Diversity and biogeography of mesophotic and bathyal Onuphidae (Annelida: Eunicida) of the Western Mediterranean sea

The Mediterranean Sea is extensively studied from a biological point of view and known for its high diversity, however benthic macro- and meiofauna of its deep ecosystems remain largely unexplored. Polychaetes are among the most abundant deep-sea benthic invertebrates, yet they are poorly understood in terms of taxonomy and biogeography. The Onuphidae family is known for its diversity in deep-sea environments, but most European studies focus on shallow-water species, with no molecular data for specimens collected at depths below 100 m in the Mediterranean. In this study, 88 specimens from 10 morphotypes were morphologically characterised. Samples came from Canyon Dohrn, Salerno bathyal plain, and Palinuro and Marsili seamounts of the Western Mediterranean Sea, as well as from the historical collections of the Natural History Museum of the University of Pisa. Phylogenetic reconstruction was performed using concatenated markers 16S rDNA, 18S rDNA and, separately, cytochrome c oxidase subunit I. This study provides the first thorough molecular phylogeny for Onuphidae in the deep Mediterranean. The combined morphological and molecular data revealed three potentially endemic Nothria species: Nothria sp. 1 (over 600 m depth off Corsica and in the Northern Tyrrhenian Sea), Nothria sp. 2 (35–90 m), and Nothria cf. jourdei (80–230 m). This study also identified Paradiopatra species with different biogeographical affinities: P. bihanica (Senegalese) and P. fiordica (Lusitanian) at different depths. Our study reveals that the overlap of Northern and Southern species ranges in shallow Mediterranean waters, known as the "biodiversity pump" effect, may also influence the deep Mediterranean biota, though further research across more invertebrate groups is needed. The deep Mediterranean polychaete fauna is likely influenced by the neighbour biogeographic districts, including Senegalese and Lusitanian, but it also includes a significant endemic component reflecting the basin's unique environmental conditions. This complex biogeographic composition of the deep-sea Mediterranean fauna should be considered when planning conservation and management of these unique and diverse ecosystems.