Transport-related enzymes and osmo-ionic regulation in a euryhaline freshwater shrimp after transfer to saline media

To understand the response of freshwater organisms to rising environmental salinity, it is essential to investigate their osmoionic regulatory physiology. Our laboratory experiment investigated the transfer of Palaemon argentinus (Nobili, 1901) from 2[per thousand] (control condition) to concentrated salinity (15 and 25[per thousand]) for short- (6 h), medium- (48 h), and long-term (>504 h) acclimation periods. We measured relevant parameters in the shrimp's haemolymph, the time course of the response of branchial V-[H.sup.+]-ATPase (VHA), [Na.sup.+], [K.sup.+]--ATPase (NKA), carbonic anhydrase (CA) activity, and muscle water content. Upon prolonged acclimation to 15[per thousand] (hyper-regulating condition), shrimp reached a new steady state of haemolymph osmolality by tightly adjusting ion concentrations to levels higher than the external medium. While NKA and CA activities recovered their pre-transfer levels, the downregulation of VHA suggests other functions rather than ion uptake after prolonged acclimation to 15[per thousand]. The activity of the three transport-related enzymes remained almost unchanged at the highest salinity (isosmotic condition), leading to increasing osmotic pressure and ion concentration after prolonged acclimation to 25[per thousand]. Although the freshwater shrimp studied here retains a certain degree of tolerance to high salinity, a common trait in palaemonid shrimps, our results highlight that 25[per thousand] represents a significant hypertonic challenge for this species. Keywords: carbonic anhydrase, gills, ionic regulation, [Na.sup.+], [K.sup.+]--ATPase, Palaemon argentinus, (V)-type H+--ATPase
Introduction Inland aquatic ecosystems from southern regions in South America as the ecoregion of the Pampas (Argentina), are vulnerable to increasing salinization due to human activities and global climate change [...]