A colony of Volvox alga swimming up against a glass coverslip, viewed from above. The fluid streamlines shown illustrate the flow driven by the flagella that cover the surface of the colony.

From the article:

Dancing Volvox: Hydrodynamic Bound States of Swimming Algae
Knut Drescher, Kyriacos C. Leptos, Idan Tuval, Takuji Ishikawa, Timothy J. Pedley, and Raymond E. Goldstein
Phys. Rev. Lett. 102, 168101 (2009)

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