The branchial chambers of the terrestrial anomuran Birgus latro L. were examined as potential sites for urine reprocessing. The antennal glands opened at the bases of the second antennae within the anterior extension of the branchial chambers. The adjacent mouth parts and scaphognathites and the cuticle lining the ventral folds of the branchiostegite bear hydrophilic hairs. The hairs retained urine in the branchial chamber and conducted it to either the gills or the mouth. Irrigation of the branchial chambers with artificial urine demonstrated the strong net uptakes of Na+ and cl− to be similar at approximately 4.5μmolh−1g−1 from full-strength urine, diluting the fluid to a threshold below 20mmollminus;1.
Na+/K+-ATPase and Ca2+-ATPase activities of gill homogenates indicated Vmax values (65–127 and 11 nmol min−1mg−1 protein, respectively) similar to those of marine species but low Km values (sodium Km=0.2-2.4mmoll−1, potassium Km=0.03-0.2mmoll−1, calcium Km=4-9μmoll−1), which are more characteristic of freshwater species. The gills are considered to be a site of ion reclamation from the urine and this is achieved by specific morphological, biochemical and behavioural adaptations.