Role of lungs and gills in an African fresh-water crab, Potamonautes warreni (Decapoda: Potamoidea), in gas exchange with water, with air, and during exercise

Respiratory gas transport and acid-base state were investigated in the crab Potamonautes warreni breathing air or water for at least 24 h to assess the role of the lungs and gills. The efferent pulmonary and arterial samples had similar O-2 content and thus either the lungs received most of the hemolymph flow or the lungs and gills were of equal importance during air-breathing. The gills were more important in immersed crabs. The O-2 partial pressure in arterial hemolymph decreased from 8.6 kPa in air-breathing crabs to 4.5 kPa in water-breathing crabs. Despite an increase in O-2 diffusion limitation during water-breathing (L-diff = 0.78) compared to air-breathing crabs (L-diff = 0.57) the arterial-venous O-2 content difference did not change. In immersed crabs, the CO2 content of the hemolymph (6.1 mmol.L-1) was half that in air-breathing crabs, but the pH remained unchanged at pH 7.4. Potamonautes warreni showed no specific adaptations to the >1400-m altitude of its habitat other than apparently relatively improved O-2 diffusion. Potamonautes warreni performed well during exercise in air. Crabs exercised at slow speed(1.8 m.min(-1)) experienced smaller hemolymph acid-base perturbations (Delta pH = 0.18) than crabs exercised at fast speed (Delta pH = 0.36; 3 m.min(-1)). While the partial pressure of O-2 in the arterial and efferent pulmonary hemolymph during 20 min of fast exercise decreased from near 9 to 6 kPa or less, the hemocyanin O-2 saturation was maintained near 80% and the arterial-venous O-2 difference doubled. An increase in Hc-O-2 affnity in exercising animals partially offset the Bohr effect and assisted in O, uptake at the gas-exchange surfaces. The relative importance of lungs and gills in gas exchange after exercise in air was apparently similar to that in resting crabs. There was little requirement for anaerobiosis during submaximal exercise. Hemolymph L-lactate levels peaked at 3.6 mmol.L-1 after 20 min of fast exercise and were similar to L-lactate levels in the muscle.