MAPPING POLYCHAETE DIVERSITY: Combining novel mega-barcoding and traditional morphology to resolve species diversity and distribution in the genus Terebellides (Annelida, Trichobranchidae) in Swedish waters

Sammanfattning

Recent genetic analyses have uncovered a hidden species complex within the genus Terebellides Sars, 1835 (Annelida, Trichobranchidae), revealing 27 North East Atlantic species where only seven were previously recognized. In the Skagerrak and Kattegat, ten species are present, eight of them are described. Because Terebellides is a dominant taxon along the Swedish West Coast, precise identification is essential for environmental monitoring, yet currently unattainable through morphology alone. This study utilized a novel molecular mega-barcoding approach to assess Terebellides diversity and distribution using 2022 and 2024 specimens from the Swedish national environmental monitoring program (PMK). Genomic DNA was extracted from 529 specimens, and a COI mini-barcode was amplified for dual-platform sequencing using Sanger and Oxford Nanopore Technologies (ONT) MinION. Taxonomic assignment was performed via phylogenetic reconstruction against reference clades established through prior multi-marker species delimitation. Morphological verification using light microscopy was conducted on undescribed lineages to identify diagnostic characters, including branchial shape and uncinus typology. ONT sequencing (313 bp COI fragment) significantly outperformed Sanger sequencing, yielding a 78% success rate compared to 36%. Integrated phylogenetic and BOLD database analyses identified six species: four described (Terebellides shetlandica Parapar, Moreira & O’Reilly, 2016, Terebellides europaea Lavesque, Hutchings, Daffe, Nygren & Londoño-Mesa, 2019, Terebellides williamsae Jirkov, 1989, and Terebellides lavesquei Barroso, Moreira, Capa, Nygren & Parapar, 2022) and two undescribed lineages (Clade 4 and Clade 12), with T. shetlandica dominating relative abundance (80%). T. shetlandica and T. europaea were the most widespread and northernmost taxa. T. shetlandica exhibited the broadest bathymetric range (12–279 m), while Clade 4 was restricted to the shallow Kattegat (12–19 m) and T. williamsae to deep offshore waters (179–295 m). Morphological characters were highly congruent with the molecular clades. This study establishes a robust framework for resolving the diversity and distribution of the Terebellides species complex, thereby significantly enhancing the accuracy of benthic biodiversity monitoring in Swedish waters.