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Cycliophorans

Cycliophora

Brief Summary

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Cycliophora is a phylum that was recognized only in 1995 with the description of Symbion pandora, found living on the mouthparts of Norwegian Lobsters (Nephrops norvegicus) in the North Atlantic (Funch and Kristensen 1995). Interestingly, the sessile (i.e., attached to substrate) feeding stage of S. pandora has been known since the 1960s, but was not described until 1995 (Funch and Kristensen 1997, cited in Kristensen 2002).

The cycliophoran body is divided into an anterior buccal funnel, an oval trunk, and a posterior acellular stalk and adhesive disc by which the animal attaches itself to setae (flexible hair-like projections) on the host's mouthparts. Females are about 350 µm long and 100 µm wide. They are suspension feeders, obtaining food by creating water currents with dense cilia around the open end of the buccal funnel. The U-shaped gut is ciliated along its entire length, ending with an anus located near the base of the buccal funnel. Circulation and gas exchange are presumably accomplished by simple diffusion. Obst and Funch (2003) reported S. pandora population densities ranging from fewer than 100 to more than 500 feeding stages per mouthpart.

The Cycliophora currently includes just two described species: Symbion pandora Funch and Kristensen, 1995 from the mouthparts of the Norwegian Lobster (Nephrops norvegicus) and S. americanus Obst, Funch, and Kristensen, 2006 from the mouthparts of the American Lobster (Homarus americanus) (an apparently distinct third species lives on the mouthparts of the European Lobster [Homarus gammarus]; Obst et al. 2005) . For at least S. americanus, there is evidence suggesting that this nominal species may in fact include several cryptic species (Obst et al. 2005, 2006; Baker and Giribet 2007; Baker et al. 2007). All three known hosts of Cycliophora are members of the lobster family Nephropidae. Reports of cycliophorans on nematodes and non-nephropid crustaceans (e.g., copepods) are apparently all in error and instead are based on observations of chonotrich ciliates. Examination by transmission electron microscopy is required to see that, in contrast to a cycliophoran, the ciliate consists of just a single cell with several nuclei. (Kristensen 2002)

Cycliophorans have a very complex life cycle that alternates between sexual and asexual phases. The most prominent stage is the asexual and sessile feeding stage, which lives attached to the setae of the host lobster's mouthparts and filters small food particles from the water. For a detailed description of the complex life cycle of Symbion pandora, see General Description on the Symbion pandora page; for a whimsical but informative account, check out the CreatureCast podcast on this topic.

A growing body of evidence suggests that the sister group to the Cycliophora is the Entoprocta (=Kamptozoa) (Fuchs et al. 2010 and references therein), consistent with the suggestion made by Funch and Kristensen (1995) in their original description, although cycliophorans share many similarities with the Rotifera and some molecular analyses have indicated a close relationship between these two groups (e.g., Winnepenninckx et al. 1998).

The phylum name Cycliophora is derived from Greek roots meaning "wheel bearing", referring to the circular mouth ring. The genus name Symbion is derived from Greek roots meaning "living together", referring to this animal's intimate association with its lobster host. The specific epithet pandora is a reference to the feeding stage, which contains both an inner bud and a Pandora larva with a miniature feeding stage inside, reminding the authors of Pandora's Box of Greek mythology. The specific epithet americanus is a reference to the host of S. americanus, the American Lobster.(Funch and Kristensen 1995)

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Comprehensive Description

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Cycliophora is a phylum that was recognized only in 1995 with the description of Symbion pandora, found living on the mouthparts of Norwegian Lobsters (Nephrops norvegicus) in the North Atlantic (Funch and Kristensen 1995). Interestingly, the sessile feeding stage of S. pandora has been known since the 1960s, but was not described until 1995 (Funch and Kristensen 1997, cited in Kristensen 2002).

The cycliophoran body is divided into an anterior buccal funnel, an oval trunk, and a posterior acellular stalk and adhesive disc by which the animal attaches itself to setae (flexible hair-like projections) on the host's mouthparts. Females are about 350 µm long and 100 µm wide. They are suspension feeders, obtaining food by creating water currents with dense cilia around the open end of the buccal funnel. The U-shaped gut is ciliated along its entire length, ending with an anus located near the base of the buccal funnel. Circulation and gas exchange are presumably accomplished by simple diffusion. Obst and Funch (2003) reported S. pandora population densities ranging from fewer than 100 to more than 500 feeding stages per mouthpart.

