Xenoturbella

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Xenoturbella
Xenoturbella japonica.jpg
Xenoturbella japonica . The white arrowhead indicates the ring furrow.
Scientific classification OOjs UI icon edit-ltr.svg
Domain: Eukaryota
Kingdom: Animalia
Phylum: Xenacoelomorpha
Subphylum: Xenoturbellida
Bourlat et al., 2006
Family: Xenoturbellidae
Westblad, 1949
Genus: Xenoturbella
Westblad, 1949 [1] [2]
Species

See text

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. [3]

Contents

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 Museum of Natural History in Stockholm.

Description

Xenoturbella bockii longitudinal section Xenoturbella bockii longitudinal section English.svg
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 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]

Phylogeny

Among species

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.
   Xenacoelomorpha   

  Acoelomorpha  

   Xenoturbella   
  'Shallow' clade  
         

  X. japonica

         

  X. bocki

  X. hollandorum

  'Deep' clade  
         

  X. monstrosa

         

  X. churro

  X. profunda

The cladogram has been reconstructed from mitochondrial DNA and protein sequences. [3] [27]

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, [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]

Related Research Articles

<span class="mw-page-title-main">Chordate</span> Phylum of animals having a dorsal nerve cord

A chordate is a deuterostomal bilaterian animal belonging to the phylum Chordata. All chordates possess, at some point during their larval or adult stages, five distinctive physical characteristics (synapomorphies) that distinguish them from other taxa. These five synapomorphies are a notochord, a hollow dorsal nerve cord, an endostyle or thyroid, pharyngeal slits, and a post-anal tail.

<span class="mw-page-title-main">Flatworm</span> Phylum of soft-bodied invertebrates

Platyhelminthes is a phylum of relatively simple bilaterian, unsegmented, soft-bodied invertebrates commonly called flatworms or flat worms. Being acoelomates, and having no specialised circulatory and respiratory organs, they are restricted to having flattened shapes that allow oxygen and nutrients to pass through their bodies by diffusion. The digestive cavity has only one opening for both ingestion and egestion ; as a result, the food can not be processed continuously.

<span class="mw-page-title-main">Bilateria</span> Animals with embryonic bilateral symmetry

Bilateria is a large clade or infrakingdom of animals called bilaterians, characterized by bilateral symmetry during embryonic development. This means their body plans are laid around a longitudinal axis with a front and a rear end, as well as a left–right–symmetrical belly (ventral) and back (dorsal) surface. Nearly all bilaterians maintain a bilaterally symmetrical body as adults; the most notable exception is the echinoderms, which extend to pentaradial symmetry as adults, but are only bilaterally symmetrical as an embryo. Cephalization is also a characteristic feature among most bilaterians, where the special sense organs and central nerve ganglia become concentrated at the front/rostral end.

<span class="mw-page-title-main">Coelom</span> The main body cavity in many animals

The coelom is the main body cavity in many animals and is positioned inside the body to surround and contain the digestive tract and other organs. In some animals, it is lined with mesothelium. In other animals, such as molluscs, it remains undifferentiated. In the past, and for practical purposes, coelom characteristics have been used to classify bilaterian animal phyla into informal groups.

<span class="mw-page-title-main">Acoelomorpha</span> Phylum of marine, flatworm-like animals

Acoelomorpha is a subphylum of very simple and small soft-bodied animals with planula-like features which live in marine or brackish waters. They usually live between grains of sediment, swimming as plankton, or crawling on other organisms, such as algae and corals. With the exception of two acoel freshwater species, all known acoelomorphs are marine.

<span class="mw-page-title-main">Acoela</span> Order of flatworm-like bilaterian animals

Acoela, or the acoels, is an order of small and simple invertebrates in the subphylum Acoelomorpha of phylum Xenacoelomorpha, a deep branching bilaterian group of animals, which resemble flatworms. Historically they were treated as an order of turbellarian flatworms. About 400 species are known, but probably many more not yet described.

