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In general, mercury (Hg) undergoes biomagnification in aquatic systems. The absence of Hg biomagnification in a certain aquatic environment constitutes an exceptional finding and this seems to be the case for Sepetiba Bay, in Rio de Janeiro state (RJ), Brazil. There are three distinct ecological populations of Guiana dolphins in the Sepetiba Bay (SB)-Ilha Grande Bay (IGB) Complex, inhabiting: (1) the inner part of SB; (2) SB entrance; and (3) IGB. In addition, there are two other delphinid species, rough-toothed dolphin and Atlantic spotted dolphin, that feed on the SB-IGB Complex. Considering the widely employed use of cetaceans as sentinels of environmental contamination by bioaccumulative toxicants, we have biopsy sampled individuals of the abovementioned ecological populations/species for measuring skin Hg concentrations. Two Bryde's whales and one humpback whale were biopsied in the SB-IGB Complex as well. Skin Hg concentrations [μg g -1 dry weight (dw)] of Guiana dolphins were the highest in IGB, followed by SB entrance and the inner part of SB (0.99-5.47; 0.09-6.00; 0.08-2.22). Considering all species investigated in the present study, skin Hg concentrations were found in the following order: humpback whale < Bryde's whale < Guiana dolphins from SB inner part < Guiana dolphins from SB entrance < Guiana dolphins IGB = Atlantic spotted dolphins < rough-toothed dolphins. The skin Hg concentrations found in Guiana dolphins from the inner part of Sepetiba Bay (0.08-2.22) and rough-toothed dolphins from the SB-IGB Complex (1.26-20.0) are among the lowest and highest ever reported for dolphins worldwide, respectively., Competing Interests: Declarations. Competing interests: The authors have no relevant financial or non-financial interests to disclose., (© 2024. The Author(s), under exclusive licence to Springer Science+Business Media, LLC, part of Springer Nature.)