Martinez MM, Full RJ, Koehl MA
Department of Integrative Biology, University of California at Berkeley, Berkeley, CA 94720, USA.
J Exp Biol. 1998 Sep;201 (Pt 18):2609-23. doi: 10.1242/jeb.201.18.2609.
As an animal moves from air to water, its effective weight is substantially reduced by buoyancy while the fluid-dynamic forces (e. g. lift and drag) are increased 800-fold. The changes in the magnitude of these forces are likely to have substantial consequences for locomotion as well as for resistance to being overturned. We began our investigation of aquatic pedestrian locomotion by quantifying the kinematics of crabs at slow speeds where buoyant forces are more important relative to fluid-dynamic forces. At these slow speeds, we used reduced-gravity models of terrestrial locomotion to predict trends in the kinematics of aquatic pedestrian locomotion. Using these models, we expected animals in water to use running gaits even at slow speeds. We hypothesized that aquatic pedestrians would (1) use lower duty factors and longer periods with no ground contact, (2) demonstrate more variable kinematics and (3) adopt wider stances for increased horizontal stability against fluid-dynamic forces than animals moving at the same speed on land. We tested these predictions by measuring the three-dimensional kinematics of intertidal rock crabs (Grapsus tenuicrustatus) locomoting through water and air at the same velocity (9 cm s-1) over a flat substratum. As predicted from reduced-gravity models of running, crabs moving under water showed decreased leg contact times and duty factors relative to locomotion on land. In water, the legs cycled intermittently, fewer legs were in contact with the substratum and leg kinematics were much more variable than on land. The width of the crab's stance was 19 % greater in water than in air, thereby increasing stability against overturning by hydrodynamic forces. Rather than an alternating tetrapod or metachronal wave gait, crabs in water used a novel gait we termed 'underwater punting', characterized by alternating phases of generating thrust against the substratum and gliding through the water.
当动物从空气进入水中时，其有效重量会因浮力而大幅减轻，而流体动力（如升力和阻力）则会增加800倍。这些力大小的变化可能会对运动以及抵抗被推翻产生重大影响。我们通过量化螃蟹在低速时的运动学来开始对水生步行运动的研究，此时浮力相对于流体动力更为重要。在这些低速情况下，我们使用陆地运动的失重模型来预测水生步行运动学的趋势。利用这些模型，我们预计水中的动物即使在低速时也会使用奔跑步态。我们假设水生步行者会（1）使用更低的 Duty 系数和更长的无地面接触时间，（2）表现出更多可变的运动学特征，并且（3）与在陆地上以相同速度移动的动物相比，采取更宽的 stance 以增加抵抗流体动力的水平稳定性。我们通过测量潮间带岩蟹（Grapsus tenuicrustatus）在平坦基质上以相同速度（9厘米/秒）在水和空气中移动时的三维运动学来测试这些预测。正如从奔跑的失重模型所预测的那样，与在陆地上运动相比，在水下移动的螃蟹腿部接触时间和 Duty 系数减少。在水中，腿部间歇性地循环，与基质接触的腿更少，并且腿部运动学比在陆地上更加多变。螃蟹在水中的 stance 宽度比在空气中大19%，从而增加了抵抗水动力推翻的稳定性。水中的螃蟹没有使用交替的四足或顺序波步态，而是使用了一种我们称为“水下撑船”的新颖步态，其特征是交替出现对基质产生推力和在水中滑行的阶段。
