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 12hr if directly transferred to 11ppt (parts per thousand) whereas tadpoles previously acclimated for 48hr in 7ppt survived at least 48hr. We reared tadpoles in 3ppt and then we transferred them to one of (a) 3ppt, (b) 11ppt, and (c) 7ppt for 48hr and then 11ppt. In the first 6hr after transfer to 11ppt, tadpole osmolality sharply increased and tadpole water content decreased. Tadpoles pre-acclimated for 48hr in 7ppt were able to maintain lower and more stable osmolality within the first 3hr after transfer. These tadpoles initially lost water content, but over the next 6hr gradually regained water and stabilized. In addition, they had a higher relative abundance of NKA proteins than tadpoles in other treatments. Pre-acclimation to 7ppt for 48hr was hence sufficient to activate NKA expression, resulting in increased survivorship and reduced dehydration upon later transfer to 11ppt. J. Exp. Zool. 321A: 57-64, 2014. (c) 2013 Wiley Periodicals, Inc.