The effects of tidal elevation on parasite heterogeneity and co-infection in the eastern oyster, Crassostrea virginica

Risk of infection by parasites can be driven by environmental heterogeneity, often at small scales. We quantified the effect of tidal elevation on infection patterns of two lethal parasites, Perkinsus marinus and Haplosporidium nelsoni, in an important coastal species, the eastern oyster, Crassostrea virginica. Within the southeastern US, oysters in Georgia and South Carolina are rarely found in the subtidal zone. Historically, it has been hypothesized that this pattern could be due to the increased exposure of hosts to waterborne parasites. We manipulated oysters at two tidal elevations (intertidal and subtidal) in a Georgia estuary to test if P. marinus, H. nelsoni, and co-infection by both parasites were different between tidal heights. We found that though P. marinus prevalence and co-infection prevalence of both parasites were not significantly different between tidal elevations, as has been found previously, P. marinus intensity and H. nelsoni prevalence were significantly higher intertidally than subtidally. These findings show that parasite infections can be higher in the host's natural (preferred) tidal height, and suggest that longer exposure to parasites in the subtidal is not a likely reason for the paucity of oysters at that tidal elevation in certain regions of the southeastern US. More broadly, our results provide further evidence that environmental effects on host-parasite interactions can vary by parasite species and across small (meter-long) spatial scales.