Xenoturbella bocki

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Xenoturbella bocki
Xenoturbella bocki.jpg
X. bocki. Black arrow indicates side furrow. a is the anterior tip. p is the posterior tip. Black triangle indicates mouth. White triangle indicates circumferential furrow. The scale bar in the bottom right is 1 cm.
Scientific classification OOjs UI icon edit-ltr.svg
Domain: Eukaryota
Kingdom: Animalia
Phylum: Xenacoelomorpha
Family: Xenoturbellidae
Genus: Xenoturbella
Species:
X. bocki
Binomial name
Xenoturbella bocki
Westblad, 1949

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. [1] 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 (likely larval forms, as opposed to hard-shelled adults).

Contents

Systematics

Etymology

For the genus name Xenoturbella, Ancient Greek xénos, means foreign or strange, and Latin turbela, means a bustle or turbulence in water. Genus Xenoturbella is a member of sub-phylum Xenoturbellida, which are known as paradoxmaskar [2] – Swedish for "paradox worms" (a term that some popular media have applied to the species), because if it is classified as a deuterostome, it would be more closely related to humans than other, more complex, invertebrates such as lobsters. [3] Deuterostomes are a superphylum of animals whose anus forms before their mouth does during embryonic development. It includes humans, other chordates, echinoderms and hemichordates.

The species signifier bocki refers to Sixten Bock, who first collected the organism in 1915. [4] It was assigned by Swedish zoologist Einar Westblad, who described the species in 1949.

Taxonomy

In 1999, examination of X. bocki specimens held at the Swedish Museum of Natural History showed that a small subset of them must belong to another species. [5] This population differed from specimens identified as X. bocki in internal fertilization, its small size of 12 mm (0.47 in) at most, and its pink coloration – in contrast to yellow-white coloration identified for X. bocki. The new taxon was named after Westblad, who collected the specimens from coarser and shallower habitats in the same range as X. bocki. However, mitochondrial DNA sequencing from the specimens identified with both species suggested that the two populations belonged to the same species, involving that X. westbladi is a junior synonym to X. bocki. [6]

Phylogeny

Species level

Comparison of mitochondrial DNA and protein sequences showed that the species Xenoturbella bocki – often found off the coast of Sweden – is the sister group to X. hollandorum, a species discovered in 2016 in eastern Pacific Ocean. [7] In turn, these two species share evolutionary affinities with X. japonica into a clade of 'shallow-water' taxa. [8]

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. [7] [8]

Above the genus level

The two competing hypotheses as to the phylogeny of Xenoturbella are shown in red. One suggests it is a deuterostome (right), while the other suggests it is a basal bilaterian (left). Xenoturbella Phylogeny.png
The two competing hypotheses as to the phylogeny of Xenoturbella are shown in red. One suggests it is a deuterostome (right), while the other suggests it is a basal bilaterian (left).

When it was discovered, X. bocki was placed in a new genus Xenoturbella . Above the genus level, the classification of this animal is controversial. Westbald placed it in the phylum Platyhelminthes in the class Turbellaria (free-living flatworms). [9] In 1999, based on genetic analysis, it was placed in Protostomia by Israelsson, grouped with the bivalves. [10] [11] Protostomia is a large clade including worms, mollusks and arthropods. In embryonic development, their mouth develops prior to the development of the anus for most protostomes, though some have evolved other developmental pathways. If placed in this clade, Xenoturbella would also be among these exceptions. [12]

However, today, this is understood as a misclassification due to contaminating DNA from its shellfish food. Swedish scientist Sarah J. Bourlat and her coauthors in 2006 placed it in its own phylum, Xenoturbellida. More recent studies suggest on the basis of genetic and developmental evidence (e.g. Hox genes) that it should be grouped with Acoela and Nemertodermatida into Acoelomorpha. These three taxa are sometimes placed within the deuterostomes (a large clade that includes humans and other chordates, sea stars and others), [12] while others classify these organisms as a basal offshoot that resembles a common ancestor of deuterostomes and protostomes. [11] A 2016 analysis of many genetic data sets supports the latter, and suggests that, like Xenoturbella bocki, the common ancestor of protostomes and deuterostomes likely had one opening, ciliated locomotion and a wormlike body. [13] However, if the deuterostome hypothesis is correct, then Xenoturbella must have lost many ancestral traits, such as an anus. [14]

