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Marine Ecology Progress Series

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MEPS 432:1-15 (2011)  -  DOI:

Declining impact of an introduced pathogen: ­Haplosporidium nelsoni in the oyster Crassostrea virginica in Chesapeake Bay

Ryan B. Carnegie*, Eugene M. Burreson

Virginia Institute of Marine Science, College of William & Mary, Gloucester Point, Virginia 23062, USA

ABSTRACT: 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 naïve sentinels has grown, H. nelsoni levels in wild oysters have fallen, with prevalence typically below 20% and advanced infections uncommon. A transplant experiment comparing naïve 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.

KEY WORDS: Haplosporidium nelsoni · Multinucleated sphere X disease · MSX · Crassostrea virginica · Disease ­resistance · Oyster restoration

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Cite this article as: Carnegie RB, Burreson EM (2011) Declining impact of an introduced pathogen: ­Haplosporidium nelsoni in the oyster Crassostrea virginica in Chesapeake Bay. Mar Ecol Prog Ser 432:1-15.

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