Low mercury (Hg) concentrations down to several nanograms Hg per gram of wet tissue are documented in certain fish species such as herbivorous fish, and the underlying mechanisms remain speculative. In the present study, bioaccumulation and depuration patterns of inorganic Hg(II) and methylmercury (MeHg) in a herbivorous rabbitfish Siganus canaliculatus were investigated at organ and subcellular levels following waterborne or dietary exposures. The results showed that the efflux rate constants of Hg(II) and MeHg were 0.104d(-1) and 0.024d(-1), respectively, and are probably the highest rate constants recorded in fish thus far. The dietary MeHg assimilation efficiency (68%) was much lower than those in other fish species (approximate to 90%). The predominant distribution of MeHg in fish muscle was attributable to negligible elimination of MeHg from muscle (<0) and efficient elimination of MeHg from gills (0.12d(-1)), liver (0.17d(-1)), and intestine (0.20d(-1)), as well as efficient transportation of MeHg from other organs into muscle. In contrast, Hg(II) was much more slowly distributed into muscle but was efficiently eliminated by the intestine (0.13d(-1)). Subcellular distribution indicated that some specific membrane proteins in muscle were the primary binding pools for MeHg, and both metallothionein-like proteins and Hg-rich granules were the important components in eliminating both MeHg and Hg(II). Overall, the present study's results suggest that the low tissue Hg concentration in the rabbitfish was partly explained by its unique biokinetics. Environ Toxicol Chem 2016;35:2074-2083. (c) 2016 SETAC

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