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.

生物通常会暴露于多种胁迫因子之下，这些因子会联合诱导其产生行为、生理或形态层面的改变。在北温带纬度地区，除冰剂（主要成分为氯化钠，即NaCl）是淡水生态系统中物种面临的主要胁迫因子之一。不同物种对道路盐毒性的响应范围覆盖致死效应至亚致死效应，但目前尚不清楚这些效应与生物胁迫因子之间的交互作用机制。形态特征对环境变化极为敏感，然而我们对道路盐暴露如何影响生物体形态仍知之甚少。本研究将林蛙（Rana sylvatica）蝌蚪暴露于两种道路盐制剂中：分别为氯化钠（NaCl）以及NaCl、氯化镁（MgCl₂）与氯化钾（KCl）的混合物，每种制剂设置三个氯离子浓度梯度：200、600及1000 mg Cl⁻/L，并与三个生物胁迫水平（捕食者线索、竞争胁迫以及无胁迫对照组）开展交叉实验设计。随后我们测定了各处理对变态发育个体相对形态指标（吻肛长（snout-vent length, SVL）、体宽、前肢长、前肢宽、后肢长、后肢宽）的影响。研究结果显示，道路盐浓度与生物胁迫因子均会对相对形态指标产生影响，但二者的效应不存在交互作用。道路盐暴露会使个体的相对吻肛长与体宽显著增加；与之相反，竞争胁迫会导致相对吻肛长与前肢长缩短，而捕食者线索则会使后肢相对变长、前肢相对变窄。本研究首次发现道路盐可诱导两栖动物形态发生改变，且此类效应与生物胁迫因子所引发的形态变化相互独立。未来研究可进一步探讨道路盐暴露对两栖动物越冬成功率及后续适合度的影响，同时评估盐污染对其他面临淡水盐化胁迫的水生类群形态的影响。