DOI: 10.5281/zenodo.14810574
title: A light-off response characterised by body contraction and ciliary arrest in Acropora coral larvae - Supplementary material

description: Video Captions

Video S1. Horizontal tracking of coral larva trajectories in shallow cuvette. The video is split into four panels to show each of the 90 s light-on and dark phases. Playback is sped up by 10x. Tracks are coloured by larva ID and persist for each light phase showing the total trajectory. Light-on periods are characterized by faster swimming, covering greater distances in the cuvette. During dark periods, the larvae often tended to spin on the spot.   

Video S2. Tethered larva performing its scotophobic (light-off) response to sudden light dimming. The larva is 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.

Video S3. Particle image velocimetry (PIV) analysis. Example of a tethered larva (shaded black) with flow velocity in the surrounding seawater represented by arrows and colour (scale on left). The white light stimulus is on when the video starts, but is dimmed 5 s later. All flow stops in the water surrounding the larva around 15 s after light dimming, suggesting ciliary arrest.

Video S4. Ectodermal ciliary arrest of larva #1 in response to white light dimming. The video begins with white light (WL) on, then at 33.4 s, the white light is dimmed and imaging continues using dim red illumination only (RL). An arrest of beating can be seen among the cilia in focus 4 s later (at 38 s). Video playback is in real time but it has been compressed (from 500 fps) to 25 fps.

Video S5. Ectodermal ciliary arrest of larva #2 in response to white light dimming. The video begins with white light (WL) on, then at 33 s, the white light is dimmed and imaging continues using dim red illumination only (RL). An arrest of beating can be seen spreading down through the cilia in focus, starting at 77 s (43 s after the light dimming). Video playback is in real time but it has been compressed (from 500 fps)  to 25 fps.

Video S6. Coral larvae contract the body in response to light dimming. Two examples of coral larvae are shown from cohorts from consecutive spawning years. The larvae are tethered on a suction micropipette as the light stimulus is turned on and off, twice. Video playback is 10x. Note the muscle contraction events in dark periods, gradually rounding the body shape. An elongated shape is resumed in bright light periods via muscle relaxation.

Video S7. 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 a light-off response and floated very slightly upward. Whereas, larvae higher in the column tended to continue active swimming as before.