  Volvox barberi is a multicellular green alga forming spherical colonies of
10000-50000 differentiated somatic and germ cells. Here, I show that these
colonies actively self-organize over minutes into "flocks" that can contain
more than 100 colonies moving and rotating collectively for hours. The colonies
in flocks form two-dimensional, irregular, "active crystals", with lattice
angles and colony diameters both following log-normal distributions. Comparison
with a dynamical simulation of soft spheres with diameters matched to the
Volvox samples, and a weak long-range attractive force, show that the Volvox
flocks achieve optimal random close-packing. A dye tracer in the Volvox medium
revealed large hydrodynamic vortices generated by colony and flock rotations,
providing a likely source of the forces leading to flocking and optimal
packing.