Description

Diagram of a longitudinal section of X. bocki Xenoturbella bockii longitudinal section English.svg
Diagram of a longitudinal section of X. bocki

This animal usually grows to 1 cm (0.39 in) in length, [11] though individuals as long as 4 cm (1.6 in) have been reported. [11] Its nervous system consists of a nerve net with no defined brain or ganglia. The nerve net is found on the basal (away from the animal's surface) side of the skin. [15] This animal lacks a coelom. It also lacks an anus, excreting waste through the same opening as it intakes food. [9] Thus, the digestive organ is sac-like. The opening is on the belly of the animal, near the front. The animal is simultaneously hermaphroditic. [4]

A furrow runs along the circumference of the body in the middle of the animal. There are also side furrows. On its sides there are numerous tiny cilia that aid in locomotion. Small cells contain vesicles which may act as glands. An organ of unknown function, preliminarily called a statocyst, has been observed on the front end of the animal. Two leading hypotheses are that it aids in balance, as statocysts do in other invertebrates, or that it has endocrine functions. Experiments in which the animal was observed to cleave into two after a wound show that the statocyst is essential for normal behavior and long-term survival. [9]

Ecology

The animal moves through the water via rhythmic muscle contraction, aided by its side cilia, and a tuft of longer cilia on its back. The organism can also use its musculature to roll up into a ball, and maintain that form for several months. [9] Adults are known to have a symbiotic relationship with Chlamydiae and Gammaproteobacteria, two bacterial endosymbionts found in their gastrodermis. [16] Genetic data confirms that its diet includes bivalve mollusks. [11] However, it has never been observed feeding, so it is unknown if it eats bivalve carcasses, eggs, sperm, mucus, feces, or live larval or adult bivalves. [9] It lacks any visible means to get through the shells of adult bivalves. Captive specimens survived for several months without food, and showed no interest in any of the proposed food items afterwards. This has led some to suggest that it feeds by absorbing dissolved organic matter through its skin. At least one specimen that has been proposed to show a consumed bivalve larvae is preserved in the Swedish Natural History Museum. This species burrows, and has been observed to make tunnels as deep as 15 cm (5.9 in) into substrate in a laboratory aquarium. [9]

Range

This species has been found in ocean habitats off the coast of Europe, most often off the coast of Sweden. It is often collected using a Warén’s dredge from mud on the sea floor, at depths of 50–200 m (160–660 ft). [9] [17]

Reproduction

X. bocki has only been observed to reproduce sexually. [9] In the wild, this species spawns in the winter. It lays small, mucus-coated eggs, which sink in the water column. [11] The eggs have a pale-orange color, and are opaque. Young, upon hatching, are yellowish, nearly spherical, and move to the surface of the water. Larvae lack a blastopore and do not feed until they are fully developed. They may derive nourishment from the yolk which would make them lecithotrophic. Within five days muscular contractions are observed in a laboratory setting, which may aid locomotion. X. bocki is a direct developer. As of 2013, this animal is extremely challenging to grow in captivity. [11]

Related Research Articles

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

A chordate is a deuterostomic bilaterial 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

The flatworms, flat worms, Platyhelminthes, or platyhelminths are a phylum of relatively simple bilaterian, unsegmented, soft-bodied invertebrates. 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">Nemertea</span> Phylum of invertebrates, ribbon worms

Nemertea is a phylum of animals also known as ribbon worms or proboscis worms, consisting of about 1300 known species. Most ribbon worms are very slim, usually only a few millimeters wide, although a few have relatively short but wide bodies. Many have patterns of yellow, orange, red and green coloration. The foregut, stomach and intestine run a little below the midline of the body, the anus is at the tip of the tail, and the mouth is under the front. A little above the gut is the rhynchocoel, a cavity which mostly runs above the midline and ends a little short of the rear of the body. All species have a proboscis which lies in the rhynchocoel when inactive but everts to emerge just above the mouth to capture the animal's prey with venom. A highly extensible muscle in the back of the rhynchocoel pulls the proboscis in when an attack ends. A few species with stubby bodies filter feed and have suckers at the front and back ends, with which they attach to a host.

<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">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.

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

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.

<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.