The Cycliophora currently includes just two described species: Symbion pandora Funch and Kristensen, 1995 from the mouthparts of the Norwegian Lobster (Nephrops norvegicus) and S. americanus Obst, Funch, and Kristensen, 2006 from the mouthparts of the American Lobster (Homarus americanus) (an apparently distinct third species lives on the mouthparts of the European Lobster [Homarus gammarus]; Obst et al. 2005) . For at least S. americanus, there is evidence suggesting that this nominal species may in fact include several cryptic species (Obst et al. 2005, 2006; Baker and Giribet 2007; Baker et al. 2007). All three known hosts of Cycliophora are members of the family Nephropidae. Reports of cycliophorans on nematodes and non-nephropid crustaceans (e.g., copepods) are apparently all in error and instead are based on observations of chonotrich ciliates. Examination by transmission electron microscopy is required to see that, in contrast to a cycliophoran, the ciliate consists of just a single cell with several nuclei. (Kristensen 2002)

Cycliophorans have a very complex life cycle that alternates between sexual and asexual phases. The most prominent stage is the asexual and sessile feeding stage, which lives attached to the setae of the host lobster's mouthparts and filters small food particles from the water. This stage generates several other life cycle stages by internal budding. These feeding individuals can produce Pandora larvae, Prometheus larvae, or females, one at a time, inside a brood chamber. When released, these stages (all of which lack a gut) swim freely. In the asexual phase of the life cycle, the Pandora larva settles close to the maternal feeding stage and develops asexually into a new feeding stage. In the sexual phase of the life cycle, feeding individuals release a free male stage, the Prometheus larva, which settles on the trunk of a feeding stage (probably the maternal individual). The Prometheus larva then produces 1 to 3 dwarf males inside its body, which fertilize females. Fertilized females are thought to settle on the mouthparts of the same host, where they encyst and eventually release sexually-generated dispersing chordoid larvae, which can settle on the same or another nearby host individual and develop into new feeding forms. Fertilization in cycliophorans has never been observed so it is not clear when, where, or how it occurs (see Obst and Funch 2003 and Neves et al. 2010 for discussion). (Neves et al. 2010 and references therein)

A growing body of evidence suggests that the sister group to the Cycliophora is the Entoprocta (=Kamptozoa) (Fuchs et al. 2010 and references therein), consistent with the suggestion made by Funch and Kristensen (1995) in their original description, although cycliophorans share many similarities with the Rotifera and some molecular analyses have indicated a close relationship between these two groups (e.g., Winnepenninckx et al. 1998).

The phylum name Cycliophora is derived from Greek roots meaning "wheel bearing", referring to the circular mouth ring. The genus name Symbion is derived from Greek roots meaning "living together", referring to this animal's intimate association with its lobster host. The specific epithet pandora is a reference to the feeding stage, which contains both an inner bud and a Pandora larva with a miniature feeding stage inside, reminding the authors of Pandora's Box of Greek mythology. The specific epithet americanus is a reference to the host of S. americanus, the American Lobster.(Funch and Kristensen 1995)

For a whimsical but informative account of the complex life cycle of Symbion pandora, see the CreatureCast podcast on this topic.

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Symbion

provided by wikipedia EN

Symbion pandora wikipedia.jpg

Symbion is a genus of commensal aquatic animals, less than 0.5 mm wide, found living attached to the mouthparts of cold-water lobsters. They have sac-like bodies, and three distinctly different forms in different parts of their two-stage life-cycle. They appear so different from other animals that they were assigned their own, new phylum Cycliophora shortly after they were discovered in 1995.[1] This was the first new phylum of multicelled organism to be discovered since the Loricifera in 1983.

Taxonomy

Symbion was discovered in 1995 by Reinhardt Kristensen and Peter Funch[2] on the mouthparts of the Norway lobster (Nephrops norvegicus), and other, related, species have since been discovered on:

The genus is so named because of its commensal relationship with the lobster (a form of symbiosis) – it feeds on the leftovers from the lobster's own meals.[5]

The genus Symbion are peculiar microscopic animals, with no obvious close relatives, and which was therefore given its own phylum, called Cycliophora. The phylogenetic position of Symbion remains unclear: originally the phyla Ectoprocta and Entoprocta were considered possible relatives of Symbion, based on structural similarities.[6] However, genetic studies suggest that Symbion may be more closely related to Gnathifera.

