Xenoturbella - WikiHQ

Xenoturbella is a genus of very simple bilaterians whose relation to other animal groups is unclear. It contains a small number of marine benthic species that look like worms. They can be up to long.

History

The first known species (Xenoturbella bocki) was collected in 1878 and 1879 from the Gullmar fjord on the Swedish west coast by August Malm and is stored in the collection of the Gothenburg Natural History Museum. A specimen is on display in the exhibition. It was collected again from the Gullmar fjord in 1915 by Sixten Bock, but it was only properly described in 1949 by Einar Westblad. These specimens are kept at the Swedish Museum of Natural History in Stockholm.

Description

thumb|Xenoturbella bockii longitudinal section

Xenoturbella has a very simple body plan. It consists of a dorsoventrally flattened acoelomate body, with a ventral furrow on each side running down from the anterior tip until it is stopped by an anterior circumferential furrow. It shows two ciliated epithelial layers: an external epidermis and an internal gastrodermis lining the simple sac-like gut. The epidermis and gastrodermis are separated by a thick and multilayered basement membrane, the "subepidermal membrane complex", a major part of the extracellular matrix. The multiciliate epidermis displays unique interconnected ciliary rootlets and a mode of withdrawal and resorption of worn epidermal cells.

The nervous system is composed of 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.

Research on the species Xenoturbella bocki has shown it to have external fertilization, with eggs and sperm being released from new openings in the body wall. Gametes released into the water through ruptures also occur in Xenoturbella<nowiki/>'s closest relatives, the acoels and nemertodermatids. No examples of hermaphroditism were reported.

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

Systematics

Etymology

The term Xenoturbella derives from the Ancient Greek word (), meaning "strange, unusual", and from the Latin word 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 six 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 two clades by their environment's depth: three species which reside at depths up to compose a shallow-water clade; and three species which reside deeper than compose a deep-water clade.

The two smallest 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. but it was probably a result of 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. This clade was named Xenacoelomorpha.

References

External links

A PCR Survey of Xenoturbella bocki Hox Genes
Movie of adult Xenoturbella bocki