As filter-feeding and benthonic organisms,mollusks provide the entry point of mercury(Hg)from sediment and water to top predators through subsequent trophic transfer.The efficiency of this water/sediment-to-food web transfer of Hg and the subsequent biomagnification within low trophic levels governs the regional exposure of marine organisms[1].There are multiple recognized mollusk species,with highly diverse structure,composition and feeding habit.However,they are usually regarded as one group lying at the second trophic level in the marine ecosystem.It still remains unknown whether the biomagnification of Hg occurs in low trophic level mollusk food webs.Due to overfishing and pollution,the Chinese Bohai Sea is confronted with the decrease of fishing yield and the changes of fishery structure[2].Mollusks have gradually constituted the predominant part of the fishery structure and the important seafood source of human Hg exposure.Studying the uptake and trophic transfer of Hg in mollusks is essential not only for human exposure but also for the bioindication of Hg pollution.In this study,9 species of bivalves and 2 species of gastropods were collected from the coastal areas of the Chinese Bohai Sea during 2007-2012,with a total of 431 samples.The contents of total mercury(THg),methylmercury(MeHg),isotopic signatures(δ15N and δ13C)and lipid contents were comprehensively analyzed.THg and MeHg levels were in the range 27.2-461.1 and 2.1-295.5 μg kg-1,respectively.The δ15N values ranged from 4.23 to 11.94‰,suggesting the trophic levels(TLs)were in the range 1.99-4.02.The biomagnification of Hg in mollusks was evident from the significant positive correlations between THg and MeHg contents and TLs,and from the trophic magnification factors(TMFs).MeHg is the main species of Hg magnifying in mollusks,while growth dilution occurs in the trophic transfer of inorganic mercury(IHg).TLs showed more effect on Hg levels in mollusks than lipid contents.