MECHANISM OF THE NET UPTAKE OF WATER IN MOULTING BLUE CRABS (CALLINECTES SAPIDUS) ACCLIMATED TO HIGH AND LOW SALINITIES

Blue crabs (Callinectes sapidus Rathbun) acclimated to a salinity of 2 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 g-1 h-1 at 1 day prior to moult to approximately 8 ml 100 g-1 h-1 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 g-1 h-1) than in crabs acclimated to low salinity (0.26±0.04 ml 100 g-1 h-1). 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 g-1 h-1) than in crabs acclimated to 2 salinity (34.0±4.7 ml 100 g-1 h-1). 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 g-1 h-1 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.