Of basins, plains, and trenches: Systematics and distribution of Solenogastres (Mollusca, Aplacophora) in the Northwest Pacific

•First records of Solenogastres from the hadal zone (>6000 m).•First phylogeny of Northwest Pacific Solenogastres reveals conflict with traditional taxonomy.•Little faunal overlap between investigated regions and depth zones.•Bathymetric range of >6000 m in an Acanthomeniidae species (bathyal to hadal depths).•Molecular evidence for dispersal between landward and seaward side of the Kuril-Kamchatka Trench. Solenogastres (Mollusca, Aplacophora) form a common part of the benthic malacofauna in the deep sea, but their diversity in the Northwest Pacific is poorly understood. In this study, we analyze the systematics and distribution of Solenogastres sampled during the Kuril-Kamchatka Biodiversity Studies II expedition to the hadal Kuril-Kamchatka Trench, in the framework of previous expeditions to the shallow bathyal areas of the Japanese coast, the abyssal Kuril Basin in the Sea of Okhotsk, and the open abyssal plain of the Northwest Pacific. This unique dataset from adjacent regions extending from bathyal to hadal zones, enables us to study bathymetric and geographic distribution ranges in this neglected deep-sea clade for the first time. Our applied species delineation in these taxonomically challenging aplacophoran molluscs relies on an integrative approach, combining scleritome characters with molecular analyses. A first molecular phylogenetic study based on two mitochondrial markers reveals conflict with traditional systematics and suggests the need to revise at least two polyphyletic families and the polyphyletic order ‘Cavibelonia’. In total, 192 specimens could be grouped within 60 candidate species, which present a surprisingly rich abyssal fauna and includes the first representatives of Solenogastres at hadal depths. We found a high proportion of singletons and little faunal overlap between the investigated regions and depths, suggesting that the diversity of the abyssal and hadal Solenogastres is not entirely determined by source-sink dynamics from the bathyal slope. Rather, there is evidence for abyssal source populations, supporting the presence of endemic abyssal and potentially even hadal species. Our molecular data reject the Kuril-Kamchatka Trench as an insurmountable barrier of dispersal for Solenogastres, with one species present at both sides of the trench. Remarkably, one species from the Kuril-Kamchatka Trench shows a vertical distribution extending over more than 6,000 m. Overall, our study more than doubles the global number of solenog