Quantitative measurement of reproductive output in the American oyster Crassostrea virginica (Gmelin), using an enzyme-linked immunosorbent assay (ELISA)
Choi, Kwang-Sik, D. H. Lewis, E. N. Powell, S. M. Ray
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A quantitative gonadal index was developed for oysters, Croassostrea virginica (Gmelin, 1791), using polyclonal antibodies from eggs and sperm. Percoll used in the purification of oyster eggs and sperm greatly improved the purity of antigens compared to filtering the egg or sperm through a fine mesh only. The antigen-antibody reaction was tested with indirect sandwich ELISA using alkaline phosphatase-conjugated goat anit-rabbit IgG as a secondary antibody. Rabbit anti-oyster egg IgG and anti-oyster sperm IgG initially exhibited a weak cross-reactivity over somatic tissue. Absorbing with acetone-dried oyster tissue powder removed this cross-reactivity. Both antisera exhibited strong specific immunological reactions to oyster eggs or sperm respectively. The quantity of eggs or sperm was measured using ELISA and a quantitative gonadosomatic index (dry wt of egg or sperm/dry wt oyster) (GSI) was calculated. GSI from ELISA correlated with gonadal stage measured histologically. Monthly mean GSI of female oysters was highest during late spring to early summer (0.157-0.201) and lowest during early winter to early spring (0.002-0.000). Maximum GSI observed during the study was 0.422 for female oysters and 0.446 for male oysters. Female oysters produce 3.7-65.4 million eggs, with an average of 12.1 million during each spawning. A positive correlation was observed between the number of eggs produced and oyster size; the number of eggs in the gonad increased as oyster size (i.e., total dry wt) increased (r=0.67); however, the relationship was non-linear. Large oysters contained proportionally fewer eggs. Prevalence of Perkinsus marinus parasitism was high, 90-100%, during the study, as was weighted incidence, 1.33 to 2.67. No statistically significant correlation was observed between infection intensity and the per cent weight of oyster eggs or egg number.