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Hydrothermal vents are surficial expressions of subsurface geological and hydrological processes. Fluids emitting from active vents are chemically distinct from bottom seawater, and are enriched in dissolved metals and metalloids, including arsenic. Vent organisms accumulate arsenic but the arsenic speciation in these non-photosynthetic organisms is largely unknown. Here, arsenic concentrations and chemical species were measured in three deep sea hydrothermal vent worms (Ridgeia piscesae, Paralvinella sulfincola and Paralvinella palmiformis) from the Juan de Fuca Ridge in the Northwest pacific. R. piscesae has similar arsenic concentrations (3.8-35 mu g g(-1)) to shallow water polychaetes while P. sulfincola and P. palmiformis have significantly higher arsenic concentrations (420-1417 and 125-321 mu g g(-1) respectively). R. piscesae contains appreciable quantities of inorganic arsenic (36 +/- 14%), monomethyl arsenic (2 +/- 2%), dimethyl arsenic (34 +/- 21%), an unknown methyl arsenical (7 +/- 16%), OSO3-arsenosugar (5 +/- 9%), TETRA (4 +/- 5%), ThioPO(4)/ThioDMAE (1 +/- 2%) and an unknown thio-arsenical (12 +/- 14%). These results suggests that host and symbionts are either involved in the methylation of arsenic, or are bathed in fluids enriched in methylated arsenic as a result of free-living microbial activity. The host carrying out methylation, however, cannot be ruled out. In contrast, 96-97% of the arsenic in P. sulfincola and P. palmiformis is inorganic arsenic, likely the result of arsenic precipitation within and upon the mucus they ingest while feeding. While all worms have oxo- and thio arsenosugars (2-30%), Paralvinella also have small amounts of arsenobetaine (< 0.001-0.21%). The presence of arsenosugars, arsenobetaine and other minor arsenic species in the absence of photosynthesising algae/bacteria indicates that they may be formed by vent animals in the absence of sunlight, but at this time their formation cannot be explained. (C) 2016 Elsevier Ltd. All rights reserved.