Effects of freshwater salinization and biotic stressors on amphibian morphology

Organisms are commonly exposed to numerous stressors that induce
behavioral, physiological, or morphological changes in some combination.
In northern temperate latitudes, de-icing agents are a major stressor to
species in freshwater ecosystems (primarily sodium chloride; NaCl).
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 on other aquatic taxa that are
being exposed to freshwater salinization.