Molecular, histological, and behavioral differences in largemouth bass (Micropterus salmoides) and topsmelt (Atherinops affinis) exposed to methylmercury

Methylmercury (MeHg) pollution in aquatic environments is recognized as a serous threat, yet the sublethal effects of MeHg exposure on fish are not well understood, even though sublethal effects may affect reproductive fitness and long-term survival. I examined the relationship between environmental MeHg exposure and two sublethal biomarkers of effect (i.e., metallothionein gene expression, histopathological assessment in liver, spleen, and kidney tissues) in largemouth bass (Micropterus salmoides) collected from the Sacramento-San Joaquin Delta, CA, USA. The relationship between MeHg exposure and histopathological parameters (i.e., immune responses, parasite density) was compared between juveniles and adults. Metallothionein gene expression was lower, yet not statistically significant (p=0.081), in high Hg exposed adults, suggesting an inhibition of the metallothionein response mechanism with increased MeHg exposure. Histopathological results indicated that juvenile largemouth bass could be more susceptible than adults to MeHg exposure, as high-exposed juveniles appeared immunosuppressed, with significantly lower macrophage density in kidney and liver tissues (p=0.018, and 0.020, respectively), higher trematode density in liver tissue (p=0.014), and increased trematode maturation. To evaluate the behavioral effects of MeHg exposure, a dosing study was performed on topsmelt (Atherinops affinis) and change in school area was examined after control, low, and high MeHg exposure treatments. Results suggested an increased school area with MeHg exposure, but these findings were not statistically significant. These results suggested that chronic, low level MeHg exposure could cause adverse effects on largemouth bass before reaching reproductive maturity, as high MeHg exposed juveniles exhibited markers of immunosuppression