High Ammonia and Low pH in the Urine of the Ghost Crab, Ocypode quadrata

Nitrogen excreted into the urine (<1 mM) has generally been considered a negligible component of total nitrogen output of crustaceans. But concentrations of ammonia >100 mM were found in the urine of laboratory-held Ocypode quadrata, suggesting that this notion might not be applicable to all crustaceans. To address this issue, hemolymph and urine were removed from freshly captured O. quadrata and analyzed for nitrogenous catabolites and major ions. Hemolymph composition was similar to that of other crustaceans, but the urine was acidic ({Xbar} pH = 5.50) and contained ammonia, often at >100 mM. Other nitrogenous catabolites in the urine (urea, amino acids, and uric acid) were much less concentrated: totaling <12 mM on average. The ionic composition of the urine was similar to that of other crustaceans, with the exception that Na was much less concentrated than Cl-. Total osmolality of hemolymph and urine was similar. The Na+/K+ ATPase activity was greater in the antennal glands than in the posterior gills of O. quadrata, suggesting that this enzyme is important for ammonia concentration and Na resorption. This pattern of enzyme activity was not present in two terrestrial brachyurans whose urine contains little ammonia. The evolutionary significance of high ammonia concentrations in the urine of ghost crabs is unclear.