The Mechanism of Rapid Running in the Ghost Crab, Ocypode Ceratophthalma

Abstract

1. Ocypode ceratophthalma has a maximum speed of 2.1 m/sec when running on a measured track with a base of hard-packed sand. Speed increases linearly with the width of the carapace up to a certain size, beyond which larger crabs run slower than smaller ones. 2. The crab does not run at these high speeds by making extremely rapid movements as these data seemed to require. The highest frequency of leg movements observed was 20 Hz. 3. Electromyographs of muscles used in running, made from the freely running, intact crab, showed asymmetry in the motoneurone discharges. Extensors and flexors in the meropodites of legs on the leading side frequently showed only a maintained tonus and could not have contributed to the running movements. Those on the trailing side showed alternation synchronous with stepping. 4. It is concluded that the crab mainly pushes itself along rather than using a push-pull combination. 5. Three pairs of legs are commonly used in running. Legs 2 and 4 of one side move together with leg 3 of the opposite side and provide a tripod of support. At the highest speeds only legs 2 and 3 of one side are used to provide thrust alternately. 6. The high speed is achieved by the crab leaping through the air as it steps, thereby increasing the effective length of the steps.