Effect of the external concentration of calcium on the postmoult uptake of calcium in the blue crabs (Calinectes sapidus

The rate of calcium uptake in blue crabs (Callinectes sapidus Rathbun) acclimated to 2 ‰ sea water with a calcium concentration of 1.4 mmol l21 was dependent on the magnitude and direction of the electrochemical gradient for calcium. When transferred to water with a high calcium concentration (6 mmol l21), the electrochemical gradient for calcium favoured diffusive influx, and calcium uptake and apparent H+ excretion increased by approximately 50 %. When transferred to water with a low calcium concentration (0.10 mmol l21), where the electrochemical gradient for calcium strongly favoured diffusive efflux, calcium uptake ceased but apparent H+ excretion continued at a reduced rate. Crabs regulated the free calcium concentration in their blood at approximately 8 mmol l21 when the external concentration of calcium was 1.4 mmol l21 or higher, but the concentration of free calcium in the blood decreased to 5.6 and 4.6 mmol l21, respectively, at external concentrations of calcium of 0.25 and 0.10 mmol l21. Crabs transferred to water with 0.10 mmol l21 calcium for the first 2 days after moult accumulated only 2.5 g calcium kg21 wet mass, about one-quarter of the mass normally accumulated. Seawater-acclimated crabs transferred to 2 ‰ salinity at 1 day postmoult took up calcium at a reduced rate, indicating that a period of acclimation is necessary for a component of the active transport system to increase its capacity, for diffusive efflux to be reduced, or for both to occur