• Pachygrapsus marmoratus
• ALAN
• artificial light at night
• light pollution
• crabs

Artificial light at night (ALAN) is an expanding pollutant involving both terrestrial and aquatic environment. Since the mid-twentieth century, artificial light has begun to spread on a global scale, but only recently researchers have begun to focus on the impact that this type of pollution has on marine and terrestrial ecosystems.
As for the rocky coast habitat, which is the habitat on which we will focus on this thesis, we know that different taxa can be positively or negatively affected by artificial night light. We know that ALAN, for example, affects the proliferation of autotrophic species within the biofilm and that it affects larval recruitment. We don’t have much information about mobile organisms such as crabs, on which artificial light at night might exert effects through physiological and/or behavioural mechanisms.
To test the hypothesis of effects of artificial light on Pachygrapsus marmoratus (Fabricius, 1787) (Brachyura, Grapsidae) distribution, diet, sex, size and parasitization along the Tuscan coast, we conducted a pilot study during spring/summer in 2022.
We identified two natural rocky shores sites in Livorno (LI): Scogli dell’Accademia (SA) and Terrazza Mascagni (TM). In each site we selected two lit areas and two dark areas. The city lighting was characterized in lux using a LAN3.
Starting from May 2022, once a month, in absence of the moon, we evaluated P. marmoratus density along a 15 m transect in each area at both sites. In June, we also estimated the sex ratio, carapace size and percentage of parasitized individuals. P. marmoratus can in fact be infected by the parasitic castrator Sacculina carcini Thompson (Cirripedia: Rhizocephala).
P. marmoratus density was also estimated during two dates of full moon, but only in SA site.
Finally, we collected three male and three female individuals, for the stomach content analysis, in two dates of new moon and in two dates of full moon in SA site. The macrobenthic assemblage at the two sites was characterized by visual sampling (20x20 cm quadrats).
Data showed only weak effects of ALAN on P. marmoratos sex ratio (potentially resulting from a different parasitization rate in lit and dark areas), as well as on abundance of a few algal taxa. Further studies addressing a larger number of replicates for estimating crab density, as well as the use of manipulative approaches could potentially help revealing additional effects of ALAN on P. marmoratus.