The effect of ambient chloride concentration changes on branchial chloride-bicarbonate exchanges and hemolymph acid-base balance of crayfish

Abstract

In crayfish Astacus leptodactylus maintained in 13°C ambient water of constant oxygenation and acid-base balance, water chloride concentration was varied while the concentration of other ions, except sulfate, were maintained at constant values. The effects of wide variations of the ambient Cl − concentration, C Cl, on the branchial titration alkalinity, ΔTA, on ΔC Cl, ΔC CO 2 between inspired and expired water, and on the prebranchial hemolymph C Cl and pH were studied. The branchial changes of TA, C Cl and C CO 2 were dependent on the ambient C Cl, and a decrease of C Cl led to a fall of CO 2 excretion. Hemolymph acid-base balance closely depended on ambient C Cl, particularly when C Cl was less than 1 meq · L −1; when ambient C Cl fell from 1 meq · L −1 to 0.1 meq · L −1, pH increased from 7.87 to 8.30. Since hemolymph P CO 2 was not affected by the variations of C Cl, the changes of pH are mainly explained by the concomitant changes of the hemolymph HCO 3 − and CO 3 2− concentrations. During an 18-day exposure of crayfish to dechlorinated water, hemolymph C Cl decreased progressively whereas its pH increased. The extreme values for C Cl were 193 meq · L −1 at day 1 and 96 meq · L −1 at day 18, and the corresponding pH extremes were 7.87 and 8.65. Since the observed modifications of the branchial exchanges and the hemolymph acid-base balance were not related to the small changes of ambient osmolarity or of the SO 4 2− or Na + concentrations, it is inferred that they depended on a modified Cl − HCO 3 − exchange mechanism in the gills.