Volvox

Volvox is a genus of spherical colonial green algae whose flagella propel the colony through water. A colony hovering below a free surface exerts a net gravitational point force on the surrounding fluid.

Two bottom-heavy Volvox colonies hovering at equal depth below a stress-free surface are drawn together by the image flow of their downward Stokeslets. At large separation $x$, their relative speed scales as $-x^{-2}$ and their collision time scales as $x_0^3$.