Swimming speed of V. rousseletii spheroids after a dark-light switch, depending on the light intensity. (a) An example of the tracks of the spheroids in the observation chamber (vertical setup A, Figure 3a). The position of each spheroid was recorded every 0.1 s starting at 1 s before the dark-light switch. The color of the position marks changed every second as indicated. The yellow arrows indicate the light source (unidirectional illumination). (b) The relative swimming speed of spheroids was plotted against the time after the light was turned on (vertical setup A, Figure 3a). The swimming speed immediately before light stimulation was set to 100%. A total of eight different light intensities were analyzed separately, and the color assignment is indicated. In addition, the average values for all intensities are given (bold red line). Error bars refer to the standard deviation of the y-value (n = 20).