A DETERMINATION OF IN-VIVO GROWTH-RATES FOR PERKINSUS-MARINUS, A PARASITE OF THE EASTERN OYSTER CRASSOSTREA-VIRGINICA (GMELIN, 1791)

Perkinsus marinus, a protozoan parasite of oysters (Crassostrea virgin ica), exerts a significant controlling influence on oyster population dynamics over much of its range. Annual mortalities are typically esti mated at greater than 50% of the host population. The rate of DNA synt hesis in P. marinus was measured by following the rate of incorporatio n of C-14-aspartic acid under field conditions. The DNA content in eac h P. marinus hypnospore was approximately 1 pg. The growth rate of P. marinus in the oyster host is dependent upon P. marinus population den sity. When the parasites occur at densities of greater than 10(4) cell s g dry wt oyster-1, P. marinus exhibited an increase in population do ubling time. At low cell density, doubling times of 1 to 10 hr were ob tained. Doubling time increased to > 10(4) hr at near-lethal infection levels. Very little mortality of P. marinus occurred during the exper iment; thus the immune system was not active against P. marinus infect ion during the summer months. One important consequence of the growth dynamics of P. marinus is the importance of the parasite in controllin g its own population levels. Infection intensity in the summer was con trolled by the feedback of P. marinus cell density on doubling time. B ecause our data suggest that many oyster populations routinely exist a few doublings from death, epizootics must be produced by mechanisms, not well understood, that destabilize this delicate balance.