Using bivalves as particle collectors with PCR detection to investigate the environmental distribution of Haplosporidium nelsoni

Recently, PCR technology has been applied to search for marine microorganisms in environmental samples. Such sampling, however, has drawbacks, including the need to collect and filter large volumes of water, and the presence of substances in environmental samples that may destroy DNA or interfere with DNA isolation and amplification. We explored the possibility of using suspension-feeding bivalves in conjunction with PCR to investigate the environmental distribution of microparasites using the oyster pathogen Haplosporidium nelsoni (the MSX disease agent) with oysters and mussels in Delaware Bay, USA, as a model system. Delaware Bay oysters Crassostrea virginica have become very resistant to H. nelsoni infection development and rarely exhibit histologically detectable lesions. The ribbed mussel Geukensia demissa is also resistant. Infections were detected in only 6% of the histologically examined oysters in the present study, although PCR-positive signals for H. nelsoni were found in feces, gill or heart samples of up to 100% of oysters. Positive signals were found in the feces and gill (but not heart) samples of up to 50% of mussels. The temporal evolution of PCR signals suggested a wave-like pattern of putative infective particles, which mimicked a pattern documented in early mortality studies. The present study demonstrated that H. nelsoni persists throughout Delaware Bay, although it is rarely detected using standard histology. We propose that the use of suspension-feeding bivalves as particle collectors in combination with PCR could be a method for detecting the presence of marine microparasites and might help answer questions about factors that prevent outbreaks of epizootics in certain regions.