Nitrogenous excretion in the amphibious crab Holthuisana transversa under wet and dry conditions

The main route of nitrogen excretion in Holthuisana transversa with access to water was across the gills. Ammonia excreted into the water in this way accounted for 72.2-85.9% of total nitrogen excreted. The feces, any excretory fluid released by the crab, and volatilization of ammonia gas were minor routes of excretion. Total nitrogen excretion ranged from 7.75 ± 3.24 SD μmol kg-1· h-1 for starved animals to 36.38 ± 33.36 SD μmol kg-1· h-1 for animals on a high nitrogen diet. Under simulated drought conditions (starved with no access to water) Holthuisana transversa excreted negligible amounts of nitrogen (0.23 ± 0.41 μmol kg-1· h-1). Reduced protein catabolism may have been facilitated by the use of fat for energy and a reduction in the metabolic rate. Any waste nitrogen that was produced was probably stored. Urate was stored in the body of Holthuisana transversa and the total body urate content was high (up to 40.41 mmol· kg-1). Given the low solubility of urate, the bulk of these reserves must have been in solid form.