Xenoturbella japonica

Xenoturbella japonica is a marine benthic worm-like species that belongs to the genus Xenoturbella. It has been discovered in western Pacific Ocean by a group of Japanese scientists from the University of Tsukuba. The species was described in 2017 in a study published in the journal BMC Evolutionary Biology,^[1] and amended in 2018.^[2]

Xenoturbella japonica
X. japonica holotype female. The white arrowhead indicates the ring furrow.
Scientific classification
Domain:	Eukaryota
Kingdom:	Animalia
Phylum:	Xenacoelomorpha
Family:	Xenoturbellidae
Genus:	Xenoturbella
Species:	X. japonica
Binomial name
Xenoturbella japonica Nakano, Miyazawa, Maeno, Shiroishi, Kakui, Koyanagi, Kanda, Satoh, Omori & Kohtsuka, 2018

Longitudinal section of a congeneric species, Xenoturbella bocki

Xenoturbella japonica is known for lacking respiratory, circulatory and excretory systems.^[3][4][1]

Description

The etymology of the species name corresponds to the locality where the specimens were sampled.

Xenoturbella japonica is 5.3 cm (2.1 in) in length, with a pale orange colouration. The body wall displays ring and side furrows. The mouth is orientated ventrally, just anterior to the ring furrow. The live specimen exhibits a conspicuous ventral epidermal glandular network.^[1] Tissues contain exogenous DNA corresponding to bivalve mollusks, the vesicomyid Acila castrensis and Nucula nucleus.^[1]

Phylogeny

Comparison of mitochondrial DNA and protein sequences showed that the species Xenoturbella japonica is the sister group to X. bocki and X. hollandorum into a clade of 'shallow-water' taxa.

Species-level cladogram of the genus Xenoturbella.

Xenacoelomorpha     Acoelomorpha     Xenoturbella     'Shallow' clade               X. japonica             X. bocki   X. hollandorum   'Deep' clade               X. monstrosa             X. churro   X. profunda

The cladogram has been reconstructed from mitochondrial DNA and protein sequences.[5][1]

Other facts

Taxonomic Classification and Unique Morphology

Xenoturbella japonica belongs to the genus Xenoturbella, which has its traits as simple organisms with simple body structures and lack complex organs such as brain, lungs, gut or excretory organs. There has been a recent debate about in location in the phylogenetic tree. Recent research suggests it belongs to the clade Xenacoelomorpha having close evolutionary relationships with early bilaterians.^[6]

Evolutionary Significance in Understanding Bilaterian Origins

Xenoturbella japonica  stands out as a better path to understanding the evolution of the evolution of bilaterians, a group that includes most multicellular animals with bilateral symmetry.^[7]

Genetic Analysis and Ancestral Traits

Genetic analysis of Xenoturbella japonica reveal unique genetic relationship with the other Xenacoelomorpha species, indicating that it an early-diverging bilaterian. This is important for scientists and interested parties on clues about the earliest stages of animal evolution.^[1]