Description

Symbion pandora has a bilateral, sac-like body with no coelom. There are three basic life stages:

  • Asexual Feeding Stage – At this stage, S. pandora is neither male nor female. It has a length of 347 μm and a width of 113 μm. On the posterior end of the sac-like body is a stalk with an adhesive disc, which attaches itself to the host. On the anterior end is a ciliated funnel (mouth) and an anus.
  • Sexual Stage
    • MaleS. pandora has a length of 84 μm and a width of 42 μm during this stage. It has no mouth or anus, which signifies the absence of a digestive system. It also has two reproductive organs.
    • FemaleS. pandora is the same size as the male in this stage. It does, however, have a digestive system which collapses and reconstitutes itself as a larva.[2]

Reproduction

Symbion reproduces both asexually and sexually, and has a complex reproduction cycle, a strategy evolved to produce as many offspring as possible that can survive and find a new host when the lobster they live on sheds its shell. The asexual individuals are the largest ones. The sexual individuals do not eat. During the autumn they make copies of themselves, where a new individual grows inside the parent body, one offspring at the time. The new offspring attach themselves to an available spot on the lobster, begin to feed and eventually start making new copies of themselves. In early winter, the asexual animals start producing males. When a male is born, it crawls away from its parent and glues itself to another asexual individual. Once attached, the male produces two dwarf males inside its body, which turns into a hollow pouch. Each of the two dwarf males are about one hundred times smaller than the asexual individual to which they are attached. Their bodies start out with about 200 cells, but this number has been reduced to just 47 by the time they reach maturity. Thirty-four of the cells form its nervous system, and three more become sensory cells used to help them feel their surroundings. Eight cells becomes mucus glands, which produce mucus that helps them move across the surface. The final two cells form the testes, which make the sperm that fertilize the female’s egg. Most of the cells of the dwarf males also lose their nucleus and shrink to almost half their size, which is an adaptation that allows two mature individuals to fit inside the body of the parent male. Two males increases their chances to fertilize a female. By late winter, when the large feeding individuals in the colony have males attached to their bodies, they start making females. Each female has a single egg inside her. When she is about the be born, one of the two dwarf males fertilizes her when she comes out. The fertilized female finds herself a place on the host's whiskers where she attaches herself. Inside her the developing embryo extracts all the nutrients it needs to grow from its mother, and by the time it is ready to be born, all that remains of the mother is an empty husk. This new offspring is a strong swimmer unlike all the other forms in the colony, and those who succeed in finding a new host will attach themselves to its mouthparts, where it will grow a stomach and mouthparts, morphing into a large, feeding and asexual type, starting the cycle all over again.[7] The larval stage may be unscientifically referred to as sea worms.[8]

References

  1. ^ Marshall, Michael (28 April 2010). "Zoologger: The most bizarre life story on Earth?". New Scientist. Retrieved 19 November 2018. ... In 1995, Peter Funch and Reinhardt Møbjerg Kristensen, both then at the University of Copenhagen, Denmark, discovered an animal so unlike any other that a new phylum – Cycliophora – had to be created just for it. ...
  2. ^ a b P. Funch & R. M. Kristensen (1995). "Cycliophora is a new phylum with affinities to Entoprocta and Ectoprocta". Nature. 378 (6558): 711–714. Bibcode:1995Natur.378..711F. doi:10.1038/378711a0. S2CID 4265849.
  3. ^ M. Obst; P. Funch & G. Giribet (2005). "Hidden diversity and host specificity in cycliophorans: a phylogeographic analysis along the North Atlantic and Mediterranean Sea". Molecular Ecology. 14 (14): 4427–4440. doi:10.1111/j.1365-294X.2005.02752.x. PMID 16313603. S2CID 26920982.
  4. ^ Neves RC, Kristensen RM, Wanninger A (March 2009). "Three-dimensional reconstruction of the musculature of various life cycle stages of the cycliophoran Symbion americanus". J. Morphol. 270 (3): 257–70. doi:10.1002/jmor.10681. PMID 18937332. S2CID 206090614.
  5. ^ P. Funch; P. Thor & M. Obst (2008). "Symbiotic relations and feeding biology of Symbion pandora (Cycliophora) and Triticella flava (Bryozoa)". Vie et Milieu. 58: 185–188.
  6. ^ "Cycliophorans - Cycliophora - Details - Encyclopedia of Life". Encyclopedia of Life. Retrieved 2017-09-02.
  7. ^ Living Mysteries: This complex beast lurks on lobster whiskers
  8. ^ Piper, Ross (2007), Extraordinary Animals: An Encyclopedia of Curious and Unusual Animals, Greenwood Press.
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Symbion: Brief Summary

provided by wikipedia EN
Symbion pandora wikipedia.jpg

Symbion is a genus of commensal aquatic animals, less than 0.5 mm wide, found living attached to the mouthparts of cold-water lobsters. They have sac-like bodies, and three distinctly different forms in different parts of their two-stage life-cycle. They appear so different from other animals that they were assigned their own, new phylum Cycliophora shortly after they were discovered in 1995. This was the first new phylum of multicelled organism to be discovered since the Loricifera in 1983.

