RESUMEN
The decapod crustacean Penaeus monodon survives large fluctuations in salinity through osmoregulation in which Na+/K+-ATPase (NKA) activity in the gills plays a central role. Adult P. monodon specimens were gradually acclimatized to 5, 25 and 35‰ salinities and maintained for 20 days to observe long term alterations in NKA expression. Specific NKA activity assayed in gill tissues was found to be 3 folds higher at 5‰ compared to 25‰ (isosmotic salinity) and 0.48 folds lower at 35‰. The enzyme was immunolocalized in gills using mouse α-5 monoclonal antibody that cross reacts with P. monodon NKA α-subunit. At 5‰ the immunopositive cells were distributed on lamellar tips and basal lamellar epithelium of the secondary gill filaments and their number was visibly higher. At both 25‰ and 35‰ NKA positive cells were observed in the inter-lamellar region but the expression was more pronounced at 25‰. Gill architecture was normal at all salinities. However, the 1.5 fold increase in NKA α-subunit mRNA at 5‰ measured by quantitative RT-PCR (qRT-PCR) using EF1α as reference gene was not statistically significant. The study confirms the osmoregulating ability of P. monodon like other crustaceans at lower salinities. It is likely that significant increase in NKA transcript level happens at an earlier time point. At higher salinities all three methods record only marginal or no change from isosmotic controls confirming the hypothesis that the animal largely osmoconforms in hyperosmotic environment.