Physiological and ultrastructural studies of NaCl transport in crustaceans gills∗

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Abstract
This review concerns some new insights from ultrastructural studies combined with fluxes and transepithelial potential measurements on preparations of isolated, perfused gills of the euryhaline Chinese crab Eriocheir sinensis. The chinese crab E. sinensis is a strong hyperosmoregulator which has two completely different types of gills. The three anterior pairs of gills have a very thin, respiratory epithelium. The three posterior ones have an epithelium characteristic of salt‐transporting tissues. The permeability to Na+ of the anterior gills is relatively high and can be regulated in two different modes: a) a ≪long‐term mode≫ related to long‐term acclimation to sea water or to fresh water b) an ≪immediate mode≫ related to the Na+ concentration in the external medium. Both types of modulation result in a decrease in permeability at low external salinity. The permeability to Na+ of the posterior gills is always extremely low, whatever the salinity of the medium. The active uptake of salt is only effective in the posterior gills implying independent transport processes for Na+ and Cl‐ Na+/H+ and Cl‐/HCO‐ 3 exchange systems at the apical side of the epithelium. The Na+ transport activity can be switched on and off by the Na+ level of the blood. Low Na+ levels, as in the blood of fresh water crabs, turn the pump activity on whereas high Na+ levels, as in the blood of sea water crabs turn the pump off. The pumping system however is not simply turned on and off with acclimation to tresh water or to sea water. The capability for transport is totally lacking in sea water acclimated animals and increases progressively upon acclimation to fresh water. This activity is related to changes in structure of the epithelium and particularly to a large development of an infolding system at the apical side. The applicability of the E. sinensis model of gill transport to other decapod crustaceans is discussed.