I can’t believe I haven’t already blogged about this, but if I have it isn’t turning up in my searches. Ravi Balasubramanian’s preprint about “flocking” behavior in Volvox barberi mentioned that

V[olvox] carteri is capable of using fluid forces created by flagellar beating to form waltzing pairs.

He’s referring to a 2009 paper by Knut Drescher and colleagues in Physical Review Letters. Drescher and colleagues analyzed the physics that cause Volvox colonies to enter a hydrodynamically bound state in which two or more spheroids orbit each other in close proximity:

Stable links:

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

Drescher, K., Leptos, K.C., Tuval, I., Ishikawa, T., Pedley, T.J. and Goldstein, R.E. 2009. Dancing Volvox: hydrodynamic bound states of swimming algae. Phys. Rev. Lett., 102: 1–4. DOI: 10.1103/PhysRevLett.102.168101