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ABSTRACT Water and salt balance in the terrestrial crab Gecarcoidea natalis were examined in the laboratory and in free-ranging animals on Christmas Island, Indian Ocean. Under laboratory conditions the drinking rate was low (1.6 ± 1.0 ml · 100 g-1 · d-1) but filtration rate (which equaled urine flow) was much higher (10.5 ± 4.71 ml·100 g―1·d―1). The isosmotic urine was released to the branchial chambers and a small portion (0.54 ± 0.48 ml· 100 g-1 ·d-1) was eventually released as dilute excretory fluid ([Na] 3.85 ± 1.25 mmol·1―1) after reabsorption of ions by the gills. The remainder was ingested and reabsorbed. The system offers flexibility to cope with varying conditions, since the filtration rate is high and both volume and concentration of released fluid can be adjusted. Water efflux, estimated by tritium exchange in field animals, was high even in dry conditions (7.1 ± 1.45 ml·100 g-1·d-1, T½ = 5.25 ± 0.95 d-1) and doubled during wet weather (13.8 ± 2.93 ml·100 g-1 ·d-1, T½ = 2.7 ± 0.56 d '). The tritium method was considered to overestimate efflux of unlabeled water under the experimental conditions, but even so measured turnover was much greater than in laboratory conditions. Sodium turnover in the field was also high ranging from 231.3 ± 24.4 µmol· 100 g-1 ·d-1, T½ = 31.7 ± 4.69 d-1 in dry weather conditions to 439.3 ± 86.3 µmol· 100 g 1 ·d―1, T½ = 18.5 ± 4.89 d-1 in rainy weather. Sodium in the diet (predominantly fallen leaves) accounted for sodium intake, while output was partitioned between feces and excreted fluid. The discrepancy between tritiated water and sodium turnovers in laboratory and field data indicated that field animals had a considerable reserve of osmoregulatory capacity. Crabs participating in the annual breeding migration to the sea underwent a substantial fall in the osmolality of the hemolymph during their journey. On arrival at the coast the crabs remained in contact with sea water for several hours during which time the ion concentrations of their hemolymph returned to the normal range. It is suggested that this "dipping" behavior functions primarily in osmoregulation.


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