Is salt the solution? Interactions between trace-metal uptake and salinity in the freshwater crab Potamonautes warreni Calman, 1918 (Decapoda: Brachyura: Potamonautidae)

Crustaceans inhabiting freshwater are more vulnerable to increased loading with dissolved trace metals than those from more saline conditions for both physicochemical and physiological reasons. Potamonautes warreni (Calman, 1918) is a true freshwater crab inhabiting rivers in proximity to mining activities in South Africa. In order to assess whether the uptake of zinc (Zn2+) and cadmium (Cd2+) were associated with Na+- and Ca2+-uptake routes, individuals of P. warreni were acclimated to different Na+ or Ca2+ concentrations before measuring trace-metal uptake into the haemolymph and tissues in an intermediate freshwater. Cd2+ uptake into the haemolymph was elevated following acclimation to ‘Na+ replete’ conditions, when compared with ‘Na+ deplete’ conditions, indicating that a physiological response to the acclimation salinity had influenced Cd2+ uptake. Zn2+ uptake by the whole body was elevated in P. warreni individuals from the ‘Na+ replete’ group when compared with crabs acclimated to Na+ free water. A general change in permeability may have occurred and indeed, the apparent water permeability (water efflux) of P. warreni was increased in elevated Na+ and Ca2+ concentrations compared with that in softer water. A comparison of the osmoregulatory ability between P. warreni and the catadromous crab Eriocheir sinensisH. Milne Edwards, 1853 highlighted the greater permeability of the latter. Lower Zn2+ and Cd2+ uptake rates were measured in P. warreni in comparison to those measured in E. sinensis by other authors, indicating that the more well adapted a species is to life in freshwater, the less susceptible they will be to metal uptake. Future investigations to understand further the mechanisms influencing Zn2+ and Cd2+ uptake are required to understand the role that salinity can play in protecting a species from trace-metals released into the environment.