Hidden diversity and barcode gaps in Balearic annelid polychaetes (Western Mediterranean) revealed by an integrative checklist

Polychaetes are among the most diverse and abundant components of marine benthic communities, yet regional inventories remain incomplete or outdated in many Mediterranean areas. Here we present the first integrated checklist of polychaetes (Annelida) from the Balearic Islands (Western Mediterranean), combining historical and recent literature, public biodiversity databases (GBIF, OBIS), and newly examined specimens from independent surveys. Our curated dataset comprises 8193 georeferenced and taxonomically standardized records, documenting 624 species across 56 families. This inventory substantially exceeds previous regional estimates and expands the Spanish national register in 115 species. Newly examined material added 110 species and 701 records, increasing known regional richness by 23% relative to literature- and database-derived records. Bioregionalization based on species presence–absence data, in a matrix of 600 km ² cells, identified 13 spatially distinct assemblages, with the richest concentrated on the Balearic shelf and compositionally unique assemblages in deeper offshore habitats. Species accumulation curve did not reach an asymptote, indicating that polychaete diversity in the archipelago remains underestimated despite considerable sampling efforts. To strengthen molecular resources for the region, we generated new COI and 28S rRNA barcodes and assessed the representation of Balearic polychaetes in GenBank and BOLD DNA sequence repositories. This analysis revealed substantial gaps in DNA barcode coverage, representing an opportunity to increase public reference databases and therefore to aid in conservation and taxonomic identification efforts. Together, these results provide the first taxonomically standardized baseline for future biodiversity monitoring, systematic, and biogeographic studies, and highlight priorities for expanding DNA barcode reference libraries to support taxonomic research, biodiversity assessments, and conservation initiatives in Mediterranean polychaete research.