Contribution of Na/Na and Cl/Cl exchanges to sodium and chloride fluxes in the crabGoniopsis cruentata

Abstract

Sodium or chloride efflux and transepithelial potentials (TEP) were measured in crabs exposed to seawater concentrations ranging from 150 to 25% SW. In crabs acclimated to 150% SW the Na+ efflux (3.8 mmol/h·100 g) was significantly higher than the Cl− efflux (2.1 mmol/h·100 g), but both fluxes decreased to about 0.6 mmol/h·100 g in crabs from 50 or 25% SW. The TEP varied linearly from −1 mV (blood negative) in 150% SW, to −11 mV in 25% SW. In 150 and 100% SW the calculated components of the ion fluxes (i.e., diffusive, urinary, active uptake or extrusion) added up to less than one-half of the isotopically measured values. In 50 and 25% SW the measured effluxes were fully accounted for by their calculated components. In crabs transferred from 150% SW to low-Na 150% SW (=TRIS ASW), the Na+ efflux decreased abruptly, from 3.7 to 0.6 mmol/h; the Cl− efflux decreased much less, from 1.9 to 1.5 mmol/h. A large fraction of the Na+ (or Cl−) fluxes in crabs from concentrated SW meets the criteria for exchange diffusion, which decreases or disappears as the external concentration of each ion is lowered. This suggests that changes of the permeability to ions, in response to alterations of environmental salinity, may not constitute an important adaptive strategy in this species.