Volvox barberi flocks, forming near-optimal, two-dimensional, polydisperse lattice packings

TL;DR

Volvox barberi colonies self-organize into dynamic, near-optimal two-dimensional lattice packings driven by hydrodynamic vortices, revealing complex collective behavior and optimal packing properties.

Contribution

This study demonstrates that Volvox barberi colonies form active, near-optimally packed flocks with irregular lattice structures, driven by hydrodynamic interactions, a novel insight into biological self-organization.

Findings

Colonies form dynamic flocks with irregular lattice structures.

Flocks achieve near-optimal random close-packing.

Hydrodynamic vortices likely drive flocking behavior.

Abstract

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.