Aquatic Gas Exchange in the Freshwater/ Land Crab, Holthuisana Transversa

Holthuisana transversa (von Martens), a freshwater/land crab from arid areas of Australia, is an efficient bimodal breather. In water, resting MO2 (1-65^molg"^") and Vw ( lS^mlg" 1 h") at 25°C were lower than in other aquatic crabs whilst percentage extraction of oxygen was quite high (47 %). MO2 was not regulated at low ambient PQ2 but could be increased at least three times during exercise. Normoxic P1O2 was low (18Torr) compared with other water-breathing crabs. The haemolymph contained haemocyanin which had a moderate affinity for oxygen (P50 = 80 Torr) and carried over 90% of the oxygen transported. Oxygen content of postbranchial haemolymph was low (346^moll"). There was a small positive Bohr shift (log Pso/pH-0-33). The strategy of gas exchange in water is discussed and compared with that of aquatic crabs.