Mercury Distribution in the Host-Parasite System of the Common Toad (Bufo bufo) Across Contrasting Habitats

The present study investigates how host-parasite interactions influence mercury (Hg) distribution in the common toad (Bufo bufo) under contrasting environmental conditions. Using road-killed specimens as an ethical sampling source, Hg concentrations were quantified in multiple host tissues (muscle, liver, kidneys, lungs, and skin) and associated parasitic nematodes from two sites in eastern Slovakia with different contamination levels. In both populations, Hg accumulation followed a tissue-specific pattern, with the highest concentrations occurring in detoxification organs, particularly the kidneys and liver. Bayesian regression models revealed that habitat was the primary driver of Hg burdens, whereas biological factors, including sex and body condition factor, had no significant effect on host Hg concentrations. Parasitological analyses showed markedly higher infection levels at the contaminated site, with nematode prevalence reaching 100%, and females harbouring significantly higher parasite abundances than males. Morphometric analysis based on 14 measured traits revealed no differences between populations, suggesting that chronic Hg exposure did not induce detectable morphological divergence. Although parasite abundance was not a strong predictor of host Hg levels, bioconcentration factors indicated a concentration-dependent host-parasite interaction: under high environmental exposure, host detoxification organs accumulated more Hg than parasites, whereas at lower contamination levels, nematodes sequestered Hg more efficiently than host tissues. These findings emphasize that integrating host-parasite dynamics into ecotoxicological frameworks is essential for a comprehensive assessment of the biological fate of Hg in amphibian populations.