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Symbion

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Symbion est le seul genre de la classe des Cycliophora.

Description

Les cycliophores sont des animaux microscopiques (0,4 mm sur 0,1 mm pour la phase fixée et 0,1 mm sur 0,05 mm pour la larve Prometheus). Ils vivent sur les pièces buccales des langoustines et homards.

On trouve 6 formes chez les cycliophores, chacune étant un stade d'un cycle complexe de reproductions asexuées et sexuées :

  • La forme fixée.
  • La larve Pandora.
  • La larve chordoïde.
  • La larve Prometheus (autrefois confondue avec le mâle).
  • Le mâle nain.
  • La femelle.

Il y a deux modes de reproductions : sexué via une larve chordoïde et asexué via une larve pandora.

Contrairement à ce que l'on peut lire parfois dans la littérature le mâle ne vit pas fixé sur la femelle, c'est la larve Prometheus qui se fixe sur la forme fixée (asexué). Aucune des formes sexuées n'est donc fixée. Au mieux elle est interne en tant que bourgeon mature.

Histoire de la découverte

Dans les années soixante, des organismes vivant fixés sur les pièces buccales de langoustine sont signalés mais supposés être des rotifères[1]. En 1995, Funch et Kristensen décrivent ces organismes comme un nouveau phylum après un examen plus précis. En premier lieu le cycle n'est pas tout à fait compris. En effet, le mâle est supposé fixé sur la "forme fixée". En effet, la partie fixée sur la "forme fixée" produit des gamètes mâles. Cependant après de plus amples observations, les auteurs ont remarqué que la forme fixée produit en fait des mâles et est le résultat d'une forme mobile appelée larve Prometheus (en référence à Pandore). Au départ la larve Promethéus a été appelée "mâle primaire" et le "mâle vrai" appelé "mâle secondaire". Cependant le mâle secondaire du fait de sa petite taille passant entre les mailles des tamis n'avait pas encore été observée [2]. Ce n'est que plus tard qu'elle a été enfin observée et étudiée. Le mâle primaire a alors été appelé "Larve Prometheus" car c'est une forme libre, transitoire et asexuée et le mâle secondaire a été appelé "mâle nain" du fait de sa petite taille : autour de 40µ. En 2006, une nouvelle espèce de cycliophore est décrite : Symbion americanus[3]. Aujourd'hui les modalités de fécondation entre le mâle et la femelle sont encore inconnus. De plus plusieurs autres espèces sont suspectées : Symbion americanus pourrait être un complexe d'espèces cryptiques[4] et une espèce semble exister aussi sur les pièces buccales du homard Européen : Homarus gammarus.

Cycle de vie

Le cycle de vie des cycliophores est particulièrement complexe et fait intervenir une reproduction sexuée et asexuée.

Phase asexuée

La forme fixée va former intérieurement un bourgeon qui va donner une larve Pandora. Cette larve, mobile, va se fixer sur le même crustacé hôte et former en son sein elle-même une forme fixée. Lors de la formation d'une larve Pandora le système nerveux et digestif de la forme fixée régressent et vont régénérer après que la larve Pandora soit expulsée. Ainsi, le cône buccal régresse lors de la formation d'une larve Pandora. Cette larve est appelée Pandora car à ce moment-là, l'individu fixé contient, comme la boîte de Pandore un organisme en son sein, voire plusieurs. En effet, il semble que la larve de Pandore elle-même en cours de gestation puisse contenir un individu fixé (qui donc n'est pas encore fixé) en formation, lui-même contenant un bourgeon de larve Pandora. Cette configuration faisant alors plutôt penser à des poupées russes.

Phase sexuée

La phase sexuée est bien plus complexe que la phase asexuée et fait intervenir 5 des 6 différentes phases du cycle de vie des Cycliophores.

