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The Journal of Experimental Biology 206, 2869-2881 (2003)
doi: 10.1242/jeb.00499

Regulation of urine reprocessing in the maintenance of sodium and water balance in the terrestrial Christmas Island red crab Gecarcoidea natalis investigated under field conditions

Stephen Morris^1,* and Mark D. Ahern²

¹ Morlab, School of Biological Sciences, University of Bristol, Woodland Road, Bristol, BS8 1UG, UK
² School of Biological Sciences, University of Sydney, Sydney, NSW 2006, Australia

^* Author for correspondence (e-mail: steve.morris{at}bristol.ac.uk)

Accepted 19 May 2003

Land crabs produce isosmotic urine but reduce salt loss by reabsorbing salt via the gills to produce a dilute excretory fluid (P). This branchial salt reclamation is regulated in response to changes in dietary salt availability. The regulation of branchial Na reabsorption and osmotic status was investigated in the terrestrial crab Gecarcoidea natalis on Christmas Island. Confinement within field enclosures had no general effect on salt and water balance compared with crabs free in the rainforest but there were seasonal effects. Extracellular fluid volume was 27.9% body mass during the wet season but only 22.7% in the dry season. Urine production was 53 ml kg^-1 day^-1 in the dry season but 111 ml kg^-1 day^-1 in the wet season, while water flux rates were 140 ml kg^-1 day^-1 and 280 ml kg^-1 day^-1, respectively. Serotonin but not dopamine increased urine production by at least 16% but only during the dry season when rates were seasonally lowered. Crabs acclimated to drinking 50% seawater increased haemolymph osmotic pressure and downregulated branchial reabsorption of salt. Net Na flux (J_net) and unidirectional Na influx (J_in) were investigated in branchial perfusion experiments. In red crabs acclimated to drinking freshwater, J_in, J_net and the activity of the Na⁺/K⁺-ATPase were increased by serotonin, indicating that the increase of sodium absorption was due to a stimulation of the ATPase. Red crabs drinking 50% seawater reduced J_net primarily due to increased passive loss (J_out), since both J_in and Na⁺/K⁺-ATPase were unchanged. Serotonin and dopamine abolished the increased diffusive loss and re-established J_net with no change in J_in. G. natalis exhibits different regulatory systems. Branchial salt uptake can be adjusted via the leak component when adequate salt is available but also by stimulated active uptake under diluting conditions. The gills are important sites of ion pumping in euryhaline aquatic crabs, and the upregulation of J_net in red crabs is reminiscent of that in marine crabs. Serotonergic stimulation of branchial uptake, independent of cAMP, and hormonally modulated ion leakage are presently unique to terrestrial species.

Key words: osmoregulation, water flux, J_net, Na flux, branchial uptake, serotonin, red crab, Gecarcoidea natalis

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