Xenoturbella is a
genus of very simple
bilaterians up to a few centimeters long. It contains a small number of marine
benthicworm-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
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 till they are stopped by an anterior circumferential furrow.[6][7] It shows two ciliated
epithelial layers: an external
epidermis and an internal
gastrodermis lining the simple sac-like
gut. The epidermis and gastrodermis is separated by a thick and multilayered
basement membrane called the "subepidermal membrane complex", a major part of the
extracellular 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 no
anus:[10][6] waste is dispelled through the same opening as food is taken in.[11]
The
nervous system is composed by a net of interconnected
neurons beneath the epidermis, without any concentration of neurons forming ganglia or nerve cords.[12][13]
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.[10][6] 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.[14]
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 occurs in Xenoturbella's closest relatives the
acoels and
nemertodermatids. No examples of hermaphroditism was reported.[15][16]
Eggs of Xenoturbella are 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 acoelomate Neochildia fusca.[17]
Systematics
Etymology
The term Xenoturbella derives from the
Ancient Greek word ξένος (xénos), meaning "strange, unusual",[18][19] and from the
Latin word turbella meaning "stir, bustle".[20] 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.[21]
Taxonomy
Currently the genus Xenoturbella contains six recognized species:[22]
To date, the genus Xenoturbella is composed of six species distributed into a shallow-water
clade—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. bocki and X. 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, and X. 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 genus Xenoturbella.
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,[29] but it was probably a result from contamination with DNA of molluscs that Xenoturbella consumes.[30]
A subsequent study suggested a placement of the genus in its own phylum, Xenoturbellida, as a
deuterostome clade and
sister group to the
Ambulacraria.[31] The deuterostome affiliations were then recovered by studies that indicate a
basal position 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 epidermal
cilia, suggested a close relationship with
Acoelomorpha, another problematic group.[35] 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.[17] Molecular studies based on the concatenation of hundreds of proteins revealed indeed a monophyletic group composed by Xenoturbella and Acoelomorpha.[36][34][37] This clade was named
Xenacoelomorpha.[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 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.[3][38]
^Raikova, O. I.; Reuter, M.; Jondelius, U.; Gustafsson, M. K. S. (2000). "An immunocytochemical and ultrastructural study of the nervous and muscular systems of Xenoturbella westbladi (Bilateria inc. sed.)". Zoomorphology. 120 (2): 107–118.
doi:
10.1007/s004350000028.
S2CID19668017.
^Lundin, K (1998). "The epidermal ciliary rootlets of Xenoturbella bocki (Xenoturbellida) revisited: new support for a possible kinship with the Acoelomorpha (Platyhelminthes)". Zoologica Scripta. 27 (3): 263–270.
doi:
10.1111/j.1463-6409.1998.tb00440.x.
S2CID85324766.
^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.
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