Abstract
ABSTRACT Calcium and magnesium ions raise the oxygen affinities of 25S hemocyanins of both first-instar juvenile and adult Cancer magister. A physiologically relevant change in magnesium concentration from 16 to 32mmol l−1 changes first-instar juvenile hemocyanin affinity by 5.6mmHg (0.7kPa) but adult affinity by only 1.1mmHg (0.15kPa). In early juvenile crabs, the higher magnesium sensitivity of the hemocyanin may be compensated for by the lower oxygen affinity, which has been shown previously to be 50% lower than that of the adult under identical experimental conditions. Furthermore, ontogeny of ionic and osmotic regulation occurs during the development of C. magister, with especially high concentrations of magnesium being found in the hemolymph of early juveniles. Intermediate-stage juveniles (fifth to eighth instars) have hemocyanins with subunit stoichiometries and P50 values approaching those of the adult. These findings are significant because they indicate that modulation of C. magister hemocyanin oxygen-affinity during development incorporates differences in intrinsic affinity and differences in divalent cation sensitivity of the stage-specific hemocyanins.
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