Nitrogenous Excretion in Two Terrestrial Crabs (Gecarcoidea natalis and Geograpsus grayi)

Abstract

Gecarcoidea natalis and Geograpsus grayi are both ammonotelic but excrete ammonia by different routes and mechanisms. In G. natalis ammonium ions in the final excretory fluid (P) constitute 67. 7% of the total nitrogenous output of 38. 11 ± 23.0 SD μmol N · kg⁻¹ · h⁻¹. The urine contains little ammonium (0.36 ± 0.081 SD mmol · L⁻¹) and is released into the branchial chambers where ammonium is added and salts are reabsorbed. When the P is finally released it contains 10.8 ± 7.79 mmol NH⁺₄ · L⁻¹, but individual values as high as 73 mmol *· L⁻¹ have been measured. Starvation lowered the concentration of urea, ammonium, and amino acids in the hemolymph and reduced output of nonprotein nitrogen compounds in the feces. In G. grayi gaseous ammonia accounts for 82.3% of the total nitrogenous excretion of 106. 6 ± 50.3 SD μmol N · kg⁻¹ · h⁻¹. Periods of excretion lasting 1-2 d alternate with similar periods of minimal excretion. Starvation did not alter the rate of ammonia excretion but caused a reduction in outpu...