Virginia Institute of Marine Science
MARINE ECOLOGY PROGRESS SERIES
Disease caused by the parasite Haplosporidium nelsoni has devastated Crassostrea virginica in Chesapeake Bay, exacerbating effects of overharvesting and adversely impacting the ecology of the bay. H. nelsoni is thought to persist as an impediment to oyster restoration because strong reproductive contributions from oysters in low-salinity refugia from parasitism have prevented development of disease resistance. On the contrary, long-term data indicate that while infection pressure on naive sentinels has grown, H. nelsoni levels in wild oysters have fallen, with prevalence typically below 20% and advanced infections uncommon. A transplant experiment comparing naive sentinels with oysters from disease-enzootic populations indicated that these observations represent true disease resistance, and its geographical distribution was revealed by annual fall surveys, and by intensive sampling in 2007 and 2008. Resistance is best developed in the small, polyhaline Lynnhaven River, where salinities are never low enough to suppress H. nelsoni. Resistance gradually decreases up the large tributaries and the bay, as revealed by infections reaching higher levels further upstream when drought allows H. nelsoni to colonize unselected oyster populations. Elevated disease in small oysters from the Rappahannock River suggests that a non-resistant component of the population persists due to reproductive contributions from unselected oysters in upriver sanctuaries from disease selection, and from susceptible but pre-disease oysters in the lower river. Contributions of these unselected oysters notwithstanding, our results point to substantial reproduction by resistant oysters in disease-enzootic waters, and suggest that reefs in mesohaline-polyhaline salinities, and not only those in disease-free lower mesohaline areas, should be the focus of conservation and restoration efforts.
Haplosporidium nelsoni; Multinucleated sphere X disease; MSX; Crassostrea virginica; Disease resistance; Oyster restoration
Carnegie, Ryan B. and Burreson, Eugene M., Declining impact of an introduced pathogen: Haplosporidium nelsoni in the oyster Crassostrea virginica in Chesapeake Bay (2011). MARINE ECOLOGY PROGRESS SERIES, 432, 1-15.