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Fractionation of rare earth elements (REE) and other trace metal concentrations (Th,U,Cd,Cr,Cu,Ni,Pb,and Zn) between mangrove sediments and claw muscles and shells of male crabs (Ucides cordatus) from a subtropical estuary highly impacted by fertilizer industry activities was investigated.This is the first record of REE distribution in these organisms,and the results showed higher accumulations of these metals,U and Th in shells,probably related to the replacement of Ca during molting.Contents of Cd,Cr and Ni were similar in both tissues,but Cu,Zn and Pb were mostly accumulated in the claw muscle with concentrations above those considered safe for human consumption according to the Brazilian legislation.REE fractionation was different in the analyzed tissues being softer in the shells.The results provided evidences that the water absorbed during molting controls the chemistry of REE in shells.In contrast,the chemistry of REE in the claw muscle,in which was observed preferential absorption of light REE,is controlled by diet.REE fractionation obtained for the claw muscles was closely correlated to the observed in the contaminated substrate and in materials related to the production of phosphate fertilizers (contamination source),which supports their transference to this Ucides cordatus tissue without fractionation by the ingestion of sediments.Our results showed the potential use of crab tissues for monitoring REE and trace element sources in mangrove areas,with claw muscle exhibiting the contaminant source fingerprint.