Apparent H+ Excretion and CO2 Dynamics Accompanying Carapace Mineralization in the Blue Crab (Callinectes Sapidus) Following Moulting

Just after moulting the blue crab, Callinectes sapidus, is soft and defenceless, but in the following days, a hard calcareous shell is formed for support and protection. The carapace of intermoult (hard) crabs constitutes 27% of the wet weight and 53% of the dry weight, with 14% of the body water contained in a carapace fluid compartment. The pH of the shell fluid compartment is maintained at 0.3 to 0.5 pH units above that of the blood. The mineral portion of the carapace is primarily CaCO3, with minor components of Mg2+ and PO4−3 plus traces of other ions. The formation of a new shell after moulting is accomplished by apparent H+ excretion and Ca2+ uptake at remarkable rates, averaging 12.5 and reaching as high as 21 mequiv kg−1 h−1. Direct bicarbonate uptake from the sea water appears to be an important component of the apparent H+ efflux, since CO2 is deposited in CaCO3 at a rate greater than metabolic production, and the animals show a net CO2 uptake for several days.