Air Breathing by the Purple Shore Crab, Hemigrapsus nudus (Dana). IV. Aquatic Hypoxia as an Impetus for Emersion? Oxygen Uptake, Respiratory Gas Transport, and Acid-Base State

Blood gas and acid-base status in the purple shore crab, Hemigrapsus nudus (Dana), was determined before, during, and after 3 h of exposure to water with a PO₂ of 20 mmHg. The ventilatory, circulatory, and O₂ uptake responses were assessed during progressive hypoxia to a PO₂ of 5 mmHg. The respiratory rate was independent of PO₂ to 10 mmHg and was achieved by increasing cardiac output rather than hyperventilation or tachycardia. In contrast to the respiratory alkalosis seen in most other hypoxic crustaceans, Hemigrapsus exhibited a large and rapid metabolic alkalosis within 30 min of exposure to water with a PO₂ of 20 mmHg. Subsequently, there was a smaller respiratory alkalosis so that by 3 h of elapsed time haemolymph pH had increased to a pH of 8.19. Return to normoxia prompted an almost immediate removal of both the respiratory and metabolic alkaloses. There was no evidence of any anaerobiosis. A small increase in circulating urate toward the end of the exposure period (∼0. 05 mmol L⁻¹ maximum) suggested that O₂ use by urate oxidase might be limiting. Urate accumulation in Hemigrapsus was shown to be temperature-dependent. Exposure to 20 mmHg PO₂ induced decreases in both arterial and venous O₂ pressure and content, while O₂ delivery was maintained, as was an arterial-venous difference. Fitting the in vivo data to O₂ equilibrium curves indicated increased haemocyanin O₂ affinity due to the alkalosis and increased urate, which served to maintain arterial oxygenation near 100%. Oxygen delivery switched from a reliance on dissolved O₂, in normoxic water to a dependency on haemocyanin O₂ transport. The apparent perfusion-based compensation for reduced O₂ availability, the rapid metabolic alkalosis, and the ability of the haemocyanin in Hemigrapsus to remain unusually well oxygenated are features quite different from those reported for other species of similar habit and habitat. This strongly implies that Hemigrapsus is well adapted to hypoxic exposure and that avoidance of hypoxic water is not a stimulus for this crab to assume air breathing. The role of aquatic hypoxia as a selective pressure in crustaceans evolving to life on land must be questioned.