OYSTER SERUM AGGLUTININS AND RESISTANCE TO PROTOZOAN PARASITES

Serum agglutinins or lectins are reported to be induced in marine molluscs by exposure to bacteria and may enhance bacterial clearance from the host; however, there is a little information on possible relationships between lectins and protozoan parasites of molluscs. Two protozoans, Haplosporidium nelsoni and Perkinsus marinus, cause mortality of eastern oysters, Crassostrea virginica. We tested the hypothesis that if hemolymph agglutinins or other hemolymph proteins are important in the defense against these parasites, oysters with high ''baseline'' (pre-exposure) levels, or oysters that produce these substances after challenge, should have lower parasite burdens and survive longer than animals without these characteristics. In May 1990, individually labelled oysters were placed in Chesapeake Bay, MD, where they were exposed primarily to P. marinus, and in Delaware Bay, NJ, where they were exposed to both parasites. Changes in parasite densities, total protein, and agglutination titers were followed by repeated sampling of hemolymph from individual animals over a year. Oysters in Delaware Bay were affected initially by H. nelsoni, but mortalities from both parasites reduced survival to only 10% by September 1990. In Chesapeake Bay, mortalities were attributed to P. marinus only and survival was 36% by May 1991. There was no correlation between survival time and individual protein concentrations in samples from both locations collected during the pre-exposure or post-exposure/pre-mortality periods. The high mortality rate in Delaware Bay precluded hemolymph sampling after September 1990, but continued collections in Chesapeake Bay showed that protein concentrations eventually became inversely correlated with parasite burdens and survival time. This happened, however, only after most oysters had systemic P. marinus infections and were beginning to die. Agglutination titers were not correlated with parasite densities or survival time. We conclude that the serum agglutinins tested play no role in defense against either H. nelsoni or P. marinus, and that differences in total protein were related to pathology rather than disease resistance.