Mercury isotopes as tracers of ecology and metabolism in two sympatric shark species

Coastal top predators are exposed to a wide variety of nutrient and contaminant sources due to the diversity of trophic chains within coastal areas. Here we measured mercury (Hg) isotopes in two co-occurring shark species (the bull shark Carcharhinus leucas and the tiger shark Galeocerdo cuvier) and their prey from a coastal ecosystem of the Indian Ocean, to (i) determine the main Hg source for these marine predators and (ii) explore the relevance of mercury isotopes as tracers of diet composition and feeding habitat. Hg isotope signatures (Δ¹⁹⁹Hg and δ²⁰²Hg) of shark prey suggested that bull sharks were exposed to methylmercury produced in the water column while tiger sharks were exposed to mesopelagic methylmercury with additional microbial transformation in shelf sediments. Δ¹⁹⁹Hg values efficiently traced the ecology of the two predators, demonstrating that bull sharks targeted coastal prey in shallow waters while tiger sharks were mainly foraging on shelf species in deeper layers. Unexpectedly, we found a large positive shift in δ²⁰²Hg between sharks and their prey, leading to extremely high δ²⁰²Hg values in the two shark species. Such δ²⁰²Hg values strongly suggest that significant Hg demethylation occurs in sharks, representing a protective mechanism to limit the accumulation of methylmercury and its toxic effects.