Water carbonate alkalinity as a determinant of hemolymph acid-base balance in the shore crab,Carcinus maenas: a study at two different ambientP CO 2 andP O 2 levels

Abstract

1. Hemolymph acid-base balance (pH,PCO2 and bicarbonate concentration) was measured in shore crabs,Carcinus maenas, exposed first for 17–18 h to normoxic seawater at three levels of titration alkalinity, TAw 0.95, 2.5 and 4.5 meq·l−1, either at low (ca. 0.3 Torr) or at high (ca. 4.3 Torr) ambientPCO2 (PwCO2), and subsequently exposed acutely for 2 h to hyperoxia in the same ambient acid-base conditions. WaterPCO2 was controlled with a pH-CO2-stat system in all experiments. 2. At low constantPwCO2 (0.3 Torr), decreased TAw led to an uncompensated hypercapnic acidosis with hemolymphPCO2 increased and hemolymph pH decreased. At highPwCO2 (4.3 Torr) however, hemolymph acid-base state was independent of TAw. 3. Acute hyperoxia induced hypercapnic acidosis in all conditions. At lowPwCO2, the hyperoxia-induced hypercapnia was more marked at low than at high TAw whereas at highPwCO2 it was of the same magnitude whatever the TAw. 4. The internal acid-base state thus depends on TAw at constant low inspiredPwCO2, probably via related changes of the water CO2 capacitance coefficient, ΔCwCO2/ΔPwCO2, due to buffering of excreted CO2 by carbonate ions at high water pH. Therefore, all the acid-base properties of the ambient water, pH,PCO2 and carbonate alkalinity, must be taken into account in studying acid-base balance of water-breathers.