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

Planula larvae of the reef-building coral Acropora millepora exhibit whole-body contraction and ciliary arrest in response to sudden light dimming. Quantitative behavioural assays revealed reduced swimming speed and changes in vertical and horizontal movement under alternating light and dark conditions. High-speed microscopy showed that ciliary activity ceases upon dimming, while the typically elongated body contracts into a rounded form, altering locomotion. This response reveals a coordinated motor output in a diffuse, non-centralised nervous system, consistent with neuropeptidergic signalling. We propose that it may contribute to regulating larval positioning within the structurally complex and heterogeneous light environment of reef habitats, a process critical for coral recruitment and survival.