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.Virginia Sea Grant funded research

Examination of the Potential for Vertical and Horizontal Transmission of the Oyster Herpes Virus from Crassostrea ariakensis Broodstock

Kimberly S. Reece, Ryan B. Carnegie (VIMS)

Our objectives are: 1) to determine the capacity of Crassostrea ariakensis to transmit the oyster Herpes virus vertically from broodstock to projeny; and 2) to determine the capacity of C. ariakensis to transmit the oyster Herpes virus to native Chesapeake Bay bivalves via the water column.

We will return to sites in Japan and China at which we have detected oyster herpes virus in C. ariakensis samples and will collect live oysters from these sites for importation into the U.S. for this study. The oysters will be brought back to VIMS and will be held under strict quarantine conditions. The oysters will be screened non-destructively for oyster herpes virus DNA by withdrawing hemolymph and tissue clips for DNA extraction. Oysters that are positive for OsHV will be selected for horizontal transmission studies by co-incubation with Crassostrea virginica or Mercenaria mercenaria spat in quarantine tanks. Water samples, as well as a few individuals (3-5) of the “challenged” species, will be taken every week or as mortality occurs and screened in the PCR assay for OsHV. Some of the oysters from Japan or China that are found to be positive in the PCR assay for OsHV will be conditioned for spawning. Progeny from these spawns will be held under quarantine conditions and embryos, larvae and spat will be collected intermittently and screened by histology, in situ hybridization and PCR for OsHV. Embryos and larvae will be collected every other day for screening until settlement. Following settlement, they will be screened weekly.

With the current interest in introducing the non-native oyster, Crassostrea ariakensis, to the Chesapeake Bay region for aquaculture, we have been screening samples collected in China and Japan for the parasites and potential pathogens of C. ariakensis in its natural range and the US hatchery stocks of C. ariakensis and C. virginica held at VIMS. Using a PCR assay for oyster herpes virus (OsHV) we have detected the viral DNA in C. ariakensis both from Japan and China, but not in hatchery stocks. The oyster herpes virus has been reported to cause massive mortalities in Pacific oyster larvae and spat in France, and in various species of oysters in New Zealand. What once was a virus confined to a few species has become a pathogen found in a growing number of bivalve species worldwide, suggesting that OsHv transmission might occur inter-specifically. In addition, vertical transmission of herpes-virus has been suggested, though it has not been proven. Until very recently, there have been no25-Jan-2008 since 1972. However, recent positive PCR results in C. gigas samples from Tomales Bay, California (Friedman et al. In press) and the positive results that we found in the Asian C. ariakensis indicate that it is critical to establish the nature and magnitude of this threat to the U.S. east coast hatchery and natural populations of bivalves. The degree and significance of OsHV risk associated with using hatchery reared triploid C. ariakensis for aquaculture in Virginia will be examined as a part of this project.