Hadal Snailfishes (Teleostei: Liparidae) Extend Across Multiple Trenches: Molecular Insights and Implications for Taxonomic Nomenclature

The hadal zone, Earth's deepest oceanic region, is defined by distinct geological features and hosts a variety of endemic species, including the Liparidae Gill, 1861 (snailfishes). Ecological understanding of snailfishes dwelling at depths greater than 6000 m remains limited due to challenges in physical specimen collection and preservation. This study employs molecular tools to assess the phylogenetic relationships and distribution patterns of hadal snailfishes by analyzing three mitochondrial DNA markers (<i>16S, Cyt-B, COI</i>) and incorporates 20 new specimens from the Japan and Tonga trenches (Pacific Ocean) and the Diamantina Fracture Zone (Indian Ocean). The phylogenetic hypotheses and species delimitation assessments were tested among a framework of six taxonomic units -<i>Pseudoliparis swirei</i> Gerringer and Linley, 2017, <i>Pseudoliparis belyaevi</i> Andriashev and Pitruk, 1993, <i>Notoliparis kermadecensis</i> Nielsen, 1964, <i>Notoliparis stewarti</i> Stein, 2016, and <i>Paraliparis selti</i> Linley, Gerringer, and Canto-Hernández, 2022. The results revealed wider geographic distributions than previously thought, particularly for <i>Notoliparis</i> c.f. <i>stewarti</i>. Further, the molecular data support the hypothesis that <i>Notoliparis</i> Andriashev, 1975 should be treated as a subjective junior synonym of <i>Pseudoliparis</i> Andriashev, 1955. Our findings do emphasize the challenges and limitations of using DNA barcoding solely to distinguish closely related or recently diverged species. Together, this study advances the biogeographic understanding of hadal snailfishes and highlights the importance of expanding sampling efforts.