Aerial and aquatic respiration in the ghost crab, Ocypode saratan II respiratory gas exchange and transport in the haemolymph

Ocypode saratan breathing humid air has a postpulmonary PO₂ of 15.7 kPa and PCO₂ of 1.0 kPa with a gas exchange ratio over the lung of 0.42. When in air, however, O. saratan perfuses the lung in parallel with the gills to generate a mixed postbranchial PaO₂ of 10.5 kPa and PaCO₂ of 1.7 kPa in humid conditions with access to water and a PaO₂ of 2.7 kPa and PaCO₂ of 1.1 kPa in dry conditions with no access to water. When submerged O. saratan does not perfuse the lungs, to give a postbranchial PO₂ of 2.7 kPa and a PCO₂ in both post and prebranchial haemolymph of approximately 1 kPa. Submerged crabs accumulate lactate ions after 5–6 h which appears to be buffered by either HCO^‐ ₃ uptake or H⁺ excretion during passage of the haemolymph through the branchial circulation. The gas exchange ratio across the gills is approximately 1. These data suggest that the main site for oxygen and CO₂ exchange in O. saratan is the lungs and that respiratory gas exchange is impaired during desiccation and during submersion, when the crab perfuses the gills alone.

Key words:

• Respiratory gas exchange
• acid‐base balance
• land crabs

Notes

Department of Zoology, University of Aberdeen.

Department of Biology, King Abdulaziz University, Jeddah, Saudi Arabia.