In many regions of the US, salts are used to clear roads of snow and ice in winter. These salts contaminate runoff and significantly increase the salinity of roadside ponds, which can have both direct and indirect effects on tadpole mortality. Toxicology studies have shown that sufficiently high salt concentrations are lethal to tadpoles, thus directly contributing to tadpole mortality. Less is known about how sub-lethal salt concentrations may affect tadpole behavior and thus contribute indirectly to mortality. Tadpoles exhibit a characteristic anti-predator escape behavior that consists of turning the head and swimming in the opposite direction from a predator, thus increasing the chances of surviving a predation attempt. The escape response is mediated by the lateral line, a sensory system that detects water movement. Normal function of the lateral line is dependent upon ion concentrations in the surrounding water. We predicted that exposure to salt would interfere with escape behaviors through disruption of lateral line function. To test this, we exposed green frog tadpoles to freshwater or sub-lethal salt concentrations (200mg/L or 800 mg/L NaCl) and measured escape behaviors elicited by water jets. We then compared response latency and swimming distance across the three experimental groups.
Tadpoles in all three treatment groups exhibited escape responses, although there were differences in the behavioral metrics we measured. Preliminary analyses suggest that tadpoles exposed to both low (200mg/L) and high (800 mg/L) salt concentrations swam shorter distances and had an increase in response latency compared to controls. There was no difference in these metrics between the low salt and high salt groups. In addition, tadpoles in the high salt group were significantly larger than those in the low salt and control groups. These results suggest that exposure to sub-lethal salt concentrations reduces anti-predator escape behaviors, making tadpoles more susceptible to predation attempts. It is likely that salt exposure disrupts the lateral line system, thus decreasing the tadpole’s ability to detect and respond to water currents produced by a predator. This represents an indirect effect of salt on tadpole mortality, and suggests that road salts commonly used for deicing may pose a serious ecological threat to amphibian populations, even at sub-lethal concentrations.