Bridging past and present: the Salento Peninsula as a case study for unveiling marine annelid diversity by integrating literature, field surveys, and DNA barcoding

Annelids represent one of the most important invertebrate groups in marine environments, yet their diversity and distribution are affected by knowledge gaps. In this study, marine annelids occurring along the Salento Peninsula were reviewed, based on 120 literature sources, 72 of which include georeferenced data, and new samples collected in the years 2022–2025 in several different environments. Aside from georeferenced occurrences and ecological traits, the data also included information on reproduction and development. In addition, barcoding data based on two mitochondrial markers (16S rDNA and COI) were obtained for a total of 208 nominal species. The evidence from literature and new data allowed the reporting of 715 species along the Salento Peninsula, 23 of which are potentially new to science, and 45 non-indigenous. The available data showed a density skewed towards the areas of Taranto and Porto Cesareo, while the remaining coast of Salento remains poorly explored. A similar unevenness accounts for taxonomic groups; while some families (such as Syllidae and Sabellidae) were overall well known according to the literature, some families (such as Scalibregmatidae) were historically almost completely unknown in the area. Barcoding data confirmed the distinctness of some of the potential new species, but revealed the occurrence of multiple mitochondrial lineages in 64 nominal species (32% of all assayed species), supporting the hypothesis of widespread occurrence of cryptic diversity in Mediterranean marine annelids and allowing us to identify possible case studies for future research. Three allegedly native species showed genetic matches with individuals sampled in other geographical areas, suggesting the occurrence of human-mediated translocation. Conversely, a minor portion of the species showed genetic homogeneity despite morphological differences. These results confirm the importance of the Salento Peninsula as a marine biodiversity hotspot and stress the need for further detailed research on marine annelids even in the relatively well-known Mediterranean Sea.