• light pollution
• ALAN
• sea urchin
• moon phase

Artificial Light At Night (ALAN) is a globally widespread form of pollution, strictly related to urbanization, and strongly affecting coastal areas. Marine organisms exposed to ALAN are facing an alteration of the natural light/dark cycle. Along Mediterranean coasts, one of the key ecological species is Paracentrotus lividus (Lamarck, 1816), a nocturnal sea urchin that shows the ability to perceive light. In this study we aim to analyse: if the nocturnal activity of P. lividus changes between moon phases; if and how these differences are affected by ALAN. Replicate marked individuals exposed to different ALAN intensities were followed during both full and new moon nights, in two different sites along the rocky coast south of Livorno (Italy). Distance travelled and direction from the north of each observed movement were measured. According to results, sea urchins move for longer distances but later during new moon nights. ALAN affects their activity pattern only during full moon nights, with distance travelled increasing at greater light intensities. This may be due to a light-adapted behaviour or to the achievement of sufficiently bright conditions that allow sea urchins to “visually” exclude predators. Understanding how ALAN impacts costal organisms’ activities is extremely important for predicting potential changes in community structure, also to shed light on potential synergistic effects with other urban-related stressors.