Multi-scale environmental influence on the spatial and bathymetric distribution of the solitary ascidian Halocynthia papillosa (Linneaus, 1767) in the Ligurian Sea (NW Mediterranean)

Halocynthia papillosa is one of the most common ascidians inhabiting coralligenous and sciaphilic benthic communities on Mediterranean rocky bottoms. Despite its ecological relevance, its broad-scale distribution patterns and environmental drivers remain poorly understood. This study investigates the spatial and bathymetric distribution of H. papillosa along the Ligurian coast (NW Mediterranean Sea) using standardized ROV video surveys conducted between 2015 and 2025 in 37 sites located at 40–100 m depth. At the regional scale, species abundance exhibited a pronounced East–West gradient, increasing towards the western Ligurian sector. This pattern is consistent with the influence of the regional hydrodynamic system, including mesoscale circulation and upwelling processes, which enhance food availability and promote larval retention. At the same scale, abundance was negatively correlated with rainfall, suggesting that riverine inputs and associated turbidity may negatively affect feeding activity. Conversely, a strong positive correlation with Population Equivalent (PE) indicates that moderate anthropogenic organic enrichment may enhance trophic resources. At a more local scale, H. papillosa distribution was further influenced by depth, with the highest abundances occurring between 40 and 80 m, and by substrate type, with a strong preference for gently sloping surfaces and medium-to-high sediment cover. Overall, these results highlight a hierarchical, scale-dependent control by environmental drivers, in which regional oceanographic processes, climatic forcing, and local habitat features interact to shape the distribution of H. papillosa. This multi-scale framework provides new insights into the ecological responses of benthic suspension feeders to natural and anthropogenic drivers in Mediterranean coastal ecosystems.