Oviparous Catsharks Accumulate Mercury in Deep-Sea Brine Pool Nurseries

Since the Eocene, oviparous deep-sea sharks and rays have used deep-sea hydrocarbon seeps and hydrothermal vents as nurseries, where they lay eggs en masse. Benthic fluxes in these extreme habitats may enrich seawater with toxic metals such as mercury (Hg). We asked whether this phenomenon may lead to Hg accumulation in elasmobranchs. We thus analyzed total Hg (THg) in muscle, liver, and kidney tissues of oviparous Galeus melastomus catsharks and their embryos, which aggregate in vast numbers near deep-sea brine pools in the southeastern Mediterranean Sea. These Hg-rich brines (≤238 ng L^-1) are a likely liable geogenic Hg source in this nursery. Shark tissues carried substantial THg [2.7 μg (g of wet weight)⁻¹, median], and extreme values were found in kidneys [≤26.7 μg (g of wet weight)⁻¹], likely due to environmental uptake. Increased THg in embryos [0.64 ± 0.31 μg (g of wet weight)⁻¹] implies substantial maternal offloading (∼20%). Our results hint at the potential adaptation of elasmobranchs to Hg-enriched environments via accumulation and elimination of Hg in kidneys.