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 ar terial samples had similar O-2 content and thus either the lungs recei ved most of the hemolymph flow or the lungs and gills were of equal im portance during air-breathing. The gills were more important in immers ed crabs. The O-2 partial pressure in arterial hemolymph decreased fro m 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 a rterial-venous O-2 content difference did not change. In immersed crab s, the CO2 content of the hemolymph (6.1 mmol.L-1) was half that in ai r-breathing crabs, but the pH remained unchanged at pH 7.4. Potamonaut es warreni showed no specific adaptations to the >1400-m altitude of i ts habitat other than apparently relatively improved O-2 diffusion. Po tamonautes warreni performed well during exercise in air. Crabs exerci sed at slow speed(1.8 m.min(-1)) experienced smaller hemolymph acid-ba se 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 t he arterial and efferent pulmonary hemolymph during 20 min of fast exe rcise decreased from near 9 to 6 kPa or less, the hemocyanin O-2 satur ation was maintained near 80% and the arterial-venous O-2 difference d oubled. An increase in Hc-O-2 affnity in exercising animals partially offset the Bohr effect and assisted in O, uptake at the gas-exchange s urfaces. The relative importance of lungs and gills in gas exchange af ter exercise in air was apparently similar to that in resting crabs. T here was little requirement for anaerobiosis during submaximal exercis e. Hemolymph L-lactate levels peaked at 3.6 mmol.L-1 after 20 min of f ast exercise and were similar to L-lactate levels in the muscle.