Shining a light on jellyfish behaviour: Enhanced motor activity and positive phototaxis in Aurelia coerulea.

Jellyfish of the genus Aurelia possess a decentralized nervous system and rhopalia that contain basic visual structures, providing a unique opportunity to explore light-driven behavior in the absence of a centralized brain. Despite their anatomical simplicity, little is known about how these animals modulate their motor activity in response to light or whether they exhibit consistent preferences for light or dark environments. In this study, we conducted two behavioral experiments to investigate the light-dependent behaviour in juveniles of Aurelia coerulea. In Experiment 1, we measured pulsation frequency under light and dark conditions as a proxy for motor activity. In Experiment 2, we adapted a scototaxis-based test to assess phototactic preferences in an environment divided into illuminated and dark compartments. Our results show that jellyfish increased pulsation rates under light conditions and spent significantly more time in the illuminated area, suggesting a consistent positive response to light. These findings provide evidence of light-mediated behavior in Aurelia coerulea and contribute to a deeper understanding of how complex sensory-guided behaviors can emerge in organisms without centralized nervous systems.