Electrolyte and Gas Exchange During the Moulting Cycle of a Freshwater Crayfish

Abstract

ABSTRACTWhole-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+ (–1000μmol kg−1 h−1) correlated with a corresponding uptake of acidic equivalents (or base output) of +2000 μmol kg−1 h−1. Following ecdysis, both fluxes switched vector; uptake of Ca2+ (+2000/mol kg−1 h−1) and basic equivalents (+4000μmol kg−1 h−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 Cl− balance from intermoult up to ecdysis. Following ecdysis, both were taken up actively at rates of around +500μmol kg−1 h−1 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 and CO2 excretion increased to peak values (double intermoult rates) immediately prior to ecdysis. While recovered during postmoult, dropped precipitously, significantly reducing the gas exchange ratio. Since the deficit agreed well with the postmoult basic equivalent uptake, the latter is probably attributable to HCO3− uptake for calcification.