Abstract
Flagella are crucial to the interactions of unicellular organisms with their surrounding aquatic environment. One ecologically important group of flagellates, the dinoflagellates, has a unique flagellar arrangement consisting of a trailing and a transversal flagellum. The latter is recessed within a groove around the cell and drives a hair‐bearing membrane that undulates with a helical beat. Dinoflagellates are further unique by having clearance rates that are an order of magnitude higher than those of other similarly sized phagotrophic flagellates, overlapping in size and swimming speed with ciliates. Here, using flow visualization and computational fluid dynamics, we show how this arrangement of just two flagella propels these large cells at high speeds and allows very high clearance rates. We find that the transverse flagellum provides most of the forward thrust, whereas the trailing flagellum is mainly for steering. The flagellar hairs and the sheet‐like structure of the transverse flagellum allow dinoflagellates to exert strong propulsive forces at high efficiency without extending a long flagellum far into the surrounding fluid. The unique flagellar arrangement of dinoflagellates may therefore be key to their evolutionary success.