New deep-sea species ofXenoturbellaand the position of Xenacoelomorpha

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The discovery of four newXenoturbellaspecies from deep waters of the eastern Pacific Ocean is reported here. The genus and two nominal species were described from the west coast of Sweden1,2, but their taxonomic placement remains unstable3,4. Limited evidence placedXenoturbellawith molluscs5,6, but the tissues can be contaminated with prey7,8. They were then considered deuterostomes9,10,11,12,13. Further taxon sampling and analysis have groupedXenoturbellawith acoelomorphs (=Xenacoelomorpha) as sister to all other Bilateria (=Nephrozoa)14,15, or placed Xenacoelomorpha inside Deuterostomia with Ambulacraria (Hemichordata + Echinodermata)16. Here we describe four new species ofXenoturbellaand reassess those hypotheses. A large species (>20 cm long) was found at cold-water hydrocarbon seeps at 2,890 m depth in Monterey Canyon and at 1,722 m in the Gulf of California (Mexico). A second large species (˜10 cm long) also occurred at 1,722 m in the Gulf of California. The third large species (˜15 cm long) was found at ˜3,700 m depth near a newly discovered carbonate-hosted hydrothermal vent in the Gulf of California. Finally, a small species (˜2.5 cm long), found near a whale carcass at 631 m depth in Monterey Submarine Canyon (California), resembles the two nominal species from Sweden. Analysis of whole mitochondrial genomes places the three larger species as a sister clade to the smaller Atlantic and Pacific species. Phylogenomic analyses of transcriptomic sequences support placement of Xenacoelomorpha as sister to Nephrozoa or Protostomia.

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