After moulting, blue crabs (Callinectes sapidus) acclimated to a salinity of 2‰ were able to calcify as rapidly and accumulate as much calcium as crabs in sea water. Immediately after moult, the total masses of calcium, magnesium and strontium present in the whole body were 4.6, 15.6 and 3.0%, respectively, of their intermoult values. During the time of most rapid calcification, calcium uptake was 5.4±1.4mmoll−1, which is comparable to the maximum rate found in seawater-acclimated crabs. The concentrations of bound and free calcium in the blood changed very little with acclimation salinity, remaining at approximately 3 and 8mmoll−1, respectively, both during intermoult and 1 day postmoult. Free calcium changed relatively little through the moult cycle, varying between 6.9 and 8.1mmoll−1, but bound calcium rose to a peak of 6.4mmoll−1 prior to moult then dropped to 2.6mmoll−1 after moult, concurrent with a decrease of approximately 80% in the protein concentration of the blood. The concentration of total magnesium in the blood increased from a premoult low of 9.0mmoll−1 to a postmoult high of 11.7mmoll−1 and remained elevated throughout the period of rapid mineralization. During the postmoult period of rapid calcium uptake, the internal-to-external concentration ratio for total calcium was 6.6 to 1. The activity ratio, however, was only 2.5 to 1 because 28% of the calcium in the blood was bound to protein, and because the lower ionic strength of the medium resulted in a 2.5-fold higher activity coefficient for the water compared to blood. The transepithelial potential at postmoult (−5.4±0.7mV) was significantly more negative than at intermoult (−3.1±0.6mV). In artificial 2‰ sea water, the transepithelial potential (−9.3±0.7mV) was higher than the equilibrium potential for calcium (−12.0±0.5mV), implicating active transport in the uptake of calcium.

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