Histological evaluations of organ tissues reveal sublethal effects in a freshwater mussel (Villosa iris) exposed to chloride and potassium concentrations below benchmark estimates

•Increasing salinization of freshwater ecosystems continues.•Microscopical evaluations of freshwater mussel organs determined effects of K+ and Cl−.•Sublethal effects to digestive glands and kidneys at concentrations below published benchmarks.•Sublethal effects to organs may influence physiological functions and energy for somatic investment. Salinization of freshwater ecosystems due to anthropogenic sources will increasingly impact biodiversity. An example of point-source industrial salinization has occurred from historical activities at a U.S. Environmental Protection Agency Superfund Site near Saltville, Virginia USA and its associated chemical waste ponds adjacent to the North Fork Holston River. These point source discharges are documented contributors to mussel declines, partially due to high concentrations of chloride (Cl−, ≤ 26,000 mg Cl−/L) and potassium (K+, ≤ 97 mg K+/L). During a chronic 61-day laboratory study, Rainbow mussels, Villosa iris, were exposed to concentrations of Cl- (0, 416, 831, and 1,663 mg/L) and K+ (0, 4, 8, and 17 mg/L) to determine effects on survival and organ tissues. All test mussels died by day-2 in the 1,663 mg Cl−/L exposure, and 50% of mussels died by day-13 in the 17 mg K+/L concentration. Significantly greater abundances of tissue abnormalities were observed in digestive glands and kidneys with exposures to the 4 and 8 mg/L concentrations of K+ versus the control, and significantly greater abundances of lesions in kidneys were observed in the 416 and 831 mg Cl−/L concentrations compared to the control. The sublethal effects to digestive glands and kidneys were below reported effect (EC50, 20, 10 and LOEC) concentrations. Significant histological differences between control and baseline (day-0 sample) mussels were observed, suggesting the need for further study on the effects of captivity during longer-term laboratory experiments.