Aspects of sodium ion and water regulation, and gill ultrastructure in Crangonyx pseudogracilis (Bousfield) (Crustacea: Amphipoda)
Crangonvx pseudogracilis is a euryhaline freshwater amphipod which is generally thought to have recently invaded the waters of Great Britain. Rates of unidirectional trans-integumental flux of sodium and water, haemolymph [Na], medium imbibition, marker clearance rate, transepithelial potential and Na+/ K+ATPase activity were determined in this hyperosmotic regulator. Comparisons of these parameters were made with other fresh water amphipods. Furthermore, the effects of low and high [Na] acclimation on some of these features of the sodium ion regulatory mechanism were investigated. The main ion permeable areas were found to be the coxal gills, sternal gills, and the ventral plates. The saturable sodium transport system was investigated in freshly-collected Crangonvx after acclimation to various external [Na]. The active transport parameters KmNa and JNamax were defined. KmNa increased with acclimation, but JNamax, however, was not changed. After acclimation to high and low environmental sodium concentrations the diffusional water permeability (Pd) did not change, whereas the integumental permeability to sodium (PNa) increased. No change in the level of Na+/ K+ATPase in the gills of Crangonvx was observed after similar acclimation. Similarly the maximum rate of sodium uptake was not affected. The gill ultrastructure was also investigated in Crangonvx after acclimation to various external sodium concentrations. In control animals and those acclimated to low [Na], the gills did not appear to show basal infoldings typical of salt-transporting epithelia. Changes in the appearance of mitochondria and in the appearance of basal infoldings were found to occur after acclimation to high sodium media. The physiological adaptation of Crangonvx to fresh water and mechanisms of acclimation to different environmental sodium ion concentration are described.