Distribution of the echiurid worm Bonellia viridis Rolando, 1822 along the Ligurian Coast, North-Western Mediterranean Sea

Bonellia viridis is a benthic echiurid commonly associated with Mediterranean rocky substrates, yet its distribution patterns and environmental drivers remain poorly understood. This study investigates its spatial patterns along the Ligurian coast (NW Mediterranean Sea) using standardised video ROV surveys conducted over the last decade (2015-2025) at multiple sites between 20 and 100 m depth, characterised by coralligenous and rocky reefs under diverse environmental conditions. Results revealed significant spatial variability, with average abundance increasing from east to west and a clear preference for depths less than 50 m. Multivariate analyses highlighted the key roles of substrate slope, bottom type and biocoverage in shaping the echiurid distribution patterns, while sediment cover showed no significant effect. A multiple linear regression confirmed that most environmental variables significantly contributed to species abundance. At the broad spatial scale, species abundance was negatively correlated with rainfall regime and positively correlated with the Population Equivalent (PE) of the coastal towns facing the sites where transects were conducted. This suggests that B. viridis may benefit from increased nutrient availability associated with urban runoff, untreated sewage, and harbour operations—common features along the highly urbanised western Ligurian coast. Seasonal comparisons indicated higher abundances in spring and autumn, likely driven by an enrichment in organic matter associated with increased primary production. These findings provide new insights into the ecological requirements of B. viridis and suggest its potential role as a bioindicator species for monitoring benthic habitat conditions in Mediterranean coastal ecosystems.