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Xenoturbella

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Xenoturbella
Xenoturbella japonica.The white arrowhead indicates the ring furrow.
Scientific classificationEdit this classification
Domain: Eukaryota
Kingdom: Animalia
Phylum: Xenacoelomorpha
Subphylum: Xenoturbellida
Bourlatet al.,2006
Family: Xenoturbellidae
Westblad, 1949
Genus: Xenoturbella
Westblad, 1949[1][2]
Species

See text

Xenoturbellais agenusof very simplebilateriansup to a few centimeters long. It contains a small number of marinebenthicworm-like species.[3]

The first known species (Xenoturbella bocki) was collected in 1878 and 1879 in the Gullmar fiord on the Swedish west coast by August Malm and is stored in the collection of the Gothenburg Natural History Museum.[4]A specimen is on display in the exhibition. It was collected again in the Gullmar fiord in 1915 by Sixten Bock, but it was only properly described in 1949 by Einar Westblad.[5]The type specimens are kept at the Swedish national museum of natural history in Stockholm.

Description

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Xenoturbella bockiilongitudinal section

Xenoturbellahas a very simple body plan. It consists of a dorsoventrally flattenedacoelomatebody, with a ventral furrow on each side running down from the anterior tip till they are stopped by an anterior circumferential furrow.[6][7]It shows two ciliatedepitheliallayers: an externalepidermisand an internalgastrodermislining the simple sac-likegut.The epidermis and gastrodermis is separated by a thick and multilayeredbasement membranecalled the "subepidermal membrane complex", a major part of theextracellular matrix.[8][9]The multiciliated epidermis displays unique interconnected ciliary rootlets and mode of withdrawal and resorption of worn epidermal cells.[6]The mouth is a mid-ventral pore leading to a gastral cavity, and there is noanus:[10][6]waste is dispelled through the same opening as food is taken in.[11]

Thenervous systemis composed by a net of interconnectedneuronsbeneath the epidermis, without any concentration of neurons forming ganglia or nerve cords.[12][13]

Species ofXenoturbellaalso lack arespiratory,circulatoryandexcretory system.In fact, there are no definedorgans,except for an anteriorstatocystcontaining flagellated cells and a frontal pore organ.[10][6]There are no organizedgonads,butgametesare produced. Adults producing sperm are very rarely observed, but eggs andembryosare known to occur in follicles.[14]

Research on the speciesXenoturbella bockihas 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 occurs inXenoturbella's closest relatives theacoelsandnemertodermatids.No examples of hermaphroditism was reported.[15][16]

Eggs ofXenoturbellaare 0.2 millimetres (0.0079 in) wide, pale orange and opaque.[17]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.[17]They are similar to the juveniles of acoelomateNeochildia fusca.[17]

Systematics

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Etymology

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The termXenoturbelladerives from theAncient Greekwordξένος(xénos), meaning "strange, unusual",[18][19]and from theLatinwordturbellameaning "stir, bustle".[20]This refers to the enigmatic, unusual taxonomic status of the animal, initially considered as related toturbellarians,a group of flatworms whose aquatic species stir microscopic particles close to their ciliated epidermis.[21]

Taxonomy

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Currently the genusXenoturbellacontains six recognized species:[22]

Phylogeny

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Among species

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To date, the genusXenoturbellais composed of six species distributed into a shallow-waterclade—three species up to 400–650 metres (1,310–2,130 ft)—and a deep-water clade—three species deeper than 1,700 metres (5,600 ft).

The two smaller species,X. bockiandX. hollandorum,which are up to 4 centimetres (1.6 in) long, are found in shallower waters less than 650 metres (2,130 ft) deep. They form a clade together with a third species,X. japonica,which is slightly over 5 centimetres (2.0 in) long and was found in waters less than 560 metres (1,840 ft) deep.[27]Three larger species,X. monstrosa,X. churro,andX. profunda,which were 10 centimetres (3.9 in) or greater long and lived in deeper waters 1,700–3,700 metres (5,600–12,100 ft), form another clade.[3]

Species-level cladogram of the genusXenoturbella.
Xenacoelomorpha
Thecladogramhas been reconstructed frommitochondrial DNAand protein sequences.[3][27]

Among animals

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The systematic and phylogenetic position ofXenoturbellaamong animals has been considered enigmatic since its discovery. An early DNA analysis suggested a close relationship tomolluscs,[29]but it was probably a result from contamination with DNA of molluscs thatXenoturbellaconsumes.[30]

A subsequent study suggested a placement of the genus in its own phylum,Xenoturbellida,as adeuterostomeclade andsister groupto theAmbulacraria.[31]The deuterostome affiliations were then recovered by studies that indicate abasalposition of this phylum within the deuterostomes[32][33]or in a sister group relationship with the Ambulacraria.[34]

However, morphological characters, such as the structure of epidermalcilia,suggested a close relationship withAcoelomorpha,another problematic group.[35]The study of the embryonic stages ofXenoturbellaalso showed that it is a direct developer without a feeding larval stage, and this developmental mode is similar to that of acoelomorphs.[17]Molecular studies based on the concatenation of hundreds of proteins revealed indeed a monophyletic group composed byXenoturbellaand Acoelomorpha.[36][34][37]This clade was namedXenacoelomorpha.[34]

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 placedXenoturbellaas the sister group of Acoelomorpha within Xenacoelomorpha, and placed Xenacoelomorpha as sister toNephrozoa(ProtostomiaplusDeuterostomia), and therefore the basalmost bilaterian phylum.[3][38]

References

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Further reading

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Wikispecieshas information related toXenoturbella.
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