Date Awarded


Document Type


Degree Name

Doctor of Philosophy (Ph.D.)


Virginia Institute of Marine Science


Standish K. Allen, Jr.

Committee Member

Fu-Lin E. Chu


American oyster (Crassostrea virginica) stocks on the East Coast in general, and in Chesapeake Bay in particular, have been in decline over the past half-century in large part due to Dermo disease, caused by the protist parasite Perkinsus marinus. Efforts to restore oysters for their ecological and economic value have centered on the development and use of hatchery-based seed selectively bred for disease resistance. Selective breeding could benefit from the incorporation of oysters from wild stocks that have developed "natural Dermo resistance", but few such stocks have been identified and verified. This dissertation describes reciprocal common-garden experiments carried out at two sites in each of the Gulf of Mexico and the Chesapeake Bay between fall 1999 and winter 2001. The experiments compared growth, disease intensity, and survival of seed from putatively disease resistant and susceptible populations in both the Gulf of Mexico and Chesapeake Bay. In the Gulf experiment, oysters from Gulf stocks showed increased survival and decreased disease intensity, determined by body burden analysis of parasite cell counts, compared to Chesapeake stocks. Survival and disease intensities showed statistically significant, but slight differences among stocks of Gulf origin and among stocks of Chesapeake origin. There was no significant difference in growth over the eighteen months of the experiment. The Chesapeake study also showed lower infection intensity and decreases mortality attributable to Dermo disease in Gulf stocks. There were also significant differences in disease intensity and survival among Chesapeake stocks. Additionally, the stock from Rappahannock River, VA showed increased growth compared to other stocks. In an additional study, oysters from the Chesapeake Bay study were used to determine if an association existed between disease intensity (body burden) and host defense activity, as measured by a suite of assays, including hemocyte counts, granulocyte percentage, hemocyte killing ability, serum protein concentration, and serum lysozyme activity. Development of two new assays, serum inhibition of P. marinus protease, and serum inhibition of P. marinus activity, was begun, but require additional refinement. No strong correlation or association was found, but a seasonal component was apparent in several defense assays, as well as in disease intensity. This work has identified and verified wild stocks that have been incorporated into existing breeding programs for both restoration and the development of commercial aquaculture in Chesapeake Bay and the Gulf of Mexico. The existence of disease resistant wild stocks also suggests that the conservation of wild oysters is a viable alternative or addition to current hatchery-based restoration efforts.



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