I find the intersection of fluid dynamics and biology fascinating. How animals manipulate their surroundings to bring nutrients to them. And coral cilia is just one of these tricks!
This shot is an incredible close-up of fluid flow. The day-glo bunches*, those are coral polyps each only 1 mm across. The lines are the tracks from fluorescent beads that move with the water, illustrating how water flows. Away from the coral, at the upper part of the picture, the water is flowing nearly horizontally. But near the coral, these lines are pointed right into the surface of the coral indicating water is flowing right to the polyps! And the hair-like cilia on the surface of the coral is what causes water, and what ever particles and nutrients are flowing along with it, to be redirected towards the hungry polyps. Clearly, I need to grow me some cilia to maximize my consumption before I hit up the next all-you-can-eat buffet.
* Normally these aren’t day-glo, but the coral has a green fluorescent protein (GFP) that lights up when you hit it with the fluorescent light that is used to illuminate the water-tracking beads.