Salt and Water Relations, and Nitrogen Excretion, in the Amphibious African Freshwater Crab Potamonautes Warreni in Water and in Air

Abstract

ABSTRACT Mechanisms of salt and water conservation, and nitrogen excretion, were investigated in the freshwater amphibious crab Potamonautes warreni from the High Veld of South Africa. Adaptations to fresh water were assessed as pre-adaptations to air-breathing, and nitrogen excretion was examined as a potential constraint to terrestrial excursions. P. warreni was able to regulate water and salt loss in water up to 40 % sea water, but not in 80 % sea water. The water permeability of P. warreni was low and, since 97 % of the haemolymph filtrate was reabsorbed in the antennal organ, urinary water loss was minimal (0.7 μl g−1 h−1). The minimum equilibrium [Na] of P. warreni was low (0.116 mmol l−1), as were the rates of both Na loss (0.22 μmol g−1 h−1) and Ca loss (0.29 μmol g−1 h−1). The low loss rates were due to urinary salt conservation of approximately 90 % or better and to low permeability , and were compensated for by a high-affinity uptake mechanism (Jmax=0.76 μmol g−1 h−1 and Km=0.18 mmol l−1). Acclimating P. warreni to low Na concentrations increased maximum net Na uptake rate to 1.77 μmol g−1 h−1. Nitrogen excretion in P. warreni was almost 100 % ammoniotelic, and there was no accumulation of haemolymph or urinary ammonia or urea when in air. P. warreni was unable to excrete ammonia to air, but in water the rate of excretion was nearly 70 μmol kg−1 h−1. Crabs in amphibious conditions showed pulses of elevated NH3 excretion (350 μmol kg−1 h−1) when subsequently submerged, while for crabs breathing air for 3 days this pulse reached 4.9 mmol kg−1 h−1. Air-breathing P. warreni with artificially irrigated branchial chambers excreted double the amount of NH3via the gills compared with crabs from amphibious conditions. Water and salt conservation form useful pre-adaptations to terrestrial forays. While the relatively low water loss extends the duration of emersion, P. warreni is required to return briefly to water to excrete stored nitrogenous waste. The nature of the store remains to be determined.