<span class="mw-page-title-main">Animal</span> Kingdom of living things

Animals are multicellular, eukaryotic organisms in the biological kingdom Animalia. With few exceptions, animals consume organic material, breathe oxygen, have myocytes and are able to move, can reproduce sexually, and grow from a hollow sphere of cells, the blastula, during embryonic development. Animals form a clade, meaning that they arose from a single common ancestor. Over 1.5 million living animal species have been described, of which around 1.05 million are insects, over 85,000 are molluscs, and around 65,000 are vertebrates. It has been estimated there are as many as 7.77 million animal species on Earth. Animal body lengths range from 8.5 μm (0.00033 in) to 33.6 m (110 ft). They have complex ecologies and interactions with each other and their environments, forming intricate food webs. The scientific study of animals is known as zoology, and the study of animal behaviour is known as ethology.

<span class="mw-page-title-main">Ambulacraria</span> Clade of deuterostomes containing echinoderms and hemichordates

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<span class="mw-page-title-main">Deuterostome</span> Superphylum of bilateral animals

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<span class="mw-page-title-main">Protostome</span> Clade of animals whose mouth develops before the anus

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<span class="mw-page-title-main">Embryological origins of the mouth and anus</span> Important characteristic for separating animals into protostomes and deuterostomes

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<span class="mw-page-title-main">Nephrozoa</span> Proposed clade of animals

Nephrozoa is a proposed major clade of bilaterian animals. It includes all bilaterians other than Xenacoelomorpha. It contrasts with the Xenambulacraria hypothesis, which instead posits that Xenacoelomorpha is most closely related to Ambulacraria. Which hypothesis is correct is controversial. Authors supporting the Xenambulacraria hypothesis have suggested that the genetic evidence used to support Nephrozoa may be due to systematic error.

<span class="mw-page-title-main">Xenacoelomorpha</span> A deep-branching bilaterian clade of animals with a simple body plan

Xenacoelomorpha is a small phylum of bilaterian invertebrate animals, consisting of two sister groups: xenoturbellids and acoelomorphs. This new phylum was named in February 2011 and suggested based on morphological synapomorphies, which was then confirmed by phylogenomic analyses of molecular data.

<i>Xenoturbella japonica</i> Species of bilaterians with a simple body plan

Xenoturbella japonica is a marine benthic worm-like species that belongs to the genus Xenoturbella. It has been discovered in western Pacific Ocean by a group of Japanese scientists from the University of Tsukuba. The species was described in 2017 in a study published in the journal BMC Evolutionary Biology, and amended in 2018.

<i>Xenoturbella bocki</i> Species of bilaterians with a simple body plan

Xenoturbella bocki is a marine benthic worm-like species from the genus Xenoturbella. It is found in saltwater sea floor habitats off the coast of Europe, predominantly Sweden. It was the first species in the genus discovered. Initially it was collected by Swedish zoologist Sixten Bock in 1915, and described in 1949 by Swedish zoologist Einar Westblad. The unusual digestive structure of this species, in which a single opening is used to eat food and excrete waste, has led to considerable study and controversy as to its classification. It is a bottom-dwelling, burrowing carnivore that eats mollusks.

<span class="mw-page-title-main">Xenambulacraria</span> Animal clade containing xenoturbellids, acoelomorphs, echinoderms and hemichordates

Xenambulacraria is a proposed clade of animals with bilateral symmetry as an embryo, consisting of the Xenacoelomorpha and the Ambulacraria.

<i>Xenoturbella churro</i> Species of bilaterians with a simple body plan

Xenoturbella churro is a marine, benthic, deep-water worm-like species that belongs to the genus Xenoturbella. It was discovered in eastern Pacific Ocean by a group of Californian and Australian scientists. The species was described in 2016 from a single specimen.

<i>Xenoturbella profunda</i> Species of bilaterians with a simple body plan

Xenoturbella profunda, the purple sock or sock worm, is a marine, benthic, deep-water worm-like species that belongs to the genus Xenoturbella. It was discovered in eastern Pacific Ocean by a group of Californian and Australian scientists. The species was described in 2016 from seven specimens.

<i>Xenoturbella monstrosa</i> Species of bilaterians with a simple body plan

Xenoturbella monstrosa, a deep-sea giant purple sock worm, is a marine, benthic, deep-water worm-like species that belongs to the genus Xenoturbella. It was discovered in eastern Pacific Ocean by a group of Californian and Australian scientists. The species was described in 2016 from several specimens.

<i>Xenoturbella hollandorum</i> Species of bilaterians with a simple body plan

Xenoturbella hollandorum is a marine, benthic worm-like species that belongs to the genus Xenoturbella. It was discovered in eastern Pacific Ocean by a group of Californian and Australian scientists. The species was described in 2016.

References

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