ION REGULATION IN CRAYFISH - FRESH-WATER ADAPTATIONS AND THE PROBLEM OF MOLTING

Crayfish have a long evolutionary history in temperate fresh water (FW ). Ion regulation is challenged by low external concentrations of Na, Cl, and Ca (<1 mM). In intermolt the primary concern is Na and Cl bala nce; around ecdysis the emphasis switches to Ca regulation as the cuti cle is decalcified/calcified. Compared with marine crustaceans, interm olt crayfish maintain a reduced extracellular (EC) osmolality and have lower permeability to both ions and water. Hyperregulation involves a ctive branchial uptake of Na and Cl and the unique ability to produce a hypotonic urine. Ion uptake involves apical electroneutral ion excha nge (Na+ for H+; Cl- for HCO3-; counterions provided from CO2 via carb onic anhydrase) followed by active basolateral transport of Na via the Na pump, with Cl following passively. Reabsorption of 95% of filtered electrolytes at the antennal gland (kidney) involves similar subcellu lar mechanisms in a morphologically differentiated region of the dista l tubule. Intermolt crayfish exhibit negative Ca balance (passive effl ux unopposed by uptake) tolerable in view of the large cuticular CaCO3 reserve. In premolt, cuticular Ca is reabsorbed. A small amount is st ored as gastroliths, the remainder is lost via branchial excretion and in the discarded exuviae. At ecdysis, FW uptake generates the physica l force for shedding, leaving the crayfish with dilute hemolymph and a Ca deficiency. Levels of EC Na and Cl are restored by intensive postm olt branchial uptake. Mineralization of the soft exoskeleton involves remobilization of stored Ca and branchial uptake of Ca and HCO3. Trans epithelial Ca transport involves Ca2+ ATPase and Ca2+/Na+ exchange. Th e importance of external electrolytes and pH in postmolt ion regulatio n is explored, as are some allometric considerations.