| Xenacoelomorpha | |
|---|---|
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| Xenoturbella japonica, a xenacoelomorph member (xenoturbellids) | |
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| Proporus sp., another xenacoelomorph member (acoelomorphs) | |
Scientific classification | |
| Kingdom: | Animalia |
| Subkingdom: | Eumetazoa |
| Clade: | ParaHoxozoa |
| Clade: | Bilateria |
| Phylum: | Xenacoelomorpha Philippe et al. 2011[1] |
| Subdivisions | |
Current classification: Traditional classification: | |
Xenacoelomorpha (/ˌzɛnəˌsɛloʊˈmɔːrfə/) is a small phylum ofbilaterianinvertebrateanimals, consisting of twosister groups:xenoturbellids andacoelomorphs. This new phylum was named in February 2011 and suggested based on morphologicalsynapomorphies (physical appearances shared by the animals in theclade),[2] which was then confirmed byphylogenomic analyses of molecular data (similarities in the DNA of the animals within the clade).[3][4]
Prior to molecular studies, xenacoelomorphs were considered to beflatworms based on their superficial similarities. Like flatworms, they do not have a coelom and aredorsoventrally flattened.[5] With the advent ofphylogenetics,Xenoturbella and Acoelomorpha were found to be sister groups and only distantly related to flatworms.[4] Initially, thisphylum was considered to be a member of thedeuterostomes,[3] but because of recenttranscriptome analyses, it was concluded that the phylum Xenacoelomorpha is the sister group to theNephrozoa, which includes both theprotostomes and the deuterostomes, making the phylum thebasalmost bilaterian clade.[6][7] This would mean they are neither deuterostomes nor protostomes.
Their larvae show similarities withcnidarianplanula larvae and poriferan parenchyma larvae, but it is not clear if the similarities are ancestral or derived.[8]
However, some studies point out that their basal placement may be caused by highmutation rates leading tolong branch attraction (LBA). These analyses suggest that the xenacoelomorphs are instead the sister group ofAmbulacraria, forming the cladeXenambulacraria, and that despite their simple body plans, they actually derive from a more complex ancestor.[9][10] Having a larger number of species within this group would allow for better conclusions and analysis to be made within the phylum and in groups closely related to the phylum.
For multiple decades, the genusXenoturbella contained only one species,X. bocki. In 2016, however, a team reported the discovery of four new species from theGulf of California and sequenced each new species'mitogenome and, upon analysis, found that the two species that lived in shallow water (X. bocki andX. hollandorum) formed a "shallow" clade and that three deep water species formed a "deep" clade.[11] The following year, another team discovered a sixth species,X. japonica, found off the coast of Japan. Their phylogenetic analysis confirmed the first team's hypothesis and placedX. japonica within the shallow clade.[12]
The other two groups,Nemertodermatida andAcoela, have less clear relationships as species-level phylogenies have not been conducted. Nemertodermatida only has two families and six total genera.Ascopariidae contains two of these genera, whileNemertodermatidae has the other four. A 2016 study analyzed three of the four Nemertodermatid genera and found thatSterreria andMeara are closer to each other than toNemertoderma, whileNemertinoides was left unplaced.[6] Acoela phylogeny is even less certain, as it is by far the most diverse part of the phylum and is very understudied. A 2011 study attempted to solve this problem and recovered numerous traditional families aspolyphyletic. They also recovered a tentative clade of various species fromActinoposthiidae andIsodiametridae, which is not shown in the cladogram below.[13] Several small basal families were not included in their study, and their position is still uncertain.
| Xenacoelomorpha |
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The phylum consists of small, flat, and worm-like creatures found inmarine and sometimesbrackish waterenvironments, on thesediments. There are species that are variously free-living,parasitic, andsymbiotic. They can be found at depths of almost 4 km (2.5 mi) and nearhydrothermal vents.
The phylum ishermaphroditic (all individuals have both male and female sex organs) and reproducessexually withdirect development, meaning they skip a potentially vulnerablelarval stage.Xenoturbella has external fertilization, and Acoelomorpha has internal fertilization.[14][15][16] All xenacoelomorphs are bilateral, meaning they have a central front-to-back body axis with mirror-image right and left sides. They aretriploblasts (meaning they have the threegerm layers:ectoderm,endoderm, andmesoderm). Theirbody plan isacoelomate – they lack acoelom – do not have a true body cavity. Also, an excretory system is absent, yet all genes related to the excretory system are present except for Osr, which is essential for the development of such a system. In acoelomorphs, which have gone through rapid evolutionary rates and chromosomal rearrangements, about 60% of the genes shared between protostomes and deuterostomes are missing. How many of these genes, which are present or absent in Xenoturbella, will require whole-genome sequencing.[17]
While other animals that arediploblastic (only have two germ layers: ectoderm and endoderm) also lack a coelom, those technically do not have anacoelomate body plan because they lack the mesoderm germ layer. Inacoels, the mouth opens directly into a large endodermalsyncytium, while innemertodermatids and xenoturbellids, there is a sack-like gut lined by unciliated cells.[18]
A defining feature is a digestive system lacking nerve cells. Because anenteric nervous system, also called the stomatogastric nervous system, is also found in many cnidarians, its absence is most likely a derived trait.[19]
Their nervous systems arebasiepidermal – located right under theepidermis – and they have no brain. The xenoturbellids' nervous system consists of a simplenerve net, with no special concentration of neurons. In acoelomorphs, the nervous system is arranged in a series of longitudinal bundles, united in the anterior region by a ring comissure of variable complexity.[20]
The sensory organs include astatocyst (for balance). Some groups have two unicellularocelli (simple eyes).[18][20]
The epidermis of all species within the phylum is ciliated. Thecilia are composed of a set of nine pairs of peripheralmicrotubules and one or two central microtubules (patterns 9+1 and 9+2, respectively). The pairs 4–7 terminate before the tip, creating a structure called a "shelf".[21]
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