How to overcome osmotic stress? Marine crabs conquer freshwater. New insights from modern electrophysiology

In the present article we review our findings on split lamella preparations of crab gills mounted in modified Ussing-chambers with respect to mechanistic and ecophysiological aspects. The leaky gill epithelium of shore crabs adapted to brackish water absorbs Na+ and Cl− in a coupled mode, and shows similarities to other salt-absorbing epithelia exposed to moderately diluted media. The results so far obtained for NaCl uptake across the gills of the shore crab are compatible with a transport model where two cell types operate in parallel, one displaying cotransport-like NaCl absorption, similar to that in the thick ascending limb of Henle's loop of the mammalian mephron, and the other one with characteristics of amiloride-sensitive, channel-mediated Na+ uptake by frog skin. Although there is no clear evidence for the apical mechanisms in this model, it may serve as a good basis for more detailed studies in the future. The moderately tight gill epithelium of freshwater adapted Chinese crabs absorbs Na+ and Cl− independently from each other, and shows similarities to other salt-absorbing epithelia exposed to freshwater. The characteristics of a positive, Na+-dependent short-circuit current with externally Cl−-free saline indicate that active Na+ uptake proceeds in a frog-skin-like mode via apical Na+-channels and the basolateral Na+/K+-pump. The nature of a negative short-circuit current with external Cl−-saline indicates that active and Na+-independent Cl− uptake is driven by an apical V-type H+-pump and proceeds via apical Cl−/ HCO3−-exchange and basolateral Cl−-channels.