Lorsque la mue de l'hote commence, par un mécanisme encore inconnu, des individus de la forme fixée vont commencer à bourgeonner soit des femelles avec un oocyte, soit des larves Prometheus. La larve Prometheus va être expulsée et se fixer sur un individu fixé contenant lui-même une femelle. Au sein de la larve Prométheus vont se former plusieurs mâles nains. Une larve Prométhéus peut contenir de un à trois mâles nains. Il n'est pas encore clair si la fécondation entre le mâle nain et la femelle a lieu sur l'animal fixé ou si elle se fait librement près de ce dernier. La fécondation va donner un zygote et la femelle libre va se fixer par l'avant pas loin de l'individu fixé qui l'a produite. Au sein de la femelle va alors se former une larve chordoïde à partir des tissus de la femelle. Tous les tissus de la femelle vont être recyclés pour former une larve chordoïde. À ce moment-là il ne reste de la femelle que la cuticule fixée avec la larve chordoïde en gestation. On parle de cyste chordoïde. Une fois la larve chordoïde formée elle va briser le cyste et aller se fixer sur un autre hôte ou le même une fois sa mue effectuée. Une fois fixée, la larve chordoïde va produire une forme fixée par bourgeonnement interne et un nouveau cycle d'infestation par reproduction asexuée se déroulera.

Listes des espèces

Notes et références

  1. Funch P. 1996. The Chordoid Larva of Symbion pandora (Cycliophora) Is a Modified Trochophore. Journal of Morphology, 230 : 231-263.
  2. Kristensen R. M. 2002. An Introduction to Loricifera, Cycliophora, and Micrognathozoa. Integrative & Comparative Biology, 42 : 641-651.
  3. Obst M., Funch P. et Kristensen R. M. 2006. A new species of Cycliophora from the mouthparts of the American lobster, Homarus americanus (Nephropidae, Decapoda). Organisms, Diversity & Evolution, 6 : 83-97.
  4. Baker J. M. et Giribet G. 2006. A molecular phylogenetic approach to the phylum Cycliophora provides further evidence for cryptic speciation in Symbion americanus. Zoologica Scripta, 36(4), 353-359.

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Symbion: Brief Summary

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Symbion est le seul genre de la classe des Cycliophora.

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구륜동물

provided by wikipedia 한국어 위키백과
Symbion pandora wikipedia.jpg

심비온속(Symbion)은 냉수 바닷가재의 몸에 부착되어 사는, 폭 ½ mm 이하의 매우 작은 해양 동물의 속이다. 다른 동물문과 너무 달라서, 1995년 발견된 후 새로운 문, 구륜동물문(Cycliophora)으로 배정되었다. 구륜동물문은 1983년에 윤형동물문(Loricifera)이 발견된 이래, 새로운 다세포 기관 중에서 최초의 동물 문(門)이다.

분류

심비온속(Symbion)은 1995년에 라인하르트 크리스텐센피터 펀치[1]에 의해 노르웨이바닷가재 (Nephrops norvegicus)의 구기(口器)에서 발견되었고, 기타 종들은 다음과 같은 곳에서 발견되었다.:

이 속은 바닷가재와 공생 관계(상부상조(symbiosis) 형태)에 있기 때문에 심비온속(Symbion)이라는 이름이 붙었다. 이들은 먹이를 바닷가재가 먹다 만 먹이에서 얻는다.

심비온속(Symbion)은 명백한 근연종이 없는 매우 작고 독특한 동물로, 그 자체로 구륜동물문(Cycliophora)이라는 문(門)을 이루고 있다. 심비온속(Symbion)의 계통발생적 위치는 불명확한 상태로 남아있다: 초기에는 심비온속(Symbion)과 구조적으로 유사한 외항동물문(Ectoprocta)과 내항동물문(Entoprocta)이 심비온속과 관련이 있는 동물 문으로 고려되었다. 그러나, 유전자 연구를 통해, Gnathifera와 좀 더 밀접한 관계에 있는 것으로 입증되고 있다.

각주

  1. P. Funch & R. M. Christensen (1995). “Cycliophora is a new phylum with affinities to Entoprocta and Ectoprocta”. 《네이처378: 711–714. doi:10.1038/378711a0.
  2. M. Obst, P. Funch & G. Giribet (2005). “Hidden diversity and host specificity in cycliophorans: a phylogeographic analysis along the North Atlantic and Mediterranean Sea”. 《Molecular Ecology》 14: 4427–4440. doi:10.1111/j.1365-294X.2005.02752.x. 2018년 12월 15일에 원본 문서에서 보존된 문서. 2009년 7월 7일에 확인함.
  3. Neves RC, Kristensen RM, Wanninger A (2009년 3월). “Three-dimensional reconstruction of the musculature of various life cycle stages of the cycliophoran Symbion americanus”. 《J. Morphol.》 270 (3): 257–70. doi:10.1002/jmor.10681. PMID 18937332.
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