Date of Award


Degree Type


Degree Name

Master of Science in Fisheries, Aquaculture and Pathology


Fisheries, Aquaculture and Pathology

First Advisor

M. A. Rice


The disease prevalence in American oysters, Crassostrea virginica, from coastal areas in Rhode Island was studied. Oysters were collected by hand or tongs from the Pawcatuck River (3 stations), Narrow River (2 stations), Charlestown Pond (3 stations) and Green Hill Pond (3 stations), during July/August 1991, November 1991 , March 1992, and May 1992. Of the forty oysters collected at each of the four sites, thirty were processed for histologic examination. The remaining 10 oysters of each site were used to determine the condition index.

Haplosporidium nelsoni was detected in four of 480 oysters (0.8%); Sphenophrya sp., 15 of 480 (3.1%); Bucephalus sp., 16 of 480 (3.3%); crustacea, one of 480 (0.4%); and basophilic inclusion bodies (possible mycoplasm-like organisms), 15 of 480 samples (3.2%). Lesions also were found in the samples including: kidney concretions, 32 of 360 (8.8%); necrosis of digestive diverticulae of gastrointestinal tract, 28 of 480 (5.8%); neoplasia of gastrointestinal tract, connective tissue, reproductive tract, and gills, 18 of 480 (3.8% ); hyperplasia of digestive diverticulae , one of 480 (0.2% ); ulceration of stomach epithelium, two of 480 (0.4% ); cysts in the kidney, three of 360 (0.8% ); atrophic adductor muscle, two of 360 (0.6% ); and inflammation of kidney, gills, gastrointestinal tract, connective tissue, 447 of 480 (93.1 % ).

Condition index ranged from 26.52 to 197.67. Condition index of oysters from Pawcatuck River was below 75 throughout the year. In the other areas, condition index was consistently lower during the summer than during other times of the year. Lesions and parasites were found at all of the sites studied, although MSX was found only in Charlestown Pond. Disease prevalence in oysters from the Pawcatuck River was not different from other sites, but the condition index was consistently lower. These findings suggest that low condition index may not necessarily correlate with higher disease prevalence.



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