FORM AND MOTION OF DONAX-VARIABILIS IN FLOW

The coquina clam, Donax variabilis, rides flow from waves, migrating s horeward during rising tides and seaward during falling tides. This me thod of locomotion, swash-riding, is controlled not only behaviorally but also morphologically. The shape of this clam causes it to orient p assively; a clam rotates in flow, usually in backwash, until its anter ior end is upstream. Rotation is about a vertical axis through a pivot al point where the shell touches the sand. The density, weight distrib ution, 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 u nderwater weight-a circumstance that results in uncontrollable tumblin g. In contrast, once oriented with its anterior end upstream, a clam e xperiences downward lift that contributes to its stability while slidi ng in backwash. Furthermore, when the anterior end is upstream, drag i s reduced relative to when the ventral, dorsal, or posterior ends are upstream. Since orientation occurs only above a minimum velocity, it h as the effect of slowing a clam's motion over the substratum in rapid flows. Stability, drag, and speed reduction enhance a clam's ability t o gain a foothold and dig in after a swash-ride, before wave flows can wash it off the beach and out to sea.