Calcium is essential for many biological processes involved in cellular motility. However, the pathway by which
calcium influences motility, in processes such as muscle contraction and neuronal growth, is often indirect and complex. We
establish a simple and direct mechanochemical link that shows how calcium quantitatively regulates the dynamics of a primitive
motile system, the actin-based acrosomal bundle of horseshoe crab sperm. The extension of this bundle requires the continuous
presence of external calcium. Furthermore, the extension rate increases with calcium concentration, but at a given concentration,
we find that the volumetric rate of extension is constant. Our experiments and theory suggest that calcium sequentially binds to
calmodulin molecules decorating the actin filaments. This binding leads to a collective wave of untwisting of the actin filaments
that drives bundle extension.