The surface of a coral is rugged. Its hard skeleton is populated by polyps that stretch their tentacles into the surrounding water to filter out food. But how exactly does the water flow over the coral surface, what eddies and flows develop, and what does this mean for the oxygen supply around the coral and its associated algae? An international research team around Soeren Ahmerkamp from the Max Planck Institute for Marine Microbiology in Bremen, Klaus Koren from the Aarhus University in Denmark and Lars Behrendt from the Uppsala University and SciLifeLab in Sweden has developed a method that allows studying the flow and oxygen concentrations simultaneously at very small scales. Now it is possible to see how the corals generate a flow with their cilia, thus increasing oxygen transport.