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Channel catfish (Ictalurus punctatus Rafinesque) were progressively acclimated to CO2 partial pressures of 7.5, 15, 30, 45 and 58 mmHg (1, 2, 4, 6 and 8% CO2 in air) and blue crabs (Callinectes sapidus Rathbun) to 15, 30 and 45 mmHg, with 24 h at each partial pressure. Measurements of both conventional acid-base parameters (pH, PCO2. total CO2) and ‘strong’ ion concentrations (Na+, K+, Mg2+, Ca2+ and Cl−) were made at various times during each treatment. Intracellular [Na+], [K+] and [Cl−] were determined for red and white muscle in control and hypercapnic (8%) catfish. Extracellular [HCO3−] and strong ion difference (SID) both rose during hypercapnic compensation, with correlation coefficients (r) of 0.97 (P&amp;lt;0.01) for catfish and 0.41 (NS) for blue crabs. Since [HCO3−] is calculated from two rapid measurements, and SID from four separate procedures, the former appears to be the measurement of choice. The results also dispel the notion of a 30 mequiv1−1 upper limit to bicarbonate compensation: [HCO3−] values over 50mequiv1−1 were achieved in both animals, and %pH regulation remained around 70% at the highest PCO2 Due to superior intracellular buffering, the large change in extracellular SID did not lead to a measurable change in the concentrations of the major intracellular ions. The primary gill filaments of hypercapnic catfish showed a 30% increase in numbers and a 75% increase in area of apical crypts of chloride cells. The chloride cell ‘patches’ in crab gills increased in staining density after hypercapnia but did not enlarge. Note: Present address: Department of Biology, Dalhousie University, Halifax, NS, Canada B3H4JI.

Compensation of Progressive Hypercapnia in Channel Catfish and Blue Crabs