Lethal and sub-lethal implications of sodium chloride exposure for adult Unionid mussel species; Eurynia dilatata and Lasmigona costata

Abstract

The elevated use of salt as a de-icing agent on roads in Canada is causing an increase in the chloride concentration of freshwater ecosystems. Freshwater Unionid mussels are a group of organisms that are sensitive to increases in chloride levels. Unionids have greater diversity in North America than anywhere else on Earth, but they are also one of the most imperiled group of organisms. This underscores the importance of understanding the effect that increasing salt exposure has on these threatened species. There is more data on the acute toxicity of chloride to Unionids than on chronic toxicity. This study investigated the effect of chronic sodium chloride exposure on the survival and filtering activity of two Unionid species (Eurynia dilatata, and Lasmigona costata,) and assessed the effect on the metabolome in mussel hemolymph. The concentration causing mortality after 28 days of exposure was similar for E. dilatata (1893 mg/L) and L. costata (1903 mg/L). After 7 days of exposure, the concentration causing a 10% decline in the filtering activity of L. costata was 482 mg/L compared to 755 mg/L for E. dilatata. Significant changes in the metabolome of the L. costata hemolymph were observed for mussels exposed to nonlethal concentrations. For example, several phosphatidylethanolamines, several hydroxyeicosatetraenoic acids, pyropheophorbide-a, and alpha linolenic acid were significantly upregulated in the hemolymph of mussels exposed to 1000 mg Cl−/L for 28 days. While no mortality occurred in the treatment, elevated metabolites in the hemolymph are an indicator of stress.