Ion Transport and the Effects of Moulting in the Freshwater Crayfish Cherax destructor (Decapoda : Parastacidae)

Abstract

Sodium and chloride were maintained in approximate equilibrium by intermoult Cherax destructor but net uptake from the water began the day before ecdysis and continued for several days in postmoult. Sodium transport in postmoult was four times as much as that during the intermoult. The apparent affinities of the Na and Cl transport mechanisms, 0.5 ± 0.12 and 0.3 ± 0.04 mmol L^-1, respectively, were relatively low compared with other reported values for freshwater crustaceans. Calcium flux was negligible in intermoult animals and net efflux was evident in premoult as calcium resorbed from the skeleton was excreted. Following ecdysis, calcium net influx was high and persisted for many days into postmoult as the new exosketon was calcified. An apparent affinity (K_m) of 0.3 ± 0.04 mmol L^-1 Ca and a maximal net transport rate (J_max) of 1.8 ± 0.29 µmol Ca g^-1 h^-1 were measured in postmoult animals. The gills contained a high-affinity Ca²⁺ ATPase (K _m= 0.040.05 µmol L^-1) with a low transport capacity (J_max = 5.6 nmol mg^-1 protein min ^-1) that did not change during the moulting cycle. It is suggested that postmoult Ca transport was facilitated by Na–Ca exchange driven by elevated branchial Na ⁺ K⁺ -ATPase activity.