The lungs are responsible for essentially all uptake of oxygen in Birgus latro. Elimination of CO2 in resting crabs appears to occur largely across the gills but during exercise approximately half the output of CO2 is pulmonary. PaO2 was high in resting crabs (43.8 mmHg; 1 mmHg = 133.3 Pa) but fell during exercise (to 27mmHg). PvO2 remained constant at 10–12 mmHg. PaCO2 rose substantially during exercise (from 7.1 to 14.6mmHg).

Haemocyanin delivered 90% of oxygen in resting crabs rising to 97%following exercise. Oxygen delivery at rest was 0.46 mmoll−1 haemolymph rising to 0.72mmoll−1 following exercise. Pigment-bound oxygen capacity was 1.1 mmoll−1. Oxygen delivery to the tissues was diffusion-limited during exercise.

Anaerobic metabolism during exercise raised the concentration of L-lactate in the haemolymph 100-fold (from 0.25 to 25 mmoll−1) and concomitantly caused a fall in pH of 0.7 units. This acidosis was partially compensated by the end of the 30-min exercise period.

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