EXPOSURE OF THE EASTERN OYSTER Crassostrea virginica TO THE HARMFUL ALGAL BLOOM SPECIES Alexandrium monilatum

Sarah K. D. Pease*, Kimberly S. Reece, and Wolfgang K. Vogelbein
 
 Virginia Institute of Marine Science
 College of William & Mary
 P.O. Box 1346
 Gloucester Point, Virginia 23062, USA
 skpease@vims.edu

 

The harmful algal bloom (HAB) species Alexandrium monilatum has been long-associated with finfish, shellfish, and marine invertebrate mortalities in the Gulf of Mexico. Alexandrium monilatum produces the lipophilic, hemolytic toxin 'goniodomin A'. Since 2007, this HAB species has bloomed almost annually in the southern Chesapeake Bay, where it has reached record high cell densities (>200,000 cells/mL). Blooms of A. monilatum in the Bay are suspected in mass mortalities of Eastern oyster larvae (Crassostrea virginica) grown for aquaculture and restoration projects. As a result, representatives from Virginia's multimillion dollar oyster aquaculture industry have expressed great concern over the potential impacts of A. monilatum.

Previous laboratory studies on the effects of A. monilatum on adult C. virginica have shown varying results, with reports of: valve closure, reduced valve gape and clearance, loss of ability to close valves, morbidity, and mortality. Many of these exposures were run for 48 hours or less, and little information is available on health impacts to the oyster from this toxin-producing HAB.

HAB toxicity bioassay methods developed at the Virginia Institute of Marine Science were modified to investigate potential adverse health impacts of five different live cultured A. monilatum cell density treatments on triploid C. virginica (~40-110 mm). Oysters were exposed to treatments for 96 hours to capture effects that may be seen during prolonged blooms. Data were collected on A. monilatum cell removal, oyster feeding behavior, and oyster morbidity and mortality. Histopathological analysis was performed on each oyster to look for tissue damage and other signs of disease due to A. monilatum and its toxins; the presence of other common local oyster parasites and diseases were recorded as well. A preliminary study indicated that A. monilatum in densities as high as 5000 cells/mL was removed from the water column within 48 hours. Histology showed one oyster (exposed to 5000 cells/mL) that appeared to have A. monilatum cells within the gills, stomach, and intestines.

Additional oysters were deployed in cages at a site in the southern Chesapeake Bay that regularly experiences natural, dense A. monilatum blooms. These cages were subsampled weekly before, during, and after the 2015 A. monilatum bloom. Surface water samples analyzed using qPCR and visual counts provided bloom density data, and oysters were processed for histological analysis as described above. Data from the field and laboratory studies will be compared.