Effects of Hypoxia and Hypercapnic Hypoxia on Oxygen Transport and Acid-Base Status in the Atlantic Blue Crab, Callinectes sapidus, During Exercise
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The responses of many estuarine invertebrates to hypoxic conditions are well established. However, many previous studies have investigated hypoxia as an isolated condition despite its frequent simultaneity with hypercapnia, or elevated CO<sub>2</sub>. Although many studies suggest deleterious effects of hypercapnia, hypercapnia has been observed to improve blue crab walking performance under hypoxic conditions. To investigate the physiological effects of combined hypercapnic hypoxia, we measured respiratory and acid-base parameters including Po<sub>2</sub>, pH, [L-lactate], total CO<sub>2</sub>, and total O<sub>2</sub> in pre-and post-branchial hemolymph sampled from blue crabs before and during light walking exercise under a range of O<sub>2</sub> and CO<sub>2</sub> conditions. Crabs walked at 8 m min<sup>−1</sup> on an aquatic treadmill in normoxic (100% air saturation), moderately hypoxic (50%) and severely hypoxic (20%) 30 ppt seawater at 25° C with and without the addition of hypercapnia (2% CO<sub>2</sub>). Respiration was almost completely aerobic in normoxic conditions, with very little buildup of lactate. During exercise under severe hypoxia, lactate increased from 1.4 mM to 11.0 mM, indicating a heavy reliance on anaerobic respiration. The % O<sub>2</sub> saturation of arterial hemocyanin was 47% in severe hypoxia after 120 minutes, significantly lower than in normoxia (80%). However, the addition of hypercapnia significantly increased the % saturation of arterial hemocyanin in severe hypoxia to 92% after 120 minutes of exercise, equivalent to normoxic levels. Hypercapnia in severe hypoxia also caused marked increases in total CO<sub>2</sub> and Pco<sub>2</sub> (around 14 mM and 1.1 kPa respectively), but caused only a minor decrease in pH of 0.1 to 0.2 pH units. We suggest that the improved O<sub>2</sub> saturation at the gills likely results from a specific effect of molecular CO<sub>2</sub> on hemocyanin oxygen binding affinity, which works independently of and counter to the effects of decreased pH.