Environmental context Knowledge of the pathways by which arsenic is accumulated and transferred in marine ecosystems is scarce. Molluscs are important keystone organisms providing a link between primary producers (micro and macroalgae) and higher trophic levels such as fish. The present study examines the accumulation and species of arsenic in common bivalve molluscs from south-east Australia to understand the cycling of arsenic in marine food webs. Abstract The present paper reports the whole-tissue total arsenic concentrations and water-soluble arsenic species in 12 common coastal Australian bivalve mollusc species. Mean arsenic concentrations ranged from 18 to 57 g g -1 dry mass. Planktivores had significantly less arsenic (20-40 g g -1; 22 ± 3 g g -1) than did suspension and deposit feeders (36-57 g g -1; 43 ± 7 g g -1), with those associated with fine clay-silt sediments (49 ± 7 g g -1) having significantly more arsenic than those associated with sand substrates (31 ± 11 g g -1). Most planktivores and suspension feeders had similar arsenic species, with high proportions of arsenobetaine (AB) (64-92 %) and relatively low proportions of other arsenic species (0.55-15.8 %). Lower proportions of AB (13-57 %) and larger proportions of inorganic arsenic (6-7 %) were found in deposit feeders, reflecting increased exposure to inorganic arsenic in sediments. The study indicated that at lower trophic levels, organisms feed on algae and suspended matter containing a range of arsenic species including arsenosugars and AB. The implications for arsenic cycling are that as all bivalve molluscs accumulate AB and are a source of AB in benthic food webs. Because all bivalve molluscs also contained appreciable concentrations of arsenoriboses, precursors are present for the de novo synthesis of AB. As well, deposit feeders have higher proportions of inorganic arsenic that can be metabolised to different end products when ingested by higher trophic organisms.