Volvox barberi “flocks”

Balasubramanian Fig 3 A&B

Figure 3 A&B from Balasubramanian 2018. (a) V. barberi flock where 56 colonies gathered over several minutes and rotated coherently and rapidly in the culture well. (b) Schematic of flock in panel a.

At the Fourth International Volvox Meeting in St. Louis, a student from Harriton High School in Rosemont, Pennsylvania presented a talk and a poster on “flocking” behavior in Volvox barberi. Now a preprint describing his work is available on bioRxiv.

I can’t think of another time I’ve seen a high school student present at an international conference. It’s a bit unusual for undergraduates to present, but it’s certainly done. High school, though?

The preprint, by Ravi Balasubramanian, describes self-organized “flocks” of Volvox colonies, in some cases over 100 colonies. The flocks are essentially two-dimensional (one colony thick) and form just below the surface of the medium. Balasubramanian shows that they achieve nearly optimal packing, given that the colonies have a range of radii:

In mathematics, objects are considered optimally packed when they gather so that as many fit into a given space as possible…

I show here that V. barberi flocks have a log-normal distribution of colony radii, and form lattices which are nearly identical structurally to random close-packings in molecular dynamics simulations of weakly-attracting spheres with the same radius distribution. The characteristic feature of both the Volvox and simulation lattices is a log-normal distribution of lattice vertex angles. This shows that Volvox barberi gather into flocks that are near-optimally packed for their polydisperse radius distribution.

He also proposes a mechanism for the flocking behavior and provides evidence that this mechanism is at least consistent with the observations:

It is known that another species of VolvoxV. carteri is capable of using fluid forces created by flagellar beating to form waltzing pairs [8]. I hypothesized that V. barberi, being one of the fastest species of Volvox [14], could exert similar attractive forces strong enough to pull V. barberi into well-packed sheets. To test for these attractive forces, a dye tracer was placed into culture wells containing either Volvox in medium or clear water (as a control).

Balasubramanian Fig 5

Figure 5 from Balasubramanian 2018. (a) Top Panel: Culture well with water and dye tracer after five minutes viewed from above. Dye is spread out evenly across culture well. Bottom panel: Same culture well viewed from the side. Dye forms a uniform layer at the bottom of the well. (b) Top panel: Medium with Volvox and a dye tracer viewed from above. Numerous streaks and patches created by Volvox motions are visible. A rapidly rotating pair of Volvox near the top forms a clearing in the liquid, while an already formed flock moves slowly at bottom right clearing a channel. Bottom panel: Dye forms a uniform layer at the bottom of the culture well. (c) Top panel: Volvox in medium with a dye tracer viewed from above. A large, rotating flock formed and created a clearing in the liquid that grew over time, here termed a vortex. Diffuse spindles from the edge to the center developed at late times. Bottom panel: Same culture well viewed from the side. Despite the large vortex, the dye is uniformly spread as seen from the side suggesting that the clearing in the upper panel is a cylindrical vortex.

Hopefully this work is in review somewhere; if so, I’ll provide an update when the final version is published.


Stable links:

Balasubramanian, R.N. 2018. Volvox barberi flocks, forming near-optimal, two-dimensional, polydisperse lattice packings. bioRxiv, 1–7. DOI: 10.1101/279059



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