Xenoturbella

''Xenoturbella'' is a genus of very simple bilaterians up to a few centimeters long. It contains a small number of marine benthic worm-like species.

The first known species ('' Xenoturbella bocki'') was discovered in 1915 by Sixten Bock, but it was only properly described in 1949 by Einar Westblad.

Description

''Xenoturbella'' has a very simple body plan. It consists of dorsoventrally flattened acoelomate animals, with an anterior circumferential furrow. It shows two ciliated epithelial layers: an external epidermis and an internal gastrodermis lining the simple sac-like gut. The multiciliated epiderm displays unique interconnected ciliary rootlets and mode of withdrawal and resorption of worn epidermal cells. The mouth is a mid-ventral pore leading to a gastral cavity, and there is no anus: waste is dispelled through the same opening as food is taken in.

The nervous system is composed by a net of interconnected neurons beneath the epidermis, without any concentration of neurons forming ganglia or nerve cords.

Species of ''Xenoturbella'' also lack a respiratory, circulatory and excretory system. In fact, there are no defined organs, except for an anterior statocyst containing flagellated cells and a frontal pore organ. There are no organized gonads, but gametes are produced. Adults producing sperm are very rarely observed, but eggs and embryos are known to occur in follicles.

Eggs of ''Xenoturbella'' are wide, pale orange and opaque. Newly hatched embryos are free-swimming (tending to stay close to water surface) and ciliated. They feature no mouth and they do not apparently feed. They are similar to the juveniles of acoelomate ''Neochildia fusca''.

Systematics

Etymology

The term ''Xenoturbella'' derives from the Ancient Greek word (), meaning "strange, unusual", and from the Latin word ''turbella'' meaning "stir, bustle". This refers to the enigmatic, unusual taxonomic status of the animal, initially considered as related to turbellarians, a group of flatworms whose aquatic species stir microscopic particles close to their ciliated epidermis.

Taxonomy

Currently the genus ''Xenoturbella'' contains 6 recognized species:
* '' Xenoturbella bocki'' Westblad, 1949 'Xenoturbella westbladi'' Israelsson, 1999* '' Xenoturbella churro'' Rouse, Wilson, Carvajal & Vrijenhoek, 2016
* '' Xenoturbella hollandorum'' Rouse, Wilson, Carvajal & Vrijenhoek, 2016
* '' Xenoturbella japonica'' Nakano, 2017
* '' Xenoturbella monstrosa'' Rouse, Wilson, Carvajal & Vrijenhoek, 2016
* '' Xenoturbella profunda'' Rouse, Wilson, Carvajal & Vrijenhoek, 2016

Phylogeny

Among species

To date, the genus ''Xenoturbella'' is composed of six species distributed into a shallow-water clade — three species up to — and a deep-water clade — three species deeper than .

The two smaller species, ''X. bocki'' and ''X. hollandorum'', which are up to long, are found in shallower waters less than deep. They form a clade together with a third species, ''X. japonica'', which is slightly over long and was found in waters less than deep. Three larger species, ''X. monstrosa'', ''X. churro'', and ''X. profunda'', which were or greater long and lived in deeper waters , form another clade.

Among animals

The systematic and phylogenetic position of ''Xenoturbella'' among animals has been considered enigmatic since its discovery. An early DNA analysis suggested a close relationship to molluscs, but it was probably a result from contamination with DNA of molluscs that ''Xenoturbella'' consumes.

A subsequent study suggested a placement of the genus in its own phylum, Xenoturbellida, as a deuterostome clade and sister group to the Ambulacraria. The deuterostome affiliations were then recovered by studies that indicate a basal position of this phylum within the deuterostomes or in a sister group relationship with the Ambulacraria.

However, morphological characters, such as the structure of epidermal cilia, suggested a close relationship with Acoelomorpha, another problematic group. The study of the embryonic stages of ''Xenoturbella'' also showed that it is a direct developer without a feeding larval stage, and this developmental mode is similar to that of acoelomorphs. Molecular studies based on the concatenation of hundreds of proteins revealed indeed a monophyletic group composed by ''Xenoturbella'' and Acoelomorpha.Hejnol, A., Obst, M., Stamatakis, A., Ott, M., Rouse, G. W., Edgecombe, G. D., et al. (2009). Assessing the root of bilaterian animals with scalable phylogenomic methods. Proceedings of the Royal Society, Series B, 276, 4261–4270. This clade was named Xenacoelomorpha.

The monophyly of Xenacoelomorpha soon became established, but its position as either a basal bilaterian clade or a deuterostome remained unresolved until 2016 when two new studies, with increased gene and taxon sampling, again placed ''Xenoturbella'' as the sister group of Acoelomorpha within Xenacoelomorpha, and placed Xenacoelomorpha as sister to Nephrozoa (Protostomia plus Deuterostomia), and therefore the basalmost bilaterian phylum.

References

Further reading

* G. Haszprunar, R.M. Rieger, P. Schuchert (1991). "Extant 'Problematica' within or near the Metazoa." In: Simonetta, A.M. & Conway Morris, S. (eds.): ''The Early Evolution of Metazoa and the Significance of Problematic Taxa''. Oxford Univ. Press, Cambridge. pp. 99–105
*

External links

A PCR Survey of ''Xenoturbella bocki'' Hox GenesMovie of adult ''Xenoturbella bocki''

{{Taxonbar, from=Q310104

Bilaterian genera
Xenacoelomorpha