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Locomotion Energetics of the Ghost Crab: I. Metabolic Cost and Endurance


Arthropods possess spectacular diversity in locomotor design. Yet it is not clear what unique constraints, if any, variation in design imposes on mechanics, metabolic energy cost or endurance during terrestrial locomotion. In the present study metabolic energy cost and endurance on a treadmill are measured for an arthropod, the eight-legged sideways travelling ghost crab, Ocypode quadrata (Fabricius). In a second paper the mechanics of locomotion are determined during walking and running over a force plate.

Severe limitations in O2 uptake during exercise are not inherent in the design of a crab's O2 transport system, which consists of gills and an open circulatory system. The ghost crab's capacity to elevate oxygen consumption (VOO2) rapidly is correlated with a lesser dependence on anaerobic sources than observed in other crab species. Accelerated glycolysis contributed at the onset of submaximal exercise, before O2 uptake adjustments were completed, but played only a minor role during steady-state exercise. O. quadrata elevated VOO2 6–4- to 8-fold above resting rates. The ghost crab's maximal oxygen consumption (VOO2max) was not different from that of an ectothermic vertebrate of the same body mass and temperature, such as a lizard, that uses lungs and a closed circulatory system.

The minimum metabolic energy necessary to move 1 g of crab 1 km (Cmin) decreased as a function of body mass and age. Cmin was comparable to that predicted for vertebrates of a similar body mass and, therefore, appears to be relatively independent of locomotor design. This is consistent with the hypothesis that a similarity in the energetic properties of muscle and elastic structures may result in similar metabolic costs of locomotion.

Endurance capacity did not increase with body mass, as predicted from interspecific comparisons of mammals and lizards. Instead, endurance capacity correlated with the speed at which oxygen consumption was maximal. Mean endurance capacity for ghost crabs was similar to that found for lizards, but was far less than the values predicted for mammals. Ghost crabs could only sustain a slow walk. Running at speeds 20 times faster is possible for short periods, but not without the aid of anaerobic metabolism.