Global-scale biogeography of the pelagic polychaete family Tomopteridae (Annelida) revealed by DNA barcoding and genome-wide single nucleotide polymorphisms

Zooplankton are essential members of marine ecosystems that respond sensitively to environmental changes. While the distribution patterns of zooplankton have been extensively investigated in the global ocean, pelagic polychaetes, particularly the Tomopteridae family, are underexplored in diversity and biogeography, due to their fragile bodies and subtle morphological traits. In the current study, mitochondrial cytochrome c oxidase subunit I (mtCOI), nuclear 28S rRNA, and genome-wide single-nucleotide polymorphisms (SNPs) were analyzed to characterize the diversity and distribution of tomopterid species globally. Phylogenetic analyses of mtCOI were conducted for 603 specimens collected from 54 stations across the global ocean. Concordance was detected between 14 mitochondrial lineages and their genome-wide SNPs, while species-level operational taxonomic units (OTUs) were defined based on mtCOI sequence similarity (≥2.4%) supported by genome-wide SNPs. The comprising the global dataset contained 35 OTUs and 17 singletons, representing at least 52 tomopterid species. The number of species we assumed in this study exceeds previous morphological reports from certain ocean regions, including the eastern and western North Pacific Ocean (18 OTUs-11 species and 9 OTUs-7 species, respectively), suggesting the existence of previously unrecognized species. No specimens in this study were classified to species by morphotaxonomy. Tomopterids exhibited distribution patterns that corresponding to the general biogeography of other zooplankton, including subarctic/boreal, subarctic/bipolar, temperate–subtropical/circumglobal, temperate-subtropical/Indo-Pacific, equatorial, and upwelling region distributions. As this is the first species-level study of the global-scale biogeography of Tomopteridae, the findings provide fundamental information for future ecological and evolutionary studies on tomopterids.