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

Ambulacraria, or Coelomopora, is a clade of invertebrate phyla that includes echinoderms and hemichordates; a member of this group is called an ambulacrarian. Phylogenetic analysis suggests the echinoderms and hemichordates separated around 533 million years ago. The Ambulacraria are part of the deuterostomes, a clade that also includes the many Chordata, and the few extinct species belonging to the Vetulicolia.

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

Deuterostomes are bilaterian animals of the superphylum Deuterostomia, typically characterized by their anus forming before the mouth during embryonic development. Deuterostomia is further divided into four phyla: Chordata, Echinodermata, Hemichordata, and the extinct Vetulicolia known from Cambrian fossils. The extinct clade Cambroernida is thought to be a member of Deuterostomia.

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

Protostomia is the clade of animals once thought to be characterized by the formation of the organism's mouth before its anus during embryonic development. This nature has since been discovered to be extremely variable among Protostomia's members, although the reverse is typically true of its sister clade, Deuterostomia. Well-known examples of protostomes are arthropods, molluscs, annelids, flatworms and nematodes. They are also called schizocoelomates since schizocoely typically occurs in them.

<span class="mw-page-title-main">Brachiopod</span> Phylum of marine animals also known as lamp shells

Brachiopods, phylum Brachiopoda, are a phylum of trochozoan animals that have hard "valves" (shells) on the upper and lower surfaces, unlike the left and right arrangement in bivalve molluscs. Brachiopod valves are hinged at the rear end, while the front can be opened for feeding or closed for protection. Two major categories are traditionally recognized, articulate and inarticulate brachiopods. The word "articulate" is used to describe the tooth-and-groove structures of the valve-hinge which is present in the articulate group, and absent from the inarticulate group. This is the leading diagnostic skeletal feature, by which the two main groups can be readily distinguished as fossils. Articulate brachiopods have toothed hinges and simple, vertically oriented opening and closing muscles. Conversely, inarticulate brachiopods have weak, untoothed hinges and a more complex system of vertical and oblique (diagonal) muscles used to keep the two valves aligned. In many brachiopods, a stalk-like pedicle projects from an opening near the hinge of one of the valves, known as the pedicle or ventral valve. The pedicle, when present, keeps the animal anchored to the seabed but clear of sediment which would obstruct the opening.

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

The embryological origin of the mouth and anus is an important characteristic, and forms the morphological basis for separating bilaterian animals into two natural groupings: the protostomes and deuterostomes.

<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.

