In nature, the green crab exhibits emersion and terrestrial activity at low tide. Treadmill exercise in air (20–23°C) of crabs acclimated to 32 ppt seawater (13°C) revealed an inverse relationship between velocity and duration: 2.0 body lengths (BL) s−1 was sustainable for several minutes, and 0.25 BL s−1 was sustainable for long periods. Fatigue was not due to dehydration. Physiological responses over an 18 h recovery in seawater after near-exhaustive exercise (0.25 BL s−1, 1 h) in air were compared with responses after quiet emersion (1 h) in air. Exercising crabs exhibited transient scaphognathite slowing and progressive increases in heart rate, whereas emersed crabs exhibited persistent inhibition of ventilation and transient heart slowing. Upon return to seawater, all these rates increased above both control and treatment levels. Post-exercise disturbances were more marked and/or longer lasting (e.g. EPOC, hyperventilation, tachycardia, metabolic acidosis, lactate elevation, ionic disturbances) than those after simple air exposure. However, an increase in net acidic equivalent excretion to the environment occurred after emersion but not after exercise. Instead, post-exercise crabs relied on carapace buffering, signalled by elevated haemolymph Ca2+ and Mg2+. Prolonged lowering of haemolymph PCO2 associated with hyperventilation also played a key role in acid–base recovery. EPOC after exercise was 3-fold greater than after emersion, sufficient to support resting O2 for >14 h. This reflected clearance of a large lactate load, likely by glycogen re-synthesis rather than oxidation. We conclude that the amphibious green crab uses a combination of aquatic and terrestrial strategies to support exercise in air, emersion in air and recovery in seawater.

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