SODIUM UPTAKE AND TRANSPORT (NA-CORDATUS (L)(+K+)ATPASE CHANGES FOLLOWING NA+ DEPLETION AND LOW SALINITY ACCLIMATION IN THE MANGROVE CRAB UCIDES)

1. Ucides cordatus in a medium isoionic for Na+ and isosmotic with the haemolymph (26 parts per thousand salinity) has a relatively small ac tive uptake component of Na+ influx (7% of J(in)). 2. In 9 parts per t housand medium (hypo-osmotic to the haemolymph) Na+ is maintained by i ncreased active uptake of Na+ (to 75% of J(in)). 3. Short-term Na+ dep letion of animals acclimated to either medium stimulated active sodium uptake, increasing this component 4-5-fold. 4. The net increase in ac tive uptake was similar in both 26 and 9 parts per thousand acclimated animals (approximately 6 muM g/hr), although the maximum rate was hig her in the latter. 5. After acclimation to 9 parts per thousand (120 h r), increases in gill and antennal gland (Na+ + K+)-ATPases were obser ved, notably in the posterior gill pairs. 6. Mechanisms for the rapid Na+ depletion response and that of longer-term acclimation to low sali nities in this euryhaline mangrove crab are discussed.