Movie 2.

Tethered larva performing its light-off response to sudden intensity dimming. The larvais held in seawater containing nanoparticle beads to allow measurement of flow speeds via PIV. The first 5 s of the video show the larva elongated with beating ectodermal cilia under bright white light. At 5 s, the bright light is dimmed leaving the larva in dim red illumination only, which prompts the response of contracting the body into a round ball, and later, all flow stops around the larva as a result of ciliary arrest. Playback is in real time.