• Paracentrotus lividus
• alan
• artificial light
• light pollution
• sea urchin
• echinoderm
• inquinamento luminoso
• echinodermi

Artificial light at night (ALAN) is a recognized source of anthropogenic disturbance, with various and not entirely discovered effects on organisms and biological systems. To date, research on ALAN has shown that light pollution affects all levels of organization, from individual physiology and behavior to community structure and trophic interactions. Within marine ecosystems, coastal areas are the most impacted by ALAN, because of the exposition to outdoor lightings along streets and within harbours and related to recreational structures. This study is focused on the potential effect of ALAN on the Mediterranean sea urchin Paracentrotus lividus, also known as the purple sea urchin. Sea urchins have a crucial role in shaping benthic ecosystems, and this has been proven by many ecological studies along the Mediterranean coast. Paracentrotus lividus is widely distributed in the Mediterranean Sea, where it inhabits crevices of rocky shores and seagrass beds of the infralittoral zone down to 20m. Paracentrotus lividus is known to be a nocturnal organism; consequently, its locomotor and feeding activity occurs mostly after sunset. Daytime observations show that individuals of P. lividus are often covered with pieces of non-living material, such as small rocks, algal fragments or shells, a behavior that is thought to represent a protection from excessive light. In fact, its light-sensitivity is related to photosensitive cells distributed on the body surface, as well as the negatively phototactic tube feet. Within this context, our objective was to investigate if ALAN affects: (1) the nocturnal locomotory behavior and (2) the diet of P. lividus. A field experiment was conducted in Castiglioncello (Livorno, Italy), along an urban coastal area characterized by a rocky shore closed to a promenade and lit at night due to streetlamps and the presence of a restaurant. We chose four sites differing in light intensity, from completely dark (0 lux) to bright (18 lux) conditions. A total of 57 individuals at the ‘Dark’ site, 69 at the ‘Medium’ site and 69 at the ‘High’ site were tested between mid-July and late September 2021, during five nights characterized by absence of the moon (i.e., during new moon periods), clear sky and calm sea conditions. We video recorded the movement of sea urchins placed singularly in the middle of a transparent plastic arena filled with sea water, which was placed on the rocky shore near the sea urchins’ collection point. We used infrared cameras in order to record in the dark without any extra light source that could alter the behavior of the individuals. The time-lapse videos were examined to assess the movement performance using the following variables: latency of locomotion, mean speed, straightness-of path and direction. Furthermore, in order to analyse the diet of sea urchin, the carbon and nitrogen isotopic values of the sea urchin and its main food sources (Padina pavonica and its epibiota, Laurencia spp. and epilithon and its infauna) were estimated at each area, at two sampling dates (August and September). Data analysis suggested a potential impact of ALAN on locomotory behavior of sea urchins. Individuals of P. lividus tended to move away from the light sources, and in presence of ALAN their locomotory performance was characterized by increased latency time, increased straightness of path and decreased mean speed in comparison to sea urchins under dark conditions. Moreover, carbon and nitrogen stable isotopes analysis on samples collected at one of the two dates highlighted that niche partitioning varied depending on different degree of ALAN pollution, and that there was a lower degree of niche diversification at sites subjected to ALAN pollution. Results on diet composition are still incomplete and analyses on samples collected in the first date will hopefully clarify the trophic position of P. lividus in the study area.