Hyperosmoregulation in the red freshwater crab Dilocarcinus pagei (Brachyura, Trichodactylidae): structural and functional asymmetries of the posterior gills.

The osmotic and ionic status of the haemolymph and the structural and ion-transport characteristics of the posterior gills of Dilocarcinus pagei, a hololimnetic crab, were investigated. Haemolymph osmolality was 386 +/- 18 mosmol kg(-1), while [Na(+)] and [Cl(-)] were 190 +/- 13 and 206 +/- 12 mmol l(-1), respectively; [K(+)], [Ca(2+)] and [Mg(2+)] were 9.7 +/- 0.7, 10.2 +/- 0.5 and 2.8 +/- 0.4 mmol l(-1), respectively (means +/- S.E.M., N=12-17). The gill lamellae possess a central, osmiophilic area, which exhibits a marked structural asymmetry. The thick (18-20 microm) proximal epithelium is characterised by basal invaginations and a few apical vesicles, while the thin (3-10 microm) distal epithelium consists of apical pillar cell flanges populated by vesicles and membrane invaginations. Isolated gills, bathed and perfused with NaCl saline, spontaneously generate a negative transbranchial potential difference (V(te)), which stabilises at positive or negative values. Ouabain shifts V(te) to more positive values. When mounted in an Ussing chamber, distal split lamellae generate a negative, Cl(-)-dependent short-circuit current (I(sc)). Na(+) substitution leads to more negative values of I(sc). Internal ouabain is without effect, while diphenylamine-2-carboxylate and acetazolamide abolish I(sc). Proximal split lamellae show a positive, Na(+)-dependent I(sc), which decreases after internal application of ouabain. These data suggest that the thin epithelium actively absorbs Cl(-), while the thick epithelium actively absorbs Na(+).