Ecotoxicity responses of the freshwater cnidarian Hydra attenuata to 11 rare earth elements

Lanthanides are the major family of rare earth elements (REEs) owing to the essential properties these metallic species provide in diverse fields of today's world economy. They are now being mined and produced as never before. This raises new environmental concerns in terms of their expected future discharges notably to aquatic systems. Interspecies studies of their ecotoxicity are sparse and effects on aquatic life are still poorly understood. Absence of such information for cnidarians, an ecologically relevant freshwater community, thus prompted the present research on REEs toxicity using Hydra attenuata as our animal model. Lethal and sublethal ecotoxicity data generated with the 11 REEs displayed LC50 values ranging from 0.21 to 0.77 mg L^-1and EC50 values ranging from 0.02 to 0.27 mg L^-1, thereby confirming the inherent sensitivity of Hydra to REE exposure at environmentally relevant concentrations. Additionally, two properties of REEs were shown to modulate Hydra (sub)lethal toxicity (LC50 and EC50) which decreases with increasing atomic number and with decreasing ionic radius. Compared to studies carried out with different taxonomic groups, Hydra toxicity responses to REEs proved to be among the most sensitive, along with those of other invertebrate species (i.e., Daphnia magna, Ceriodaphnia dubia, Hyalella azteca), suggesting that members of this community are likely more at risk to eventual REE discharges in aquatic environments. Demonstrated Hydra sensitivity to REE exposure strongly justifies their future use in toxicity testing battery approaches to evaluate liquid samples suspected of harbouring REEs.