From Inshore to Offshore: Rare Polychaete Species are Informative for Soft Bottom Ecology in the Southern Adriatic

Marine soft-bottom habitats are dynamic ecosystems that support biodiversity and key ecosystem functions. This study explores the polychaete assemblages’ structure and functional traits along a $\mathbf{1 0 0 ~ k m}$ inshore-offshore transect in the Gulf of Manfredonia (Southern Adriatic Sea), spanning depths from 3 to 423 meters. A total of 28 polychaete taxa were recorded at 31 sampling stations, most of which were low in abundance yet ecologically significant. Multivariate analyses revealed a clear shift in community composition between continental shelf and slope stations, mainly driven by depth gradients rather than sediment type. Eight trophic-functional groups were identified, reflecting a broad range of feeding strategies, motility levels, and substrate interactions. Despite taxonomic differences, the distribution of trophic-functional groups - encompassing a wide range of feeding strategies, mobility, and sediment interactions - remained broadly consistent, indicating functional redundancy. Diverse trophic-functional roles even among rare species suggest a complex and resilient benthic food web. These findings reinforce the ecological relevance of rare polychaete species and advocate for long-term monitoring programs to better capture the dynamics and role of rare benthic fauna. Ultimately, rare polychaetes emerge as sensitive ecological indicators, shedding light on the health of benthic habitats.