Mercury (Hg) is a global contaminant that biomagnifies in food webs, raising concerns for food safety, fisheries exploitation, and wildlife conservation. Fish, including apex predators like sharks, are the primary source of human Hg exposure, yet species-specific speciation data remain scarce. Most studies rely on total Hg (THg) as a proxy for methylmercury (MeHg), but direct MeHg measurement is essential for accurate risk assessment due to neurotoxicity and bioavailability. This study presents a comprehensive assessment, quantifying THg-MeHg in 18 species from the Mediterranean, Indian, and Atlantic Oceans, nine measured for the first time. Concentrations varied widely, with deep-sea and pelagic sharks showing highest levels. THg and MeHg strongly correlated (R2 = 0.99), but MeHg-THg ranged 65-101%, demonstrating substantial interspecific variability and challenging the assumption of near-complete methylation. Bioaccumulation increased with body size and trophic level, and biomagnification was pronounced in Mediterranean deep-sea assemblages. Nearly half of the species exceeded the 1 mg kg-1-EU Hg limit. Target Hazard Quotients exceeded 1 for deep-sea and large pelagic sharks, highlighting tangible health risks. Elevated MeHg levels in commercial fillets confirm consumer exposure. Species with the highest MeHg burdens are heavily exploited and threatened, identifying globally traded sharks as hotspots of human Hg exposure.
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