Rubidium uptake is increased in shore crabs, Carcinus maenas, acclimated to dilute seawater

In dilute seawater, Carcinus maenas hyperosmoregulates by actively absorbing Na, K, and Cl. Here we characterize K uptake using a novel technique. Rb was used as a tracer for K transport, and hemolymph Rb levels were measured using cation chromatography. Hemolymph Rb was detectable at 0.1 mmol L-1, which enabled determination of initial rate of Rb uptake. Crabs maintained for 3 wk in dilute artificial seawater (35% ASW crabs) maintained Na and K above the level of the external media and had elevated Na-K-ATPase activity in the posterior gills. In assay conditions matched to 100% ASW, Rb uptake was the same in 35% ASW crabs (0.45 +/- 0.04 mu mol g(-1) h(-1)) and in crabs acclimated to normal seawater (100% ASW crabs, 0.49 +/- 0.05 mu mol g(-1) h(-1)). In assay conditions matched to 35% ASW, Rb uptake was greater in 35% ASW crabs (0.28 +/- 0.03 mu mol g(-1) h(-1)) compared with 100% ASW crabs (0.10 +/- 0.04 mu mol g(-1) h(-1)). Low external [Rb] or reduced salinity were found to contribute independently to the difference between 100% ASW and 35% ASW crabs. Thus, whole-body Rb uptake in crabs can be measured by cation chromatography, and Rb uptake is greater in 35% ASW crabs than in 100% ASW crabs.