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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

View larger version (14K): [in a new window]   Fig. 1. Water efflux determined from 3H2O clearance from G. natalis during the dry season and the wet season in the rainforest of Christmas Island. Determinations were made in both seasons for free-ranging crabs (N=8) and for crabs in field enclosures (N=9). Asterisks indicate a significant increase in wet season compared with dry season animals.

View larger version (23K): [in a new window]   Fig. 2. The volume rates of 51Cr-EDTA clearance, urine flow, drinking and production of the final excretory product P by G. natalis either free ranging in the rainforest or after 6 days confinement in field enclosures. Clearance was determined over 6 days, and urine flow using the appropriate U:H ratio of 51Cr in the urine and haemolymph. Drinking and P production rates were determined on separate animals over a 24 h period after 6 days in the chambers described in the Materials and methods, which were held within the rainforest on Christmas Island. In the dry season, the crabs provided no urine at all and thus 51Cr measurements in the urine could not be made. The vertical arrows indicate the range of possible urine flow calculated using a U:H ratio of 1.04 from Greenaway (1994) or the values obtained from free-ranging and confined animals in the wet season. Values are means + S.E.M. (N=6–9). * indicates an effect of confinement compared with free-ranging crabs; indicates a difference between dry and wet season values; indicates that the animals confined during the dry season drank the entire volume of water provided and thus a minimum rate is available.

View larger version (17K): [in a new window]   Fig. 3. Branchial sodium flux in G. natalis acclimated to drinking freshwater and then infused with either a saline carrier (control), dopamine or serotonin at 2x10-4 mol l-1 or dibutyryl cyclic AMP (db-cAMP; membrane-permeable cAMP) at 6.1x10-4 mol l-1. The branchial chambers were perfused with artificial urine labelled with 22NaCl sufficient to provide 6000 c.p.m. in a 400 µl sample. Flux rates were determined over 90 min. Changes in total [Na] were used to provide rates of net uptake (Jnet); uptake of 22Na provided rates of unidirectional influx (Jin); and unidirectional loss was calculated for each crab as Jin–Jnet=Jout. For details, see Materials and methods. N=8 for each treatment. * indicates significant elevation compared with saline-infused control crabs.

View larger version (17K): [in a new window]   Fig. 4. Branchial sodium flux in G. natalis acclimated to drinking seawater (50% SW) and then infused with either a saline carrier (control), dopamine or serotonin at 2x10-4 mol l-1 or dibutyryl cyclic AMP (db-cAMP; membrane-permeable cAMP) at 6.1x10-4 mol l-1. The branchial chambers were perfused with artificial urine labelled with 22NaCl sufficient to provide 6000 c.p.m. in a 400 µl sample. Flux rates were determined over 90 min. Changes in total [Na] were used to provide rates of net uptake (Jnet); uptake of 22Na provided rates of unidirectional influx (Jin); and unidirectional loss was calculated for each crab as Jin–Jnet=Jout. For details, see Materials and methods. N=8 for each treatment. * indicates significant depression compared with saline-infused control crabs; indicates significant difference to corresponding rate in crabs acclimated to freshwater in Fig. 3.

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