Abstract

Full, R.J. 1987. Locomotion energetics of the ghost crab: I. Metabolic cost and endurance. J. exp. Bio. 130, 137-154.

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 Iocomotion. 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, V(O2), 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 uptake adjustments were completed, but played only a minor role during steady-state exercise. O. quadratae elevated V(O2) 6.4- to 8-fold above resting rates. The ghost crab's maximal oxygen consumption, V(O2max) 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, C(min), decreased as a function of body mass and age. C(min) was comparable to that predicted for vertebrates of a similar body mass and, therefore, appears to be relativly 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 inter-specific 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 30 times faster is possible for short periods, but not without the aid anaerobic metabolism.