Salinity acclimation enhances salinity tolerance in tadpoles living in brackish water through increased Na⁺ , K⁺ -ATPase expression.

Amphibians are highly susceptible to osmotic stress but, nonetheless, some species can adapt locally to withstand moderately high levels of salinity. Maintaining the homeostasis of body fluids by efficient osmoregulation is thus critical for larval survival in saline environments. We studied the role of acclimation in increased physiological tolerance to elevated water salinity in the Indian rice frog (Fejervarya limnocharis) tadpoles exposed to brackish water. We quantified the effects of salinity acclimation on tadpole survival, osmolality, water content, and gill Na⁺ , K⁺ -ATPase (NKA) expression. Tadpoles did not survive over 12 hr if directly transferred to 11 ppt (parts per thousand) whereas tadpoles previously acclimated for 48 hr in 7  ppt survived at least 48 hr. We reared tadpoles in 3 ppt and then we transferred them to one of (a) 3 ppt, (b) 11  ppt, and (c) 7  ppt for 48 hr and then 11 ppt. In the first 6 hr after transfer to 11 ppt, tadpole osmolality sharply increased and tadpole water content decreased. Tadpoles pre-acclimated for 48 hr in 7 ppt were able to maintain lower and more stable osmolality within the first 3 hr after transfer. These tadpoles initially lost water content, but over the next 6 hr gradually regained water and stabilized. In addition, they had a higher relative abundance of NKA proteins than tadpoles in other treatments. Pre-acclimation to 7 ppt for 48 hr was hence sufficient to activate NKA expression, resulting in increased survivorship and reduced dehydration upon later transfer to 11 ppt. J