Hyperosmoregulation in the red freshwater crab

and highly successful decapod group, their osmoregulatory capability has been studied mainly from the whole-animal perspective, and little information is available concerning their physiological mechanisms of osmotic and ionic regulation. Like the diadromous crabs that migrate between sea water and fresh water during their life cycle, the hololimnetic Brachyura also maintain large, outwardly directed osmotic and ionic gradients (see Mantel and Farmer, 1983). However, diffusive salt loss and osmotic water entry across the body surfaces of these crabs are reduced by their low permeability to passive salt and water movements (Shaw, 1959; Harris, 1975; Greenaway, 1981; Morris and Van Aardt, 1998). Unlike the freshwater Macrura, the freshwater crabs do not appear to have evolved the ability to produce dilute urine. Their particularly low rate of iso-osmotic urine production seems not to depend solely on the low water permeability of the body surfaces since reabsorption of iso-osmotic fluid by the antennal gland also reduces urine volume. This strategy, apparently typical of freshwater crabs, may be a water-conserving adaptation to amphibious life (Greenaway, 1981; Harris, 1975; Morris and Van Aardt, 1998). However, since a reduced flow of iso-osmotic urine also conserves salt, this same adaptation reduces dependence on the mechanisms of active NaCl absorption from the freshwater medium that counterbalance diffusive losses. Hyperosmoregulating Crustacea compensate for passive salt loss in dilute media by actively absorbing NaCl across their gill epithelia (for reviews, see Péqueux et al., 1988; Péqueux, 1995). In diadromous crabs from marine and brackish waters, these organs, which play vital roles in gas exchange, in osmotic and ionic regulation, in pH regulation and in the excretion of N2 compounds, have been investigated from the whole gill to the molecular level, employing a wide variety of techniques (for a review, see Taylor and Taylor, 1992). In contrast, investigations of the gills of the hololimnetic or true freshwater crabs have been limited, and gill ultrastructure has been examined only in Potamon niloticus (Maina, 1990). No structural differences regarding the gills of other hyperosmoregulating crabs are evident. Freshwater crabs absorb salt against considerable ionic gradients: the external sodium concentration at which half-maximal uptake occurs is less than 0.2 mmol l–1, which is clearly lower than for brackishwater animals. Consistent with the reduced passive salt loss 167 The Journal of Experimental Biology 205, 167–175 (2002) Printed in Great Britain © The Company of Biologists Limited 2002 JEB3494