Lisa M. Ragone Calvo

Dual disease resistance in a selectively bred eastern oyster, Crassostrea virginica, strain tested in Chesapeake Bay

Selective breeding efforts have yielded oyster strains, Crassostrea virginica, with improved survival and resistance against Haplosporidium nelsoni (MSX); however, because of susceptibility to the oyster pathogen Perkinsus marinus (Dermo), their utility has been limited in areas where the two parasites co-occur. Dual resistance to H. nelsoni and P. marinus was achieved through four generations of artificial selection of wild Delaware Bay oyster progeny at a site in the lower York River, Virginia, USA where both diseases are enzootic. During 1993–1995, survival, growth, and disease susceptibility of third generation Delaware Bay (F3-DEBY) oysters were evaluated at the York River site in comparison to that of similarly selected third generation James River, Virginia oysters (F3-JR), and first generation Louisiana oysters (F1-LA), whose parents were naturally selected in the wild for resistance to P. marinus. During 1997–1999, the performance of F4-DEBY was evaluated at three sites in Virginia in comparison to two groups of first generation oysters whose parents originated from Mobjack Bay and Tangier Sound, Virginia where both P. marinus and H. nelsoni are enzootic. In the presence of high infection pressure from both H. nelsoni and P. marinus, the F3-DEBY stock showed significantly higher survival and growth than either the F3-JR or F1-LA strain. After 15 months of deployment, 79% of F3-DEBY, 11% of F3-JR and 17% of F1-LA oysters were market size (≥76.2 mm) and cumulative mortality was only 16% in F3-DEBY as compared to 42% in F3-JR and F1-LA. At the termination of the study, F3-DEBY oysters exhibited 22% lower mortality than the F1-LA stock, which outperformed the F3-JR stock. Relative performance in respect to disease varied considerably with sample date; however, average H. nelsoni weighted prevalence varied such that F3-DEBY<F1-LA=F3-JR and average P. marinus weighted prevalence varied such that F1-LA<F3-DEBY<F3-JR. In the 1997–1999 trial, F4-DEBY oysters experienced 34–61% lower mortality, greater growth rate, and consistently lower prevalence and intensity of P. marinus than either Mobjack Bay or Tangier Sound oysters. H. nelsoni prevalences were very low (<12%) in all three stocks. This is the first study to demonstrate that reduced susceptibility to both P. marinus and H. nelsoni can be achieved through selective breeding.

INFLUENCE OF HOST GENETIC ORIGIN AND GEOGRAPHIC LOCATION ON QPX DISEASE IN NORTHERN QUAHOGS (=HARD CLAMS), MERCENARIA MERCENARIA

Abstract QPX (Quahog Parasite Unknown) a protistan pathogen of northern quahogs (=hard clams), Mercenaria mercenaria, has caused disease outbreaks in maritime Canada, and in Massachusetts, New York, New Jersey, and Virginia, USA. Although epizootics have occurred in wild hard clam populations, the parasite has most seriously affected cultured hard clams, suggesting that aquaculture practices may promote or predispose clams to the disease. In this investigation the influence of clam genetic origin and the geographic location at where they are grown on QPX disease susceptibility was examined in a common garden experiment. Aquaculture stocks were acquired from hatcheries in Massachusetts, New Jersey, Virginia, South Carolina, and Florida and spawned at a single hatchery in Virginia. All stocks were originally, although not exclusively, derived from wild hard clam populations from each state. The seed clams were deployed at two sites, New Jersey and Virginia, and evaluated during the subsequent 2.5 y for growth, survival, and QPX disease. At both sites, South Carolina- and Florida-derived clam stocks exhibited significantly higher QPX prevalence and lower survival than New Jersey and Massachusetts clam stocks. Levels in the Virginia stock were intermediate. In Virginia, mortality at the termination of the experiment was 78%, 52%, 36%, 33%, and 20% in the Florida, South Carolina, Virginia, Massachusetts, and New Jersey hard clam stocks, respectively. Mortality was significantly correlated with QPX prevalence. Maximum QPX prevalence in the South Carolina and Florida stocks ranged from 19% to 21% and 27% to 29%, respectively, whereas in the Virginia, New Jersey, and Massachusetts stocks prevalence was 10% or less. Similar trends were observed in New Jersey where mortality at the termination of the experiment was estimated to be 53%, 40%, 20%, 6%, and 4% in the Florida, South Carolina, Virginia, Massachusetts, and New Jersey clam stocks, respectively. QPX prevalence peaked at 18% in the Florida stock, 38% in the South Carolina, 18% in the Virginia, and 5% in the New Jersey and Massachusetts stocks. These results suggest that host genotype is an important determinant in susceptibility to QPX disease. As such, hard clam culturist should consider the genetic origin of clam seed stocks an important component of their QPX disease avoidance/management strategies.

Acute osmotic tolerance of cultured cells of the oyster pathogen Perkinsus marinus (Apicomplexa : Perkinsida)

Cultured Perkinsus marinus cells were exposed for 24 hr to salinities of 0, 3, 6, 9, 12 and 22 ppt at temperatures of 1, 5, 10, 15 and 28 degrees C in artificial seawater (ASW) and to the same salinities at 28 degrees C in ASW with the osmotic concentration adjusted with sucrose to the equivalent of 22 ppt. At 28 degrees C mortality increased as salinity decreased below 22 ppt. Mortality was greater than 99% at 0 ppt and greater than 90% at 3 ppt. Mortality was 70% at 6 ppt, 43% at 9 ppt and 20% at 12 ppt. Mortality was low (< 5%) and equal to that at 22 ppt in all treatments where osmotic concentration was maintained with sucrose. Mortality occurred rapidly, within 5 min of exposure to experimental conditions. In the region where mortality was most sensitive to salinity changes (6-12 ppt), lower temperature caused an increase in mortality, but the temperature effect was significant only at 9 ppt.