Gaseous ammonia evolution is coupled to reprocessing of urine at the gills of ghost crabs

Abstract

Urine of terrestrial and aquatic crabs generally is of neutral pH and contains negligible waste nitrogen. Ammonia is added to urine during its reprocessing by the gills of some terrestrial species. The ghost crab, Ocypode quadrata, produces an acidic urine (pH < 6.0) that is high in ammonia (>lo0 mM). This prompted us to study the chemical changes which occur during urine reprocessing by this species. Urine and reprocessed urine were collected from fed animals maintained on sand moistened with diluted seawater, and several chemical variables were measured. During urine reprocessing, C 1~ was reclaimed, and C 0 2 concentration and pH increased. Evolution of gaseous ammonia was then measured in crabs which could and could not pass urine onto their gills. Animals with occluded nephropores released gaseous ammonia at a much lower rate (-300 pmol-kg -'-day-') than animals with open nephropores (-2,000 pmol.kg-.day-I ) . We hypothesize that the gaseous ammonia released by control crabs is predominantly due to degassing of NH, from the fluid (urine) upon the pH increase which occurs during reprocessing by the gills. o 1993 Wiley-Liss, I~C .

Ammonia excretion in aquatic and terrestrial crabs

“…In all land crabs examined to date, the gills have become adapted for reabsorption of salt from primary urine directed through the branchial chamber Wolcott, 1985, 1991;Morris, 2001), allowing diffusive NH 3 loss and NH 4 + extrusion in exchange for required ions from the urine. Ocypodid crabs seem exceptional in utilizing the antennal gland for increasing urinary ammonia, although the gills are required to complete the excretory process (DeVries and Wolcott, 1993). Generally, while the gills are initially bathed with a fluid isosmotic with the hemolymph, osmotic concentration may decline by as much as 90% (Wolcott and Wolcott, 1985;Varley and Greenaway, 1994;Greenaway, 1999;Morris et al, 2000;Taylor and Greenaway, 2002;Morris and Ahern, 2003).…”

“…Other lineages of terrestrial crabs have not been investigated to the same extent as the gecarcinids, but at least one airbreathing ocypodid, Ocypode quadrata, has been shown to sustain ammonia excretion (DeVries and Wolcott, 1993).…”

“…However, the mechanism of NH 4 + excretion is quite different from that of gecarcinids because the concentration in the primary urine of the ghost crab is extraordinarily high. For example, in O. quadrata (DeVries and Wolcott, 1993;DeVries et al, 1994) Wolcott, 1993), if the pH is increased to pH·7 it is possible to estimate PNH 3 using the pK and solubility for NH 3 provided by Kormanik and Cameron (1981) as used by Varley and Greenaway (1994). In the primary urine PNH 3 =11.6·Pa whereas at pH·7 the PNH 3 =460·Pa, which is a 40-fold increase in potential diffusive gradient.…”

“…In the primary urine PNH 3 =11.6·Pa whereas at pH·7 the PNH 3 =460·Pa, which is a 40-fold increase in potential diffusive gradient. In view of the concomitant increase in the fluid CO 2 concentration and the uptake of Cl -, the most obvious candidate for the net base excretion is transport by an apical HCO 3 -/Cl -exchanger (DeVries & Wolcott, 1993). Reclamation of urinary Na + appears to be accomplished within the antennal gland (DeVries et al, 1994).…”

Dogmas and controversies in the handling of nitrogenous wastes: Ammonia tolerance in the oriental weatherloachMisgurnus anguillicaudatus

“…This process of NH3 volatilization is actually better understood in arthropods. For example, there is periodic NH3 volatilization in the isopod Porcellio scaber (Wright and O'Donnell, 1993), and NH3 volatilization has also been reported in certain terrestrial crabs, which carry out the process by alkalinization of urine (De Vries and Wolcott, 1993;Varley and Greenaway, 1994).…”