Cr III and Cr VI, but Not Cr II, Enhance Cr Incorporation Into the Exoskeleton and Inhibit Ca and Mg Flux to the Exoskeleton During Postecdysial Mineralization of the Blue Crab, <scp> <i>Callinectes sapidus</i> </scp>

ABSTRACT Cr is reportedly present in crustacean exoskeleton. Using Callinectes sapidus as the model, this study addressed during which phase of the molting cycle Cr is deposited to the exoskeleton and how Cr in different valencies impacts this process and exoskeletal mineralization. We hypothesized that due to Cr II and Ca being both divalent, Cr II would promote Cr incorporation into the shell during postecdysial mineralization, thereby adversely affecting mineralization more than either Cr III or Cr VI. One‐day postecdysial blue crabs were injected with each of Cr II, Cr III, and Cr VI at 2 μg Cr/g wet weight at days 0 and 2. At day 4, carapace and tissue samples were collected. Cr III and Cr VI treatments led to a 2.53‐ and 2.96‐fold increase in exoskeletal Cr content, respectively, while Cr II treatment had no effect. Both Cr III and Cr VI, not Cr II, significantly reduced the Ca and Mg content per unit shell area, suggesting that both Cr III and Cr VI inhibit the flux of Ca and Mg to the shell during mineralization. We postulate that the entry of Cr III and Cr VI from the hemolymph to epidermal cells is mediated by transferrin and a sulfate transporter, respectively, and both Cr species are presumably reduced to Cr II in epidermal cells before being transported to the hardening exoskeleton via competitive binding to Ca 2+ /2H + antiporter in the apical membrane, resulting in inhibition of calcification.