After 3h(50 m) of voluntary walking, the haemolymph pH of the land hermit crab Coenobita compressus (H. Milne Edwards) decreased by 0.4units. This was accompanied by increases in CO2 tension (Pcoco2). bicarbonate (HCO3 + CO32-) and lactate concentrations. The hypercapnic acidosis was partially compensated by metabolic bicarbonate accumulation and an H+ deficit developed. Unloaded crabs accumulated less of a proton load than crabs transporting mollusc shells. During activity, oxygenation of the haemocyanin (HCy) accounted for the release of 0.3 mmol CO2l−1, via the Haldane effect, which was seven times more than in settled crabs. Control acid-base balance was re-established within 1 h of recovery. At this time, acidic equivalents were excreted at a mean flux rate of 5 mequiv kg−1 h−1 into a source of external water. [Na+] and the ratio of [Na+]:[Cl] increased during exercise.

Coenobita haemolymph had a high O2-carrying capacity (CmaxHCyOHCyO2 = l.55 mmol 1−1). HCy oxygen-binding characteristics were typical of other decapods (φ= −0.44), yet no lactate sensitivity was apparent. Settled in vivo values of O2 tension (Poo2) and content (Coo2) were located around the half-saturation tension (P50) of the dissociation curve. During exercise, POO2 increased and an unopposed Bohr shift decreased the O2-binding affinity, thereby reducing postbranchial saturation. Quantitatively, however, compensations in cardiac output (V·b) were more instrumental in increasing the O2 delivery to respiring tissues. During recovery, haemolymph POO2 remained high and the venous reserve doubled.

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