HIGH AMMONIA AND LOW PH IN THE URINE OF THE GHOST CRAB, OCYPODE QUADRATA

Nitrogen excreted into the urine (< 1 mM) has generally been considere d a negligible component of total nitrogen output of crustaceans. But concentrations of ammonia > 100 mM were found in the urine of laborato ry-held Ocypode quadrata, suggesting that this notion might not be app licable to all crustaceans. To address this issue, hemolymph and urine were removed from freshly captured O. quadrata and analyzed for nitro genous catabolites and major ions. Hemolymph composition was similar t o that of other crustaceans, but the urine was acidic ((X) over bar pH = 5.50) and contained ammonia, often at > 100 mM. Other nitrogenous c atabolites in the urine (urea, amino acids, and uric acid) were much l ess concentrated: totaling < 12 mM on average. The ionic composition o f the urine was similar to that of other crustaceans, with the excepti on that Na was much less concentrated than Cl-. Total osmolality of he molymph and urine was similar. The Na+/K+ ATPase activity was greater in the antennal glands than in the posterior gills of O. quadrata, sug gesting that this enzyme is important for ammonia concentration and Na resorption. This pattern of enzyme activity was not present in two te rrestrial brachyurans whose urine contains little ammonia. The evoluti onary significance of high ammonia concentrations in the urine of ghos t crabs is unclear.