Earthworms influence metal transfer from soil to snails

Most of the studies focusing on metal transfer from soil to biota ignore the possible non-trophic influence of an organism on pollutant transfer to other species. We hypothesised that an earthworm (Aporrectodea tuberculata) might modify the bioavailability of metals in soil and thus, their transfer to the snail Helix aspersa. Snails were exposed for 2 weeks to a multicontaminated field soil with or without earthworms, under controlled conditions. When exposed with earthworms, snails had higher concentrations of Cd, Cu and Zn than when they were exposed alone, while no difference was detected for Pb. For Cd only, the difference in snail bioaccumulation corresponded to an increase in its water-soluble fraction. Internal concentrations of metals in earthworms remained similar in the presence or absence of snails. Two non-exclusive possible mechanisms, including variations in bioavailable fractions and/or total accessible pools of metals, are proposed to explain how earthworms could modulate the transfer of metals from soil to snails. This work demonstrated that metal transfer from soil to one invertebrate species was influenced by another invertebrate. We conclude that the concept of intermediary species, usually used to describe interactions among species, should be extended to the interactions between biota and pollutants in non-biotic compartments.