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.
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Sequence data have been deposited in GenBank; accession numbers can be found in Supplementary Tables 3–5.
We thank the crew of the R/V Western Flyer and pilots of the ROVs Tiburon and Doc Ricketts for their skill and patience during hunts for these ‘purple socks’. We also thank S. Johnson for verifying bivalve sequences obtained from Xenoturbella and L. Lundsten for hunting through many video files for imagery. We acknowledge the Cyberinfrastructure for Phylogenetic Research (CIPRES) Science Gateway26 for computing resources, and thank M. Miller for additional resources, S. Mirarab for discussions on species tree methods and N. Holland for comments on the manuscript. This work was supported by the David and Lucile Packard Foundation via the Monterey Bay Aquarium Research Institute, Scripps Institution of Oceanography and the National Science Foundation Assembling the Tree of Life program (DEB1036368 to G.W.R.).
Extended data figures
ROV Tiburon dive 610, October 25, 2004. Collection of a X. monstrosa specimen (type) and images of other Xenoturbella in a field of vesicomyids (mainly Archivesica diagonalis in this clip).
Near Pinkie’s Vent at ~1,700 m depth, a cold-water seep in the Guaymas Transform Fault, Gulf of California, Mexico.
ROV Doc Ricketts dive 385, April 13, 2012. The Mexican paratype of X. monstrosa is shown initially followed by the holotype of X. churro and then the collection of both specimens.
Sediment near a hydrothermal vent at ~3,700 metres in the Pescadero Basin, Gulf of California, Mexico.
ROV Doc Ricketts dive 757, April 24, 2015. The video shows the female paratype of X. profunda gliding over the sediment in an S-pattern amongst polynoid scaleworms, sea anemones and vesicomyid clams. The video is speeded up to x6 times.
About this article
PIWI genes and piRNAs are ubiquitously expressed in mollusks and show patterns of lineage-specific adaptation
Communications Biology (2018)