Swimming, Stirring, and Hydrodynamic Scaling in the {\it Volvocales}

The {\it Volvocales} constitute a family of colonial algae ranging in size from tens to hundreds of microns. The surface of these nearly spherical algae is packed with cells, each having two flagella. Their incessant, periodic flailing moves the water in which the organisms live. The {\it Volvocales} and especially their largest member {\it Volvox} constitute a model system to study the coordinated action of multiple flagella in self-propulsion and the fluid mixing they produce. Using flow visualization and micromanipulation we have measured the swimming speed, fluid velocities, and propulsive forces for {\it Volvocales} over several orders of magnitude in organism size. The associated Peclet number varies from $\sim 0.01$ for the smallest species to $\sim 100$ for the largest, spanning the range from diffusion-dominated to advection-dominated transport of vital molecules dissolved in the suspending medium. Over this same range we quantify scaling relations for swimming speed and propulsive efficiency. These results are interpreted in terms of metabolic and physical tradeoffs.