The invertebrate shredder Gammarus pulex L. is a key species for aquatic carbon turnover via litter decomposition and can thrive in high-arsenic (As) environments. To understand their strategies for coping with increased As concentrations while fulfilling their ecosystem functions, we analyzed the As concentration and speciation in their aquatic habitat and in leaves with heterotrophic biofilms as their natural food source. We also followed the As distribution and speciation on the cuticle and within the body of G. pulex by X-ray absorption spectroscopic imaging. Half of the total As on G. pulex was found to be associated with the cuticle but was not taken up. Removing this externally bound As yielded only arsenate in the wash solution which reflects the speciation of the surrounding aquatic phase and shows that this As does not undergo any biotransformation. The major pathway into the organism is suggested to be incorporation via food intake, but only very low amounts of As were taken up or translocated from the gut system to other tissues. In one of the main food sources, leaves, 68 % arsenate and 29 % monomethylarsenate were found. After ingestion into the gut system, up to 23 % of the more toxic arsenite was seen, but a substantial share was methylated to dimethylarsenate (46-56 %). Little arsenate and arsenite were found in the adjacent tissues. . Besides 76-80 % mono- and dimethylarsenate, 10-21 % of the As was complexed as As(III)-S species. G. pulex plays an important role in As cycling and our results indicate that it As translocation from the gut to other tissues is minimized , but a transformation to other As-species occurred.