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

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 microl 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 micromol g-1 h-1) and Ca loss (0.29 micromol g-1 h-1). The low loss rates were due to urinary salt conservation of approximately 90 % or better and to low permeability (K'Na=0.0025; K'Ca=0.0521), and were compensated for by a high-affinity uptake mechanism (Jmax=0.76 micromol 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 micromol g-1 h-1. <P> 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 micromol kg-1 h-1. Crabs in amphibious conditions showed pulses of elevated NH3 excretion (350 micromol 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 NH3 via the gills compared with crabs from amphibious conditions. <P> 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. <P>

A Multi-Species Comparison and Evolutionary Perspectives on Ion Regulation in the Antennal Gland of Brachyurans

“…In addition, the bimodal/intertidal species Ucides cordatus has a relatively low urine to hemolymph ratio (U/H) with Na + concentration in diluted seawater ( Harris and Santos, 1993 ). On the other hand, the following only showed a limited ability to reabsorb Na + from urine in hypo-osmotic mediums: the terrestrial species Gecarcoidea natalis and G. lateralis (Gecarcinidae) ( Gross, 1964 ; Taylor and Greenaway, 2002 ); semi-terrestrial/intertidal crabs Chiromantes dehaani (Sesarmidae), Helice formosensis and Hemigrapsus oregoncnsis (Varunidae) ( Gross, 1964 ; Tseng et al, 2020 ; Allen and Weihrauch, 2021 ); intertidal/marine species Cancer antennarius and Cancer magister (Cancridae) ( Gross, 1964 ; Hunter and Rudy, 1975 ), Carcinus maenas and Callinectes sapidus (Portunidae) ( Carmeron and Batterton, 1978 ; Harris and Santos, 1993 ) and Macrophthalmus banzai (Macrophthalmidae) ( Tseng et al, 2020 ); and freshwater species Candidiopotamon rathbunae ( Wang and Lin, 2011 ) and Potamonautes warreni ( Morris and Van Aardt, 1998 ) ( Figure 2B ). Two Varunidae species, Pachygrapsus crassipes (Grapsidae) and two Cancridae species can reabsorb Na + from urine to some degree in hyper-osmotic environments ( Prosser et al, 1955 ; Gross, 1964 ; Hunter and Rudy, 1975 ; Harris and Santos, 1993 ; Tseng et al, 2020 ; Allen Weihrauch, 2021 ).…”