Movie 7.

Vertical tracking of larval trajectories in tall cuvette. Fading trajectory tails span 30 frames and are coloured according to mean track speed (blue represents slow travel, while red is fast). Video playback is sped up by 10x. A) A batch of deciliated and immobile larvae float upwards over 50 s after mixing to disperse them in the column. B) Larvae swimming actively with white light on. Magenta box in the second repeat of the video highlights the larvae probing the base of the cuvette. C) Same batch of larvae as in B, in darkness after the sudden dimming of white light. Yellow box in second video repeat highlights that most larvae near the bottom surface showed alight-off response and floated very slightly upward. Whereas larvae higher in the column tended to continue active swimming as before.