Physiological Compensation to Short-Term Air Exposure in Red Rock Crabs, Cancer productus Randall, from Littoral and Sublittoral Habitats. I. Oxygen Uptake and Transport

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Respiratory and circulatory responses to 4 h emersion were studied in large (subtidal) and small (intertidal) red rock crabs, Cancer productus. Pre- and postbranchial hemolymph O₂ tensions (Po₂) decreased rapidly during emersion, suggesting that O₂ uptake at the gill is diffusion limited under these conditions. Oxygen uptake (Mo₂) could not be maintained during emersion in either group of crabs, and hemolymph lactate levels rose substantially. Fick estimates of cardiac output indicated that tissue oxygen supply is perfusion limited-more in small than in large crabs. Collapse of the gill lamellae in air may limit both gas exchange and hemolymph flow at the gill. In large crabs, scaphognathite pumping frequency (fsc) increased 250% in air, yet heart rate changed little, despite the shortfall of oxygen supply. In contrast, small crabs exhibited bradycardia and alteration of the pattern of scaphognathite activity, eventually leading to reduced fsc. The altered scaphognathite activity pattern in emersed small crabs was associated with large subambient (negative) branchial pressures, possibly serving to aspirate seawater from the substrate into the branchial chamber. The respiratory and circulatory responses of large and small C. productus to an emersion-induced internal hypoxia differ markedly from the responses of other decapods to acute hypoxia while submerged. The present study demonstrates that air breathing makes complex demands of all components of the oxygen uptake and transport system that may not be possible in basically aquatic animals.