Schematic representation of the phototactic movements in V. rousseletii. (a) Straight-ahead swimming in the dark. (b) A sudden dark-light switch causes the flagellar beating to reverse in the anterior hemisphere and the deceleration of the spheroid's forward movement (photophobic response). (c) After approximately 2 seconds, only cells on the illuminated side of the anterior hemisphere of the rotating spheroid show the reversed flagellar beating direction, resulting in an acceleration of the spheroid's forward movement and turning toward the light source. Gravity assists the phototactic movements because it pulls more on the posterior hemisphere due to an anisotropic mass distribution caused by the denser daughter spheroids within the posterior hemisphere and probably also by the closer spacing of the somatic cells in the posterior hemisphere. (d) Time scale.