Ion Regulation in Crayfish: Freshwater Adaptations and the Problem of Molting

Abstract

SYNOPSIS. 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 m M ). In intermolt the primary concern is Na and Cl balance; around ecdysis the emphasis switches to Ca regulation as the cuticle is decalcified/calcified. Compared with marine crustaceans, intermolt crayfish maintain a reduced extracellular (EC) osmolality and have lower permeability to both ions and water. Hyperregulation involves active branchial uptake of Na and Cl and the unique ability to produce a hypotonic urine. Ion uptake involves apical electroneutral ion exchange (Na$ for H$; Cl− for HCO3−; counterions provided from CO2 via carbonic anhydrase) followed by active basolateral transport of Na via the Na pump, with Cl following passively. Reabsorption of 95% of filtered electrolytes at the antennal gland (kidney) involves similar subcellular mechanisms in a morphologically differentiated region of the distal tubule. Intermolt crayfish exhibit negative Ca balance (passive efflux unopposed by uptake) tolerable in view of the large cuticular CaCO3 reserve. In premolt, cuticular Ca is reabsorbed. A small amount is stored as gastroliths, the remainder is lost via branchial excretion and in the discarded exuviae. At ecdysis, FW uptake generates the physical force for shedding, leaving the crayfish with dilute hemolymph and a Ca deficiency. Levels of EC Na and Cl are restored by intensive postmolt branchial uptake. Mineralization of the soft exoskeleton involves remobilization of stored Ca and branchial uptake of Ca and HCO3. Transepithelial Ca transport involves Ca2$ ATPase and Ca2$/Na$ exchange. The importance of external electrolytes and pH in postmolt ion regulation is explored, as are some allometric considerations.