Running in the surf: hydrodynamics of the shore crab Grapsus tenuicrustatus

When locomoting in water, animals experience hydrodynamic forces due to ambient water motion and their own motion through the water. Because an aquatic pedestrian must maintain contact with the substratum to locomote, hydrodynamic forces which can dislodge an animal have the capacity to constrain the postures, gaits and speeds an animal can use. This study measured hydrodynamic forces on the amphibious shore crab Grapsus tenuicrustatus in aquatic and terrestrial postures. The crabs' locomotory speeds and ambient water velocities in their habitat were considered in predicting the conditions under which a crab is likely to overturn or wash away. A non-moving crab can withstand 200% faster flow in the aquatic posture than in the terrestrial posture. A crab using the terrestrial posture while locomoting through still water experiences 132% greater drag and 17% greater acceleration reaction forces than it does in the aquatic posture. Due to the lower hydrodynamic forces in the aquatic posture, a crab could locomote up to 50% more quickly or through a faster water flow environment than it could in the terrestrial posture. In faster flow environments like wave-swept rocky shores, a crab in either posture would have to actively grasp the substratum to keep from being dislodged, preventing it from using a punting gait. In slower flow environments, animals can locomote faster and take advantage of different gaits that are not available to them in faster flow environments.