THE EFFECT OF AMBIENT PH ON ELECTROLYTE REGULATION DURING THE POSTMOLT PERIOD IN FRESH-WATER CRAYFISH PROCAMBARUS-CLARKII

The effects of ambient pH on postmoult net fluxes of Ca, acidic/basic equivalents (H+, NH4+/OH-, HCO3-), Na and Cl-, total body Ca, haemolym ph pH and electrolyte status were assessed in the freshwater crayfish Procambarus clarkii (Girard). Variables were monitored for 5 days post moult in acidic (pH 5.2; H2SO4) or alkaline (pH 9.2; KOH) artificial t ap water (ATW) and compared with those in control (pH 7.4) tap water. In control ATW there was an initial net influx of Ca (+2700 mumol kg-1 h-1) and titratable basic equivalents (+5000 mumol kg-1 h-1) that dec lined with time. Calcium uptake accounted for 40 % of total body Ca (0 .49 mmol g-1); haemolymph Ca concentration remained constant. Haemolym ph pH was initially relatively alkalotic (7.7) but recovered within 24 h. A 20 % haemolymph dilution by water uptake at ecdysis necessitated uptake of Cl- and Na for the first 2-3 days postmoult (+1000 mumol kg -1 h-1). In acidic ATW, Ca and basic equivalent uptake were both 60 % reduced during the first 3-4 days and total body Ca was reduced by 37 %. Chloride and Na uptake and haemolymph [Cl-] were decreased. In alka line ATW, Ca and basic equivalent uptake were elevated by 30 % for the first 2 days and haemolymph alkalosis was maintained. Sodium and Cl- balance were unaffected. Thus, ambient pH affects Ca and basic equival ent fluxes associated with postmoult calcification. Regulation of Na a nd Cl- levels is also impaired in acidic ATW.