COMPARISON OF INFECTIVITY AND PATHOGENICITY OF MERONT (TROPHOZOITE) AND PREZOOSPORANGIAE STAGES OF THE OYSTER PATHOGEN PERKINSUS-MARINUS INEASTERN OYSTERS, CRASSOSTREA-VIRGINICA (GMELIN, 1791)

Two experiments were conducted to compare the infectivity and pathogen icity of two life stages of the parasite Perkinsus marinus, meronts (t rophozoites) and prezoosporangia (hypnospores), in eastern oysters, Cr assostrea virginica. Oysters were inoculated with 5 X 10(4) meronts or prezoosporangia per oyster by injection into the shell cavity. Preval ence and intensity of P. marinus infections, condition index, serum pr otein concentrations, and lysozyme activities were measured in oysters after 15, 25, 40, and 65 days in experiment 1 and after 20, 40, 50, 6 5, and 75 days postchallenge by P. marinus cells in experiment 2. Cont rols were injected with filtered York River water. In the first experi ment, P. marinus infections were initially detected in oysters exposed to prezoosporangia after 15 days postchallenge. In the second experim ent, infection was not detected in oysters until 40 days postchallenge with either meronts or prezoosporangia. Intensity and prevalence of P . marinus infection were significantly higher (p < 0.002) in oysters c hallenged by meronts compared with prezoosporangia-challenged oysters at the end of both experiments. In experiment 1, a significant decreas e (p < 0.05) was observed in serum protein in infected oysters challen ged by prezoosporangia compared with uninfected oysters. Condition ind ex was higher in uninfected oysters compared with infected oysters cha llenged by prezoosporangia. The differences in condition index and pro tein were insignificant between oysters infected by meronts or prezoos porangia. Lysozyme activities were significantly lower (p < 0.05) in i nfected oysters than in uninfected oysters challenged with meronts. No significant differences were observed in condition index, protein con centrations, and lysozyme activities between oysters challenged by mer onts and prezoosporangia in experiment 2. Lower condition index and pr otein concentrations in the groups of oysters infected with prezoospor angia compared with the groups infected by meronts and nonchallenged a t the end of experiment 1 suggest a higher energetic demand on these o ysters.