Mercury speciation and selenium dynamics across marine tetrapods: a cross-taxa assessment

Mercury (Hg) is a globally distributed contaminant that bioaccumulates and biomagnifies in marine food webs, yet total mercury (THg) concentrations alone often provide limited insight into the toxicological significance of Hg exposure because toxicity depends strongly on Hg speciation. Here, we investigated hepatic concentrations of THg, methylmercury (MeHg), and selenium (Se) in five marine tetrapod species: the green turtle (Chelonia mydas), kelp gull (Larus dominicanus), Magellanic penguin (Spheniscus magellanicus), franciscana dolphin (Pontoporia blainvillei), and Guiana dolphin (Sotalia guianensis). A total of 100 liver samples spanning a broad gradient of hepatic THg concentrations were analyzed. Species differed primarily in Hg speciation patterns (%MeHg) and Se:Hg stoichiometry rather than in absolute mercury burdens. Median THg concentrations ranged from 2.72 to 38.97 mg kg^-1 dw, whereas median %MeHg values ranged from only 0.3% in S. guianensis to 15.9% in L. dominicanus. Sotalia guianensis presented the highest THg concentrations but the lowest %MeHg, whereas S. magellanicus exhibited the highest detected MeHg concentrations despite comparatively lower THg levels. Selenium concentrations varied less than Hg concentrations and were positively associated with THg (R² = 0.294, p < 0.001), but not MeHg. Descriptively, seabirds showed stronger THg-MeHg associations than cetaceans, although the species × THg interaction for MeHg was not statistically significant. These findings indicate that THg alone does not consistently reflect the methylmercury fraction across species and that Hg speciation provides additional ecological and physiological information relevant to the potential toxicological significance of Hg exposure. Our results highlight contrasting Hg speciation and processing patterns among marine tetrapods and support incorporating Hg speciation into biomonitoring programs.