<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

  1. ↑ Westblad, E. (1949). "Xenoturbella bocki n. g., n. sp., a peculiar, primitive Turbellarian type". Arkiv för Zoologi. 1: 3–29.
  2. ↑ "Xenoturbellida". Dyntaxa. Retrieved 31 December 2018.
  3. ↑ Jauregui, Andres (28 March 2013). "'Paradox Worm' Xenoturbella Bocki Lacks Brain & Sex Organs, But Could Be Mankind's 'Progenitor'". HuffPost . Retrieved 26 April 2019.
  4. 1 2 Telford, Maximilian J. (2008). "Xenoturbellida: The fourth deuterostome phylum and the diet of worms". Genesis. 46 (11): 580–586. doi: 10.1002/dvg.20414 . ISSN   1526-968X. PMID   18821586.
  5. ↑ Israelsson, Olle (22 April 1999). "New light on the enigmatic Xenoturbella (phylum uncertain): ontogeny and phylogeny". Proceedings of the Royal Society of London. Series B, Biological Sciences. 266 (1421): 835–841. doi:10.1098/rspb.1999.0713. PMC   1689910 .
  6. ↑ Rouse, Greg W.; Wilson, Nerida G.; Carvajal, Jose I.; Vrijenhoek, Robert C. (2016). "New deep-sea species of Xenoturbella and the position of Xenacoelomorpha". Nature. 530 (7588): 94–97. Bibcode:2016Natur.530...94R. doi:10.1038/nature16545. PMID   26842060. S2CID   3870574.
  7. 1 2 Rouse, Greg W.; Wilson, Nerida G.; Carvajal, Jose I.; Vrijenhoek, Robert C. (4 February 2016). "New deep-sea species of Xenoturbella and the position of Xenacoelomorpha". Nature. 530 (7588): 94–97. Bibcode:2016Natur.530...94R. doi:10.1038/nature16545. ISSN   0028-0836. PMID   26842060. S2CID   3870574.
  8. 1 2 Nakano, Hiroaki; Miyazawa, Hideyuki; Maeno, Akiteru; Shiroishi, Toshihiko; Kakui, Keiichi; Koyanagi, Ryo; Kanda, Miyuki; Satoh, Noriyuki; Omori, Akihito; Kohtsuka, Hisanori (18 December 2017). "A new species of Xenoturbella from the western Pacific Ocean and the evolution of Xenoturbella". BMC Evolutionary Biology. 17 (1): 245. doi: 10.1186/s12862-017-1080-2 . ISSN   1471-2148. PMC   5733810 . PMID   29249199.
  9. 1 2 3 4 5 6 7 8 Nakano, Hiroaki (2015). "What is Xenoturbella?". Zoological Letters. 1 (22): 22. doi: 10.1186/s40851-015-0018-z . PMC   4657256 . PMID   26605067.
  10. ↑ Perseke, Marleen; Hankeln, Thomas; Weich, Bettina; Fritzsch, Guido; Stadler, Peter F.; Israelsson, Olle; Bernhard, Detlef; Schlegel, Martin (April 2007). "The Mitochondrial DNA of Xenoturbella bocki: Genomic Architecture and Phylogenetic Analysis". Theory in Biosciences. 126 (1): 35–42. doi:10.1007/s12064-007-0007-7. PMID   18087755. S2CID   17065867.
  11. 1 2 3 4 5 6 7 Nakano, H.; Lundin, K.; Bourlat, S. J.; Telford, M. J.; Funch, P.; Nyengaard, J. R.; Obst, M.; Thorndyke, M. C. (2013). "Xenoturbella bocki exhibits direct development with similarities to Acoelomorpha". Nature Communications. 4: 1537. Bibcode:2013NatCo...4.1537N. doi:10.1038/ncomms2556. PMC   3586728 . PMID   23443565.
  12. 1 2 de Mendoza, A.; Ruiz-Trillo, I. (2011). "The Mysterious Evolutionary Origin for the GNE Gene and the Root of Bilateria". Molecular Biology and Evolution. 28 (11): 2987–2991. doi:10.1093/molbev/msr142. PMC   4342547 . PMID   21616910.
  13. ↑ Cannon, J. T.; Vellutini, B. C.; Smith, J.; Ronquist, F.; Jondelius, U.; Hejnol, A. (2016). "Xenacoelomorpha is the sister group to Nephrozoa". Nature. 530 (7588): 89–93. Bibcode:2016Natur.530...89C. doi:10.1038/nature16520. PMID   26842059. S2CID   205247296.
  14. ↑ Philippe, H.; Brinkmann, H.; Copley, R. R.; Moroz, L. L.; Nakano, H.; Poustka, A. J.; Wallberg, A.; Peterson, K. J.; Telford, M. J. (2011). "Acoelomorph flatworms are deuterostomes related to Xenoturbella". Nature. 470 (7333): 255–258. Bibcode:2011Natur.470..255P. doi:10.1038/nature09676. PMC   4025995 . PMID   21307940.
  15. ↑ Schmidt-Rhaesa, Andreas; Steffen, Harzsch; Günter, Purschke; Thomas, Stach (2016). Structure and Evolution of Invertebrate Nervous Systems (1st ed.). Oxford: Oxford University Press. ISBN   9780191066214.
  16. ↑ Kjeldsen, K. U.; Obst, M.; Nakano, H.; Funch, P.; Schramm, A. (2010). "Two Types of Endosymbiotic Bacteria in the Enigmatic Marine Worm Xenoturbella bocki". Applied and Environmental Microbiology. 76 (8): 2657–2662. Bibcode:2010ApEnM..76.2657K. doi:10.1128/aem.01092-09. PMC   2849209 . PMID   20139320.
  17. ↑ Nakano, Hiroaki; Miyazawa, Hideyuki; Maeno, Akiteru; Shiroishi, Toshihiko; Kakui, Keiichi; Koyanagi, Ryo; Kanda, Miyuki; Satoh, Noriyuki; Omori, Akihito; Kohtsuka, Hisanori (2017). "A new species of Xenoturbella from the western Pacific Ocean and the evolution of Xenoturbella". BMC Evolutionary Biology. 17 (1): 245. doi: 10.1186/s12862-017-1080-2 . PMC   5733810 . PMID   29249199.

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