A light-off response characterised by body contraction and ciliary arrest in Acropora coral larvae

Planulae of the reef-building coral Acropora millepora exhibit whole-body contraction and ciliary arrest after exposure to sudden light dimming. Quantitative behavioural assays confirmed reductions in swimming speed and alterations in vertical and horizontal movement under alternating light and dark conditions. High-speed microscopy revealed that ciliary activity ceases upon dimming, while the typically elongated body contracts into a rounded form, altering swimming behaviour. These larval responses suggest an adaptive mechanism to regulate positioning within the highly structured and complex light environment of reef habitats, critical for coral recruitment and survival.