RESUMO
Since 2000, a well-established population of the invasive oriental shrimp Palaemon macrodactylus has been present in fully marine conditions in the southwestern Atlantic Ocean (~38° S). To assess the physiological performance of this atypical population restricted to fully marine conditions, we conducted a laboratory experiment in which individuals were transferred from 35 S (local seawater) to 2 S; 5 S; 10 S; 20 S; 50 S and 60 for short (6 h), medium (48 h), and long (>504 h) acclimation periods. We measured the time course response of relevant parameters in the shrimp's hemolymph; activity of Na+, K+-ATPase (NKA), and V-H+-ATPase (VHA); and muscle water content. Shrimp showed great osmoregulatory plasticity, being able to survive for long periods between 5 S and 50 S, whereas no individual survived after transfer to either 2 S or 60 S. Shrimp hyper-regulated hemolymph osmolality at 5 S and 10 S, hypo-regulated at 35 S and 50 S, and isosmoticity was close to 20 S. Compared to 35 S, prolonged acclimation to 5 S caused a decrease in hemolymph osmolality (~34%) along with sodium and chloride concentrations (~24%); the NKA and VHA activities decreased by ~52% and ~88%, respectively, while muscle water content was tightly regulated. Our results showed that the atypical population of P. macrodactylus studied here lives in a chronic hypo-osmo-ion regulatory state and suggest that fully marine conditions contribute to its poor performance at the lower limit of salinity tolerance (<5 S).