Field evidence for interactive effects of artificial light at night and moon phase on sea urchin Paracentrotus lividus locomotion

Artificial Light at Night (ALAN) disrupts natural light-dark cycles and alters biological processes tuned to lunar rhythms. We investigated the nocturnal locomotor behaviour of the sea urchin Paracentrotus lividus , a key grazer of the Mediterranean coasts, in relation to ALAN and the moon phase. We hypothesised different ALAN effects under new versus full moon conditions. Low ALAN levels during new moon may mimic full moonlight, whereas higher intensities may mask natural full moon cues inducing disorientation, and/or negative phototaxis independently of moon phase. Field observations were conducted under new and full moon phases across a gradient of ALAN intensities originating from street lamps. Sea urchin positions were marked at sunset, and their movement was followed every hour until midnight. A total of 180 tagged individuals were monitored during nine sampling campaigns. Three response variables were obtained: path speed, maximum displacement, and straightness. Our results showed that animals moved more rapidly and covered greater distances as ALAN intensity increased, but only during full moon nights. This response likely reflects light-escape behaviour and/or disorientation possibly caused by the additive intensity as well as the directional and spectral interference of ALAN with full moonlight cues. The differential behavioural response to ALAN among moon phases highlights the importance of deepening our understanding of moon – ALAN interactions when assessing ecological impacts and the need for targeted mitigation strategies to minimise the effects of artificial light in coastal ecosystems.