Effects of Freshwater Salinization and Biotic Stressors on Amphibian Morphology

Abstract

Organisms are commonly exposed to numerous stressors that induce behavioral, physiological, or morphological changes in some combination. At northern temperate latitudes, de-icing agents (primarily sodium chloride, NaCl) are a major stressor to species in freshwater ecosystems. Species-specific responses to road salt toxicity range from lethal to sublethal effects, but it remains unclear how these effects interact with biotic stressors. Morphology can be quite sensitive to environmental changes, yet we know little about how it is affected by road salt exposure. We exposed Wood Frog tadpoles (Rana sylvatica) to two road salt formulations (NaCl and a mixture of NaCl, MgCl2, and KCl), each at three concentrations (200, 600, and 1000 mg Cl–/L), crossed with three biotic stressor levels (predator cue, competition, and a no-stressor control). We then measured the impacts on relative morphology (snout–vent length, body width, forelimb length, forelimb width, hindlimb length, hindlimb width) of the emerging metamorphs. Salt concentration and biotic stressors both impacted relative morphology, but their effects did not interact. Exposure to road salts increased relative snout–vent length (SVL) and body width. In contrast, competition induced relatively shorter SVL and forelimb length while predator cues induced relatively longer hindlimbs and narrower forelimbs. This is the first discovery that road salts can induce changes in amphibian morphology and that these effects are independent of changes induced by biotic stressors. Future research should examine the effects on overwintering success and future fitness in amphibians as well as the impacts of salt on the morphology of other aquatic taxa that are being exposed to freshwater salinization.