A mystery tale: Nickel is fickle when snails fail. Investigating the variability in Ni toxicity to the great pond snail.

Dissolved nickel (Ni) concentrations inhibiting the growth of juvenile great pond snails (Lymnaea stagnalis) have been documented to vary from about 1 to 200 µg L-1 Ni. This variability makes L. stagnalis either a moderately sensitive or the most sensitive freshwater species tested to date to chronic Ni exposure. Given the role of sensitive species in environmental risk assessment frameworks, it is particularly important to understand this variability, i.e. to characterize the factors that modulate Ni toxicity and that may confound toxicity test outcomes when uncontrolled. In the present study, we tested if this variability is due to analytical (growth calculation: biomass versus growth rate), environmental (water quality), lab-specific practices and/or source population differences among earlier studies. Specifically, we re-analyzed previously published Ni toxicity data and performed additional measurements of Ni aqueous speciation, short-term Ni uptake and chronic Ni toxicity with test waters and snail cultures used in previous studies. Corrections for nickel bioavailability and growth calculations explained a large degree of variability in the published literature. However, a residual 16-fold difference remained puzzling between two studies: Niyogi et al. (2014) (low ECxs) and Cremazy et al. (2018) (high ECxs). Indeed, differences in metal bioavailability due to water chemistry, lab-specific practices, and in snail population sensitivity could not explain the large variation in Ni toxicity in these two very similar studies. Other potentially important toxicity-modifying factors were not directly evaluated in the present work: test duration, diet, snail holding conditions and snail age at onset of testing. The present analysis highlights the need for further studies to elucidate i) the mechanisms of growth inhibition in Ni-exposed L. stagnalis and ii) the important abiotic and biotic factors affecting this biological response. Until these processes are understood, substantial uncertainties will remain about inclusion of this species in Ni environmental risk assessment. This article is protected by copyright. All rights reserved.