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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, 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 pretransfer 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.

Studies on digestive adjustments at the biochemical level in relation to salinity in euryhaline crabs are lacking. Moreover, knowledge of biochemical digestive characteristics of euryhaline crabs (i.e. occurrence and characteristics of key digestive enzyme activities) is still scarce and fragmentary. We studied the occurrence, characteristics and response to low salinity of amylase, maltase and sucrase activities in the hepatopancreas of the euryhaline crab Neohelice (Chasmagnathus) granulata. Maximal amylase and maltase activities were found at pH 5.2. Sucrase activity was maximal within the pH range 3.6-5.2. Amylase, maltase and sucrase activities showed a Michaelis-Menten kinetics (km = 0.41±0.10 mg ml-1; 1.37±1.03 mM and 0.55±0.45 mM, respectively). In crabs acclimated to low salinity (10 psu; hyperregulating conditions), amylase activity (7263±980 μg maltose min-1 mg prot-1) was higher than in 35 psu (osmoconforming conditions) (3605±340 μg maltose min-1 mg prot-1). Maltase and sucrase activities (497±98 and 64±16 μg glucose min-1 mg prot-1, respectively) were similar in both salinities. The response of amylase activity to low salinity suggests a role in digestive adjustments upon hyperregulation. This study contributes to a better understanding of the complexity of the biochemical adaptations to low salinity in euryhaline crabs., El conocimiento sobre las características digestivas a nivel bioquímico en cangrejos eurihalinos es aún escaso y fragmentario. Así, faltan estudios sobre identificación y caracterización de actividad de enzimas digestivas clave en el hepatopáncreas y sobre el posible efecto de la salinidad sobre las mismas. Se estudió la existencia, características bioquímicas y respuesta a baja salinidad de amilasa, maltasa y sacarasa en el hepatopáncreas del cangrejo eurihalino Neohelice (Chasmagnathus) granulata. La mayor actividad de amilasa y maltasa se encontró a pH 5.2. La actividad de sacarasa fue máxima dentro de un rango de pH de 3.6-5.2. Las actividades de amilasa, maltasa y sacarasa mostraron una cinética michaeliana (km=0.41±0.10 mg ml-1; 1.37±1.03 mM y 0.55±0.45 mM, respectivamente). En cangrejos aclimatados a baja salinidad (10 psu, condición de hiperregulación), la actividad de amilasa fue mayor (7263±980 μg maltosa min-1 mg prot-1), que en individuos aclimatados a 35 psu (condición de osmoconformación) (3605±340 μg maltosa min-1 mg prot-1). Las actividades de maltasa y sacarasa (497±98 y 64±16 μg glucosa min-1 mg prot-1, respectivamente) fueron similares en ambas salinidades. La respuesta de la actividad de amilasa a baja salinidad permite sugerir un rol en mecanismos de ajustes bioquímicos secundarios a la hiperregulación. Los resultados constituyen un aporte relevante al conocimiento sobre las complejas adaptaciones a nivel bioquímico en respuesta a baja salinidad en cangrejos eurihalinos.