Whole-animal net electrolyte fluxes (Ca2+, apparent H+, titratable acidic equivalents, ammonia, Na+, Cl, K+, Mg2+, phosphate and sulphate) and respiratory gas exchange were monitored throughout the moulting cycle in juvenile freshwater crayfish Procambarus clarkii (Girard) at 21 °C. Intermoult crayfish were essentially in ion balance. As crayfish approached ecdysis (−3 days, where t=0 is the day when the cuticle is shed), there was a net efflux of Ca2+ (−1000μmolkg−1 h−1) correlated with a corresponding uptake of acidic equivalents (or base output) of +2000μmolkg−1 h−1. Following ecdysis, both fluxes switched vector; uptake of Ca2- (+2000μmolkg h) and basic equivalents (+4000μmolkg−1h−1) were completed within 6 days. The moulting cycle also affected fluxes of electrolytes other than those involved in CaCO3 resorption and deposition. Crayfish remained in Na+; and C; balance from intermoult up to ecdysis. Following ecdysis, both were taken up actively at rates of around plus;500μanolkg−1h1 for 3 days, presumably restoring the haemodilution that would have resulted from water loading. A premoult efflux of K+ was partially offset by postmoult uptake. Meanwhile, crayfish experienced increased efflux of phosphate following ecdysis, probably because of increased integumentary permeability. Rates of O2 uptake (MO2) and CO2 excretion (Mco2) increased to peak values (double intermoult rates) immediately prior to ecdysis. While MO2 recovered during postmoult, Mco2 dropped precipitously, significantly reducing the gas exchange ratio. Since the Mco2 deficit agreed well with the postmoult basic equivalent uptake, the latter is probably attributable to HCO3 uptake for calcification.

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