Walking of the Shore Crab Pachygrapsus Crassipes in its Two Natural Environments

Video analyses of crabs walking on smooth and rocky terrains when in air and when submerged in water were conducted. Modifications of walking in the two environments, between which the animal's weight changes sixfold, are consistent with the goals of maximizing ease of walking and minimizing risk of injury. The duty factor (fraction of step cycle in which the dactylus is in contact with the substratum) was greater than 50 % when in air compared to less than 46 % when submerged, indicating a need for greater stability against the destabilizing vertical force of gravity when in air. The duty factors of the trailing and leading leg rows were the same for the two terrains in air but the trailing leg row had a larger duty factor when submerged, indicating a greater pushing effort to overcome drag forces. Width of stance differed among the four conditions and was narrowest in animals walking over rocky terrain in air, the condition which has the greatest potential for injury. The mean phase difference (percentage of a step cycle by which ipsilateral legs differ) between leading and trailing rows did not differ under any condition except for submerged smooth terrain, meeting the unique requirements of that condition. The observed walking speed range had no effect on stance, duty factor or phase difference. Note: Present address: U.S. Fish & Wildlife Service, c/o Wildlife & Fisheries Biology Department, University of California, Davis, CA 95616, USA.