Osmoregulation in the terrestrial Christmas Island red crab Gecarcoidea natalis (Brachyura: Gecarcinidae): modulation of branchial chloride uptake from the urine.
Crabs generally produce urine iso-osmotic to their haemolymph, but terrestrial crabs are able to vary the composition of their final excretory fluid (termed P) postrenally, in the branchial chambers. Regulatory aspects of branchial urine processing were investigated in the Christmas Island red crab Gecarcoidea natalis acclimated to drinking either freshwater (FW crabs) or 70% seawater (SW crabs). FW crabs released dilute P (mean [Cl(-)] 8.8 mmol l(-1)). Drinking 70% seawater caused the mean [Cl(-)] of the P to rise to 376 mmol l(-1) over 5 days, approaching the haemolymph [Cl(-)]. FW crabs with saline-perfused branchial chambers absorbed chloride at a high rate (10 mmol kg(-1) h(-1)), and haemolymph [Cl(-)] increased at approximately 20 mmol l(-1) h(-1). SW crabs exhibited elevated haemolymph osmolalities and ion concentrations and zero branchial chloride uptake. FW crabs that were salt-loaded by branchial chamber perfusion over several hours downregulated, and eventually ceased, chloride uptake. The rate of downregulation, but not the initial chloride flux, was dependent on initial haemolymph [Cl(-)]. Intravascular infusion of NaCl caused immediate reduction in branchial [Cl(-)] of 80%. Crabs ingested and regurgitated the perfusion saline, supporting suggestions that reingestion of urine could conserve water and ions. Dopamine upregulated branchial chloride transport in G. natalis. This is consistent with the ion-regulatory effects of dopamine in euryhaline marine brachyurans but contrasts with its inhibitory effects in the terrestrial anomuran Birgus latro. Dopamine increased the rate of urine release in FW crabs. Urine composition appears to be unimportant in ionic regulation, except in the case of magnesium, levels of which were elevated in the urine of SW crabs.
Taylor
H H
HH
Department of Zoology, University of Canterbury, Private Bag 4800, Christchurch, New Zealand. h.taylor@zool.canterbury.ac.nz
Greenaway
P
P
D000042:Absorption; D000064:Acclimatization; D000818:Animals; D003386:Brachyura / Q000502:physiology*; D002712:Chlorides / Q000378:metabolism / Q000652:urine*; D000242:Cyclic AMP / Q000494:pharmacology; D004298:Dopamine / Q000494:pharmacology; D005618:Fresh Water; D005880:Gills / Q000378:metabolism*; D006458:Hemolymph / Q000737:chemistry / Q000378:metabolism; D007700:Kinetics; D009994:Osmolar Concentration; D012965:Sodium Chloride / Q000008:administration & dosage; D014882:Water-Electrolyte Balance
D002712:Chlorides; D012965:Sodium Chloride; D000242:Cyclic AMP; D004298:Dopamine