Ellers O
Biol Bull. 1995 Oct;189(2):138-147. doi: 10.2307/1542464.
The coquina clam, Donax variabilis, rides flow from waves, migrating shoreward during rising tides and seaward during falling tides. This method of locomotion, swash-riding, is controlled not only behaviorally but also morphologically. The shape of this clam causes it to orient passively; a clam rotates in flow, usually in backwash, until its anterior end is upstream. Rotation is about a vertical axis through a pivotal point where the shell touches the sand. The density, weight distribution, and wedge-like shape are all important in effecting orientation. Such orientation is significant because it contributes to stability of motion. On an unoriented clam, upward lift can be higher than its underwater weight--a circumstance that results in uncontrollable tumbling. In contrast, once oriented with its anterior end upstream, a clam experiences downward lift that contributes to its stability while sliding in backwash. Furthermore, when the anterior end is upstream, drag is reduced relative to when the ventral, dorsal, or posterior ends are upstream. Since orientation occurs only above a minimum velocity, it has the effect of slowing a clam's motion over the substratum in rapid flows. Stability, drag, and speed reduction enhance a clam's ability to gain a foothold and dig in after a swashride, before wave flows can wash it off the beach and out to sea.
宝石文蛤（Donax variabilis）借助海浪的水流移动，涨潮时向岸迁移，落潮时向海迁移。这种移动方式，即乘浪移动，不仅受行为控制，还受形态控制。这种蛤蜊的形状使其能够被动定向；蛤蜊在水流中旋转，通常是在回流中，直到其前端朝上。旋转围绕着一个垂直轴，通过贝壳接触沙子的枢轴点进行。密度、重量分布和楔形形状在实现定向方面都很重要。这种定向很重要，因为它有助于运动的稳定性。对于一个未定向的蛤蜊，向上的升力可能高于其水下重量——这种情况会导致无法控制的翻滚。相比之下，一旦前端朝上定向，蛤蜊在回流中滑动时会受到向下的升力，这有助于其稳定性。此外，当前端朝上时，相对于腹端、背端或后端朝上时，阻力会减小。由于定向仅在最低速度以上才会发生，它会使蛤蜊在快速水流中在底层上的移动速度减慢。稳定性、阻力和速度降低增强了蛤蜊在乘浪之后、海浪水流将其冲上海滩并冲入大海之前站稳脚跟并钻入沙中的能力。
