  The spherical alga Volvox swims by means of flagella on thousands of surface
somatic cells. This geometry and its large size make it a model organism for
studying the fluid dynamics of multicellularity. Remarkably, when two nearby
Volvox swim close to a solid surface, they attract one another and can form
stable bound states in which they "waltz" or "minuet" around each other. A
surface-mediated hydrodynamic attraction combined with lubrication forces
between spinning, bottom-heavy Volvox explains the formation, stability and
dynamics of the bound states. These phenomena are suggested to underlie
observed clustering of Volvox at surfaces.
