The influence of multiple environmental stressors on susceptibility to parasites: An experimental determination with oysters

A large-scale field experiment was used to test whether exposure to a suite of potential environmental factors (flow speed, temperature, salinity, and low dissolved oxygen) influences the level of parasitic infection of the o yster Crassostrea virginica. The parasite was the protozoan Perkinsus marin us, which has decimated populations of oysters in estuaries of North Americ a. The environmental factors were considered stressors because they influen ce the physiological condition of either the host or parasite. Between Dece mber 1994 and July 1995, flow speed, temperature, salinity, dissolved oxyge n concentration (DO), Perkinsus infection, and mortality of oysters were mo nitored across 24 experimental oyster reefs in the Neuse River estuary, Nor th Carolina. Eight reef treatments were created consisting of an orthogonal combination of three factors: water depth (3 m vs. 6 m deep), reef height (2 m vs. 1 m tall), and position on reef (base vs, crest). Principal compon ent analysis revealed that there was clear separation of environmental fact ors among reefs and that a majority of the variation (96.2%) among treatmen ts could be explained by two principal component axes (PCs): one (24.3% of variation explained) was formed by flow and the other (71.9% of variation e xplained) by temperature, salinity, and DO. Oysters with the highest propor tion of individuals infected (prevalence), highest intensity of infection, and highest mortality were located at the base of reefs, where flow speeds and food quality were lowest and sedimentation rates highest. However, ther e was no significant effect of hydrographic conditions on Perkinsus infecti on or mortality of oysters, despite large differences in mean salinity, DO, and exposure to hypoxia-anoxia. Temperature did not vary among treatments. Correlation of disease responses (infection prevalence and intensity, and mortality) with the first two PCs showed that these response variables were significantly (and negatively) correlated with flow only. Oysters in low f low were hypothesized to have:the greatest susceptibility to Perkinsus infe ction because of their poor physiological condition. The restoration of oys ter reefs increases reef height and thus flow speeds (by elevating oysters higher above the boundary and by actively influencing flow), thereby reduci ng the negative effects of disease by restoring reef morphology to its natu ral size.