OSMOTIC REGULATION AND ADAPTIVE REDUCTION OF WATER-PERMEABILITY IN A BRACKISH-WATER CRAB RHITHROPANOPEUS HARRISI (BRACHYURA, XANTHIDAE)

¹ Department of Zoology, University of California, Berkeley, California 94720

1. The osmotic performance of the small cancroid crab Rhithropanopeus harrisi (Brachyura) has been surveyed in order to assess the mechanisms mainly responsible for its success in colonizing waters of low salinity.

2. This crab shows hyper-regulation of chloride and osmotic pressure in media up to about 60-70% sea water, and a slight tendency to hypo-regulate in higher salinities.

3. Like other crabs, Rh. harrisi maintains a relatively high blood concentration and produces a blood-isotonic urine.

4. Urine production, estimated by an indirect method, approximates 24% of the body weight per day in low salinities, implying a high rate of water turnover.

5. Approximately of the total salt loss is via the urine.

6. Inward permeability to water, as judged by D₂O influx rate, is decreased at lower salinities. This mechanism, here demonstrated for the first time in crustaceans, is suggested as being of adaptive significance.

7. Urinary output of water exceeds the diffusional net (osmotic) input of water as calculated by D₂O influx, suggesting the possibility that a differential diffusional permeability to water or some form of non-diffusional water transport may be involved.