Adaptive physiological strategies in the Crustacea: the invasion of fresh water and the molecular and structural bases of osmoregulation

Abstract

The survival of aquatic organisms has resulted in part from the selection of regulatory mechanisms that maintain the osmotic and ionic equilibria of their internal milieu, thus protecting tissues and cells against the intervening gradient and oscillations in the external medium. The Crustacea have developed many adaptive osmotic and ionic patterns that underlie their ample distribution and notable diversity; thus, crustaceans are often used as comparative models to better comprehend osmoregulatory mechanisms that employ ion transporting proteins such as the Na+/K+- and V-ATPase which participate in volume regulation and ionic composition of the intra- and extracellular fluids. The palaemonid shrimps and brachyuran crabs exhibit numerous adaptive strategies clearly manifest in their life cycles, physiology, biochemistry and morphology that reflect the wide variety of environments occupied, and include species particularly useful to analyze adaptive osmoregulatory strategies. This investigation compares the levels of mRNA expression of the α and B subunits of the Na+/K+- and V-ATPase, the Na+/H+ exchanger and the Na+K+2Cl- symporter, and Na+/K+- and V-ATPase activity, in the gills of a series of marine, estuarine and freshwater palaemonid shrimps (Palaemon northropi, Macrobrachium acanthurus e M. brasiliense, respectively) and in the marine crab Callinectes danae as a function of the external medium salinity. Gill Na+/K+- and V-ATPase activity also will be localized by immunofluorescence microscopy in M. amazonicum and in C. danae in which the ultrastructure of the posterior gill ion-transporting cells will be characterized after adaptation to dilute salinity. The data generated will aid in clarifying the molecular mechanisms and structural rearrangements that underpin osmotic and ionic regulation in the decapod Crustacea, and should help elucidate the molecular phylogeny of íon-transporting proteins in the Palaemonidae as well as illustrate the diversity of osmoregulatory mechanisms present in different crustacean taxa that confront similar osmotic challenges. (AU)