PREVALENCE OF THE OYSTER PARASITE HAPLOSPORIDIUM NELSONI IN COMMERCIAL AND NATURAL-BED POPULATIONS OF THE EASTERN OYSTER CRASSOSTREA VIRGINICA IN THE DAMARISCOTTA ESTUARY, MAINE.  

In 2010 there was an outbreak of disease associated with the oyster parasite Haplosporidium nelsoni, commonly know as MSX, in commercially grown populations of the Eastern oyster Crassostrea virginica in the Damariscotta Estuary, Maine. In 2011 the industry made a management decision to change the strain of oyster being cultivated from the old Maine strain that was cold water tolerant but not MSX-resistant to a newer strain that had been generated to be more MSX tolerant. In the summer of 2012 we conducted a prevalence study of the parasite in both commercial and natural-bed populations. This showed that the prevalence of the parasite was about 50%. At the same time we sampled biofouling organisms found in close proximity to the commercial sites and investigated them for the presence of the parasite in the hope of identifying possible reservoir species for this parasite. This study revealed no obvious notable reservoir species although the parasite was detected, at moderate levels, in tunicates and in plankton samples. Finally, in the summer of 2014 we repeated the prevalence study in commercial and natural-bed oyster populations. This revealed a switch in parasite prevalence, suggesting that another parasite, SSO, was a significant player in the oyster populations.