GASEOUS AMMONIA EVOLUTION IS COUPLED TO REPROCESSING OF URINE AT THE GILLS OF GHOST CRABS

Urine of terrestrial and aquatic crabs generally is of neutral pH and contains negligible waste nitrogen. Ammonia is added to urine during i ts reprocessing by the gills of some terrestrial species. The ghost cr ab, Ocypode quadrata, produces an acidic urine (pH < 6.0) that is high in ammonia (>100 mM). This prompted us to study the chemical changes which occur during urine reprocessing by this species. Urine and repro cessed urine were collected from fed animals maintained on sand moiste ned with diluted seawater, and several chemical variables were measure d. During urine reprocessing, Cl- was reclaimed, and CO2 concentration and pH increased. Evolution of gaseous ammonia was then measured in c rabs which could and could not pass urine onto their gills. Animals wi th occluded nephropores released gaseous ammonia at a much lower rate (almost-equal-to 300 mumol.kg-1.day-1) than animals with open nephropo res (almost-equal-to 2,000 mumol.kg-.day-1). We hypothesize that the g aseous ammonia released by control crabs is predominantly due to degas sing of NH3 from the fluid (urine) upon the pH increase which occurs d uring reprocessing by the gills. (C) 1993 Wiley-Liss, Inc.