ION-TRANSPORT AND THE EFFECTS OF MOLTING IN THE FRESH-WATER CRAYFISH CHERAX DESTRUCTOR (DECAPODA, PARASTACIDAE)

Sodium and chloride were maintained in approximate equilibrium by inte rmoult Cherax destructor but net uptake from the water began the day b efore ecdysis and continued for several days in postmoult. Sodium tran sport in postmoult was four times as much as that during the intermoul t. 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. Calciu m flux was negligible in intermoult animals and net efflux was evident in premoult as calcium resorbed from the skeleton was excreted. Follo wing ecdysis, calcium net influx was high and persisted for many days into postmoult as the new exosketon was calcified. An apparent affinit y (K-m) of 0.3 +/- 0.04 mmol L-1 Ca and a maximal net transport rate ( J(max)) of 1.8 +/- 0.29 mu mol Ca g(-1) h(-1) were measured in postmou lt animals. The gills contained a high-affinity Ca2+ ATPase (K-m = 0.0 4-0.05 mu mol 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. I t is suggested that postmoult Ca transport was facilitated by Na-Ca ex change driven by elevated branchial Na+K+-ATPase activity.