Macroparasite infection and mercury contamination as coupled stressors in Eastern oysters.

Contaminants and parasites are ubiquitous in aquatic environments and their interactions can have detrimental effects on host physiological and ecological responses. Yet, few studies have investigated the combined influence of chemical stress and parasite infections on bivalves. We studied the relationships between mercury (Hg) and methylmercury (MeHg) and macroparasite infections of pea crabs (Zaops ostreum) on Eastern oysters (Crassostrea virginica). Between 2022 and 2023, pea crabs (n = 95) were opportunistically collected during monthly sampling of oysters (n = 674) deployed at four coastal sites in the Long Island Sound (CT, USA). Hg concentrations in oysters with pea crabs (least-squares mean ± SE: 28.8 ± 1.07 ng g-1 ww) were 13 % higher than in oysters without pea crabs (25.4 ± 1.08 ng g-1 ww), with no observed difference in MeHg concentrations nor %MeHg. We did not observe significant relationships between pea crab and oyster tissue concentrations of Hg, MeHg, or %MeHg. Pea crab infection and oyster tissue Hg and MeHg concentrations were independently associated with reduced oyster condition index, suggesting an additive but not synergistic influence of these two stressors on oyster health. These results motivate additional research into the dynamics of Hg bioaccumulation in oyster-pea crab systems and their implications for the health of oysters and their consumers.