Maternal-embryonic mercury transfer in two Northwest Atlantic sharks with differing reproductive strategies

Mercury (Hg) is a pervasive contaminant in marine ecosystems, and early-life Hg exposure in fishes is largely driven by maternal provisioning. However, understanding of maternal contaminant transfer in cartilaginous fishes is limited, particularly regarding how reproductive strategies influence Hg dynamics during embryonic development. This study quantified total Hg in muscle, reproductive, and embryonic tissues of two coastal sharks from southern New England (Rhode Island, USA): the spiny dogfish (Squalus acanthias), a lecithotrophic ovoviviparous species, and smooth dogfish (Mustelus canis), a placental viviparous species. Comparisons of Hg concentrations among maternal and embryonic tissues were used to assess the effects of reproductive mode on bioaccumulation and early-life exposure. Muscle Hg in subadult-adult females increased with body size and age in both species, indicating progressive bioaccumulation. Smooth dogfish exhibited higher Hg concentrations and stronger age-related increases than spiny dogfish, consistent with differences in trophic ecology. Maternal transfer of Hg to embryos occurred in both species, but the strength and pathways of transfer differed markedly. Smooth dogfish showed strong associations among maternal muscle, ovarian yolk, and embryonic Hg concentrations, along with greater transfer to embryos, indicating sustained gestational exchange facilitated by placental viviparity. In contrast, spiny dogfish exhibited weaker maternal-embryo relationships and lower transfer efficiency, consistent with reliance on pre-fertilization yolk provisioning. Embryonic Hg concentrations declined with body size, reflecting intrauterine growth dilution, but remained higher in smooth dogfish. These findings demonstrate that reproductive mode strongly influences Hg transfer pathways and embryonic exposure in sharks, with implications for developmental toxicity and population vulnerability in contaminated coastal systems.