Data from: New deep-sea species of Xenoturbella and the position of Xenacoelomorpha

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 Sweden1, 2, but their taxonomic
placement remains unstable3, 4. Limited evidence placed Xenoturbella with
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 grouped Xenoturbella with 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 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.