Date Awarded


Document Type


Degree Name

Doctor of Philosophy (Ph.D.)


Virginia Institute of Marine Science


Beverly Weeks-Perkins


The effects of an organophosphorus pesticide, methyl parathion, on cellular immune defense mechanisms of the giant black tiger shrimp (Penaeus monodon) were studied. Animals were exposed for 96 hours at concentrations equal to, above and below the LC&\sb{lcub}50{rcub}& (3 ppb). Phagocytes were obtained from the heart and circulating hemolymph. Cellular immune responses, including chemotaxis, phagocytosis and the respiratory burst were examined. Chemotactic activity was determined by means of a modified Boyden double chamber technique. There was a dose-dependent decrease in the chemotactic activities of both circulating hemocytes and cardiac phagocytes. Phagocytic activity (percent phagocytosis) was examined by the microscopic enumeration of phagocytes which had internalized yeast cells. For both circulating and cardiac phagocytes, there was a decreasing trend in the phagocytosis of yeast cells as the concentration of methyl parathion was increased. Exposure to methyl parathion at and above 2 ppb resulted in a significant reduction in phagocytic activity for both cell types as compared to the control group. The phagocytic index for circulating and cardiac cells showed a decreasing trend with increasing concentrations with significant differences at and above 6 ppb. Since the measurement of superoxide has been accepted as an accurate way to quantify the intensity of the respiratory burst, superoxide production by hemocytes was measured by the reduction of the redox dye nitroblue tetrazolium (NBT). Both circulating and cardiac phagocytes exhibited significant increases in superoxide production at 6 and 10 ppb, as well as at 3 ppb in the case of cardiac phagocytes. Histopathological changes of the cells were observed in the gill, hepatopancreas, heart, muscle and ventral nerve, with the hepatopancreas containing the most pronounced changes. Electron microscopy revealed granular damage of the circulating hemocytes at 3, 6 and 10 ppb. These experiments indicate that methyl parathion alters cellular immune responses of Penaeus monodon in a dose-dependent manner. Further investigation of these immunological mechanisms is needed to explain the phenomenon of shrimp survival in contaminated environments.



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