Peculiar new-founded seafloor species sheds light on primitive animal evolution.

Xenoturbella Japonica is new species found by the Japanese (University of Tsukuba)

A new species of the unknowable marine worm Xenoturbella, scoured from the seafloor of the western pacific, plays a part to understanding of the early on evolution of the Bilateria.

Japanese researchers have discovered and revealed a new species of the mysterious marine worm Xenoturbella, which they have named Xenoturbella Japonica, as reported in a new study published in BMC Evolutionary Biology.

Xenoturbella lacks selected features common among more complex animals, such as a centralized nervous system, kidneys, and an anus which is its digestive system has simply a single opening. Thus, these primitive worms are valuable for understanding the origins of these structures. The classification of Xenoturbella in the tree of life has been controversial, but it is normally regarded as a basal associate or sister group of the Bilateria, a class that includes most complex animals.

The researchers, centred at the University of Tsukuba, described two specimens of X. Japonica dredged from the seafloor of the western Pacific a female about 5 cm in length, and a juvenile about 1 cm in length. Both specimens are pale orange in colour with an oval-shaped mouth a glandular system on the ventral (bottom) surface.

Micro CT scanning, a method not previously applied to Xenoturbella, revealed the specimens' internal structures, and a new attribute not visible using regular methods: the frontal pore.

"We furthermore extracted DNA and sequenced the mitochondrial genome and partial Histone H3 gene sequences", co-author Hideyuki Miyazawa explains. "Molecular phylogenetic analysis confirmed that X. Japonica is different from previously described species of Xenoturbella." DNA contamination from several species of bivalve was also detected, which indicates that, like other species of Xenoturbella, X. Japonica likely feeds on marine bivalves.

"Species within this genius have previously been divided into 'shallow' and 'deep' subgroups, and our results place X. Japanica in the 'shallow' subgroup, "lead author Hiroki Nakano says. "Interestingly, X. Japanica shares features with both subgroups, however. Thus, features of this species may be ancestral for this genus, and this new species may be particularly significant for unravelling the descent of Xenoturbella and the early history of the Bilateria."

Research on Xenoturbella has been limited by the inaccessibility of specimens in their seafloor habitats hundreds or thousands of meters below the surface. This new discovery may offer a solution. As co-author Hisanori Kohtsuka explains, "because one habitat was X. Japonica was found is easily reachable from a marine station, this new species promises to be valuable for potential research on bilaterian and deuterostome evolution."