Martinez M M
Department of Integrative Biology, University of California at Berkeley, 94720, USA.
J Exp Biol. 2001 Sep;204(Pt 17):3097-112. doi: 10.1242/jeb.204.17.3097.
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.
动物在水中移动时，会因周围水体的运动以及自身在水中的移动而受到水动力的作用。由于水生步行者在移动时必须与水底保持接触，那些可能将动物冲走的水动力就能够限制动物可采用的姿势、步态和速度。本研究测量了两栖岸蟹细甲相手蟹在水生和陆生姿势下所受的水动力。在预测螃蟹可能被掀翻或冲走的条件时，考虑了螃蟹在其栖息地的移动速度和周围水体的流速。静止不动的螃蟹在水生姿势下能够承受比陆生姿势快200%的水流。一只螃蟹在静水中以陆生姿势移动时，所受的阻力比在水生姿势下大132%，加速度反作用力大17%。由于水生姿势下水动力较小，螃蟹以水生姿势移动的速度可比陆生姿势快50%，或者能够在流速更快的水流环境中移动。在诸如受海浪冲刷的岩石海岸这类流速较快的环境中，无论处于哪种姿势，螃蟹都必须主动抓紧水底以防止被冲走，这使得它无法采用蹬踏步态。在流速较慢的环境中，动物能够移动得更快，并利用在流速较快的环境中无法采用的不同步态。
