EFFECT OF THE EXTERNAL CONCENTRATION OF CALCIUM ON THE POSTMOLT UPTAKE OF CALCIUM IN BLUE CRABS (CALLINECTES-SAPIDUS)

The rate of calcium uptake in blue crabs (Callinect es sapidus Rathbun) acclimated to 2 parts per thousand sea water with a calcium concentration of 1.4 mmoll(-1) was dependent on the magnitude and direction of the electrochemical gradient for calcium. When transferred to water with a high calcium concentration (6 mmoll(-1)), 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 mmoll(-1)), 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 mmoll(-1) when the external concentration of calcium was 1.4 mmoll(-1) or higher, but the concentration of free calcium in the blood decreased to 5.6 and 4.6 mmoll(-1), respectively, at external concentrations of calcium of 0.25 and 0.10 mmoll(-1). Crabs transferred to water with 0.10 mmoll(-1) calcium for the first 2 days after moult accumulated only 2.5 g calcium kg(-1) wet mass, about one-quarter of the mass normally accumulated. Seawater-acclimated crabs transferred to 2 parts per thousand 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.