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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). Research Highlights: Shrimp lives in a chronic hypo‐osmo‐ionic regulatory state.Shrimp can tolerate brackish water but not freshwater.An incipient local physiological adaptation to full‐strength seawater is suggested. [ABSTRACT FROM AUTHOR]

To understand the response of freshwater organisms to rising environmental salinity, it is essential to investigate their osmo-ionic regulatory physiology. Our laboratory experiment investigated the transfer of Palaemon argentinus (Nobili, 1901) from 2‰ (control condition) to concentrated salinity (15 and 25‰) 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+–ATPase (VHA), Na+, K+–ATPase (NKA), carbonic anhydrase (CA) activity, and muscle water content. Upon prolonged acclimation to 15‰ (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‰. 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‰. 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‰ represents a significant hypertonic challenge for this species. [ABSTRACT FROM AUTHOR]

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In spite of its inherent physiological importance, studies on the occurrence, characteristics, and modulation of amylase in euryhaline crabs are lacking. We investigated the occurrence of amylase forms and the effect of acclimation to different salinities on their number and made a partial purification and characterization of the major form present in the hepatopancreas of Neohelice granulata. Zymogram analysis revealed 5 amylase forms in crabs acclimated to 35 psu (seawater) and 37 psu, and an additional band at 10 psu, but with a major form (29 kDa) in all cases, which was partially purified and characterized. Amylolytic activity was maximal between 30 and 40 °C; maintained at high NaCl concentrations (up to 4 M); increased by 5 mM K+, Li+, Co2+, and Mg2+ (36%-45%); inhibited by Cu2+, Zn2+, Cd2+, Fe2+, and Mn2+ (92.4% and 23.7%); not affected by Ni2+ or Ba2+; and enhanced almost 100% by Ca2+. Amylase exhibited Michaelis-Menten kinetics (starch: Km = 1.24 mg mL-1; glycogen: Km = 16.19 mg mL-1). The potential physiological significance and relationship to habitat conditions of the extra form in low salinity and the biochemical characteristics of the partially purified amylolytic activity (halotolerant, differential sensitivity to ions, capability to hydrolyze starch and glycogen) are discussed. [ABSTRACT FROM AUTHOR]