A new Xenoturbellid: Xenoturbella monstrosa

A pink flatworm-like animal known by a two species found in waters off Sweden has puzzled biologists for nearly six decades. The animal's position on the tree of life shifted ever since the first species, named Xenoturbella bocki, was found in 1950. It was classified as a flatworm, then, in the 1990s as a simplified mollusk. In recent years, Xenoturbella has been regarded as either close to vertebrates and echinoderms, or as a more distant relative on its own branch further away. Knowing where Xenoturbella belongs is important to understand the evolution of organ systems, such as guts, brains and kidneys, in animals.

Xenoturbella has a very simple body plan: it has no brain, no through gut, no excretory system, no organized gonads, or any other defined organs. 

The new species, Xenoturbella monstrosa, was found in Monterey Bay and the Gulf of California, and measured 20-centimeters. It was named for its extraordinary size (20 cm).

For the experts: 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.