Physiological variables for ventilation (scaphognathite frequency, branchial chamber pressure and branchial air flow) and for circulation (heart rate, intracardiac and pericardial haemolymph pressure) were measured in the land crab Cardisoma guanhumi (Latreille). Crabs were studied both in air alone and in air with access to a shallow layer of fresh, brackish or sea water.

During complete air exposure, forward scaphognathite beating predominated and reversed scaphognathite beating was very infrequent. Periods of apnoea were rare. When crabs were able to immerse the Milne-Edwards openings to the branchial chambers in water, scaphognathite reversal occurred much more frequently, and most air flow through the branchial chambers was generated by this mode of ventilation. Changes in water salinity had no effect on respiratory patterns. The cyclic variation between forward and reversed scaphognathite beating appears to serve not only to ventilate the branchial chambers with air, but also to flush water through the branchial chambers for non-respiratory purposes such as ion, water and acid-base regulation.

Haemolymph pressures were comparatively low (14mmHg systolic, 6mmHg diastolic). During diastole a pressure gradient of approximately 0.6 mmHg existed between the pericardial space and the heart lumen. Pauses in heart beat were never observed. Circulatory events were closely coordinated with adjustments in ventilation. Reversed scaphognathite beating produced a transient increase in systolic and diastolic haemolymph pressure due to the rise in branchial air pressure acting directly upon the large, haemolymph-filled spaces lining the branchial chambers. A transient brady-cardia accompanied this brief rise in central haemolymph pressures. Possible mechanisms for the regulation of haemolymph pressure are discussed.

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