The fiddler crab, Uca pugilator, used sideways octapedal locomotion during 15 min of treadmill exercise. At each velocity tested (0.06, 0.11 and 0.16 km h−1), oxygen consumption (VOO2) showed only a modest, sluggish elevation; a ‘steady-state’ was never attained. The highest VOO2 recorded, 0.22 mlO2g−1h−1, was 4.4 times the resting rate. Net whole body lactate (WBL) was found to increase at a constant rate throughout the exercise period.

During recovery, VOO2 and WBL removal followed a similar time course and returned to pre-exercise rates in 30–45 min. Although the fate of lactate after exercise is unknown for crustaceans, calculations suggest that not enough oxygen is consumed by the crab during recovery to oxidize lactate completely to CO2 and H2O. A gluconeogenic fate is compatible with the data.

As running velocity was increased, VOO2 increased only slightly, while the net rate of WBL production showed a substantial elevation. At low velocity aerobic metabolism accounted for 60% of the ATP produced when aerobic metabolism and anaerobic fermentation are considered. Anaerobic fermentation dominated at medium and high velocity and produced 60 and 70% of the ATP, respectively.

The minimum cost of transport, the least amount of energy required to transport a given mass a distance, was determined using both aerobic and anaerobic sources. This estimation of locomotion economy for Uca pugilator was within the range predicted for a vertebrate of a similar mass.

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