1.3 days compared to diploids. Despite these differences, sub-lethal responses between adult ploidies were largely comparable in all treatments, suggesting diploids and triploids use similar strategies to cope with hypoxia and anoxia stress at high temperatures. Combined, this research shows that hypoxia and high temperature (>28 °C) can cause significant oyster mortality if conditions persist longer than three weeks. Under anoxia exposure, significant mortality can occur within several days. Population-specific tolerances suggest that adaptation to local environmental conditions has occurred, and that selectively breeding populations for better environmental tolerance is possible. The relatively comparable responses between diploid and triploid oysters suggest the risks to co-occurring hypoxia and high temperature events are the same, regardless of ploidy, when considering their use in aquaculture. As environmental conditions shift due to climate change and other anthropogenic activities, testing single and multi-stressor tolerance, and potential population and ploidy tolerances will be critical to develop better restoration and production strategies.

Date

6-22-2022

Committee Chair

Megan La Peyre

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