The rate and mechanism of nitrogen excretion were examined in Geograpsus grayi. This species excretes waste nitrogen as gaseous NH3 in periodic bursts. The mean concentration of total ammonia ([NH3]+[NH4+]) in the primary urine during bursts of excretion (1.72 mmol l-1) was similar to that of haemolymph (2.07 mmol l-1) but was significantly lower (P<0.005) than that of branchial fluid (80.6 mmol l-1). The effects of ion exchange inhibitors on the apical membrane of the gill epithelium in Geograpsus grayi were examined. The presence of an amiloride-sensitive Na+/NH4+ exchanger was confirmed and a SITS-sensitive Cl- influx suggested Cl-/HCO3- exchange. Thus, the site of nitrogenous excretion in this species is the branchial chamber, which is also the site of reprocessing of urine for ion regulation in other terrestrial crabs. Gaseous ammonia excretion is achieved by volatilisation of NH3 from the branchial fluid. High partial pressures of ammonia in the branchial fluid are produced by apical Na+/NH4+ exchange and elevation of the pH.

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