Enhanced salt tolerance of euryhaline tadpoles depends on increased Na+, K+-ATPase expression after salinity acclimation

Understanding physiological responses and osmoregulatory mechanisms for dealing with salinity stress is essential to clarify how amphibians living in coastal areas adapt to fluctuating salinity levels. Euryhaline species are rare among reported tadpole species inhabiting saline habitats, and few studies addressed the osmoregulatory mechanisms. We quantified the effects of salinity acclimation on survival, osmolality, water content, ion concentration, and gill Na+, K+-ATPase (NKA) expression of euryhaline tadpoles of Fejervarya cancrivora, to examine time-course changes of osmoregulatory responses of tadpoles subjected to salinity stress and how osmoregulatory mechanisms were involved in the process. Acclimation to 10 ppt for 24 h increased tadpole survival of F. cancrivora in 15 ppt, and it activated osmoregulatory mechanisms such as increase in NKA expression, which enabled them to maintain a stable osmolality below that of the surrounding media, to reach lower sodium and chloride concentrations of body fluid, and to modulate dehydration at higher salinities. The minimum required acclimation period is shorter than that reported previously on this species and non-euryhaline tadpoles. This study highlights that these physiological mechanisms are ecologically relevant and critical for tadpoles living in coastal brackish waters, improving their survival in coastal microhabitats with highly variable salinity levels.