Kinematics of flagellar swimming in Euglena gracilis: Helical trajectories and flagellar shapes

The flagellar swimming of euglenids, which are propelled by a single anterior flagellum, is characterized by a generalized helical motion. The three-dimensional nature of this swimming motion, which lacks some of the symmetries enjoyed by more common model systems, and the complex flagellar beating shapes that power it, make its quantitative description challenging. In this work, we provide the first quantitative, three-dimensional, highly-resolved reconstruction of the swimming trajectories and flagellar shapes of specimens of Euglena gracilis. We achieved this task by using high-speed twodimensional image recordings taken with a conventional inverted microscope, combined with a precise characterization of the helical motion of the cell body to lift the 2D data to 3D trajectories. The propulsion mechanism is discussed. Our results constitute a basis for future biophysical research on a relatively unexplored type of eukaryotic flagellar movement.