title: New deep-sea species of Xenoturbella and the position of Xenacoelomorpha.
abstract: The discovery of four new Xenoturbella species from deep waters of the eastern Pacific Ocean is reported here. The genus and two nominal species were described from the west coast of Sweden, but their taxonomic placement remains unstable. Limited evidence placed Xenoturbella with molluscs, but the tissues can be contaminated with prey. They were then considered deuterostomes. Further taxon sampling and analysis have grouped Xenoturbella with acoelomorphs (=Xenacoelomorpha) as sister to all other Bilateria (=Nephrozoa), or placed Xenacoelomorpha inside Deuterostomia with Ambulacraria (Hemichordata + Echinodermata). Here we describe four new species of Xenoturbella and 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.
authors: Scripps Institution of Oceanography, University of California, San Diego, La Jolla, California 92037, USA.
authors: Greg W
authors: GW
authors: Rouse
authors: Scripps Institution of Oceanography, University of California, San Diego, La Jolla, California 92037, USA.|Western Australian Museum, Locked Bag 49, Welshpool DC, Western Australia 6986, Australia.|School of Animal Biology, University of Western Australia, Crawley, Western Australia 6009, Australia.
authors: Nerida G
authors: NG
authors: Wilson
authors: Scripps Institution of Oceanography, University of California, San Diego, La Jolla, California 92037, USA.
authors: Jose I
authors: JI
authors: Carvajal
authors: Monterey Bay Aquarium and Research Institute, Moss Landing, California 95039, USA.
authors: Robert C
authors: RC
authors: Vrijenhoek
mesh_terms: D000818:Animals; D059001:Aquatic Organisms / Q000145:classification* / Q000235:genetics; D001267:Atlantic Ocean; D001499:Bayes Theorem; D002140:California; D005260:Female; D005796:Genes; D054629:Genome, Mitochondrial / Q000235:genetics; D060086:Hydrothermal Vents; D016013:Likelihood Functions; D008297:Male; D008800:Mexico; D008954:Models, Biological; D010140:Pacific Ocean; D010802:Phylogeny; D013045:Species Specificity; D013548:Sweden; D059467:Transcriptome / Q000235:genetics