Osmotic and Ionic Regulation

Abstract

The spatial and temporal changes in environmental salinity faced byintertidal estuarine crabs require a variety of strategies at different levels,for controlling movements of both water and ions between the organismsand their environment. The ability to osmoionoregulate is a primaryphysiological determinant for euryhalinity of these animals, and forinhabiting different zones within an intertidal area. Hyper- and hypoosmoionoregulationabilities allow maintenance of the osmotic and ionichemolymphatic concentrations within a stable range, either above or belowthat of the aquatic environment, this is, at low or high salinities,respectively (McNamara and Faria 2012). These abilities involve theactivation of their osmoionoregulatory machinery, from the molecularlevel to the entire organism. The gills of osmoregulating decapodcrustaceans are considered to be the main site of the biochemicaladaptations involved in ion transport processes (Lucu and Towle 2003,Larsen et al. 2014, for reviews). Euryhaline crabs compensate for theirpassive salt loss by active NaCl absorption across the epithelium of theposterior gills. So far, in most studied species, adaptive changes in gillNa+/K+ATPase activity and mRNA expression, have been clearly shownto occur and to be hormonally and non-hormonally regulated, being this enzyme one of the central molecular components of the ionoregulatoryprocess at the biochemical level (reviewed by Lucu and Towle 2003).Early studies, conducted on isolated perfused gills, suggest that NaClabsorption proceeds through basolateral Na+/K+-ATPase, and apicalNa+/H+andCl-/HCO3-exchangers (e.g. Siebers et al. 1985, Gilles andPéqueux 1986). Later studies with more refined techniques, expanded thelist of known ion transport proteins and proposed different mechanisticmodels for explaining ion absorption across the gill epithelium ofestuarine, euryhaline, freshwater crabs, and other decapod crustaceans(reviewed by e.g. Péqueux 1995, Henry et al. 2012, McNamara and Faria2012, Larsen et al. 2014).The biochemical adaptations in other organs or tissues of euryhalinecrabs to varying environmental conditions are scarcely known. Severalresponses have been shown to occur in muscles of crabs during osmoticadaptation (Whiteley et al. 2001, Pinoni and López Mañanes 2004, Larsenet al. 2014, Asaro et al. 2018a). On the other hand, adaptive compensationin response to different environmental conditions is an energy-demandingprocess which requires the mobilization of energy substrates formetabolically support osmoionic active transport (Wang et al. 2016,Pinoni et al. 2018).Neohelice granulata is a euryhaline crab considered an emergentanimal model for biochemical, physiological, and ecological research(Spivak 2010). This crab inhabits estuarine and oligohaline habitats and isfrequently found in rainwater pools or concentrated seawater pools. N.granulata has been identified as hyper-hypo-osmoionoregulator (MañéGarzón et al. 1974, Luquet et al. 1992, 1998, Schleich et al. 2001, Pinoniet al. 2013).In this chapter, we describe current knowledge about osmotic and ionicregulation at the biochemical and physiological levels, in adults of N.granulata. Aspects covered will include tolerance to salinity,osmoregulatory patterns, active transport, and functional morphology ofthe gills, as well as biochemical and metabolic responses in extrabranchialtissues to changes in environmental salinity.