Mechanism of the net uptake of water in moulting blue crabs (Callinectes sapidus) acclimated to high and low salinities

approximately doubled in wet mass (excluding carapace) during the period from 10 h before moult to 2 h after moult. Both in blue crabs acclimated to 2 ‰ salinity and in crabs acclimated to 28 ‰ salinity, the drinking rate increased from approximately 0.4 ml 100 g21 h21 at 1 day prior to moult to approximately 8 ml 100 g21 h21 during the first hour after moult. The drinking rate had decreased 1 day after moult in both salinities, but was significantly higher in crabs acclimated to high salinity (1.84±0.16 ml 100 g21 h21) than in crabs acclimated to low salinity (0.26±0.04 ml 100 g21 h21). Drinking accounted for two-thirds of the weight gain during the first hour after moult at both acclimation salinities, indicating that water enters the body at moult primarily through the gut rather than through the gills. [14C]polyethylene glycol, added as a tracer in the bath water, was concentrated in the midgut gland rather than in the stomach, implicating the midgut gland as the primary site of water absorption. The rate of water efflux was significantly greater in crabs acclimated to 30 ‰ salinity (66.4±9.0 ml 100 g21 h21) than in crabs acclimated to 2 ‰ salinity (34.0±4.7 ml 100 g21 h21). The osmotic uptake of water is equal at both salinities as a result of the decreased water permeability at low salinity. The rate of urine formation was estimated to be between 0.5 and 1 ml 100 g21 h21 during the first hour after moult in crabs acclimated to both low and high salinities, suggesting that the antennal gland plays a relatively small role in water regulation during this period