関: Mesozoa

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出典(authority):フリー百科事典『ウィキペディア（Wikipedia）』「2015/12/04 10:02:18」(JST)

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The Mesozoa (singular: mesozoon) are enigmatic, minuscule, worm-like parasites of marine invertebrates. As of 2012^[update] it was still unclear whether they are degenerate platyhelminthes (flatworms) or truly-primitive, basal metazoans. Generally, these tiny, elusive creatures consist of a somatoderm (outer layer) of ciliated cells surrounding one or more reproductive cells. Decades ago, Mesozoa were classified as a phylum. Molecular phylogeny studies, however, have shown that the mysterious mesozoans are polyphyletic. That is, they consist of at least two unrelated groups.^[1]

As a result of these recent findings in molecular biology, the label mesozoan is now often applied informally, rather than as a formal taxon. Some workers previously classified Mesozoa as the sole phylum of the lonely subkingdom Agnotozoa. Cavalier-Smith argued that at least some of the mesozoans are in fact protistans, not animals.^[2]

In the 19th century, the Mesozoa were a wastebasket taxon for multicellular organisms which lacked the invaginating gastrula which was thought to define the Metazoa.^[3]

Evolution

Mesozoa were once thought to be evolutionary intermediate forms between Protozoans and Metazoans, but now they are thought to be degenerate or simplified metazoa. Their ciliated larva are similar to the miracidium of trematodes, and their internal multiplication is similar to what happens in the sprocysts of trematodes. Mesozoan DNA has a low GC-content (40%). This amount is similar to ciliates, but ciliates tend to be binucleate. Others relate mesozoa to a group including annelids, planarians, and nemerteans.

Groupings

The two main mesozoan groups are the Dicyemida and the Orthonectida. Other groups sometimes included in the Mesozoa are the Placozoa and the Monoblastozoa.

Monoblastozoans consist of a single description written in the 19th century of a species that has not been seen since. As such, many workers doubt that they are a real group.^[4] As described, the animal had only a single layer of tissue. ^[5]

Rhombozoan mesozoans

Rhombozoa, or dicyemid mesozoans, are found in the nephrid tracts of squid and octopuses.^[6] They range from a few millimeters long with twenty to thirty cells that include anterior attachment cells and a long central reproductive cell called an axial cell. This axial cell may develop asexually into vermiform juveniles or it may produce eggs and sperm that self-fertilize to produce a ciliated infusiform larva.

There are three genera: Dicyema, Pseudicyema and Dicyemennea.

Orthonectid mesozoans

Orthonectida are found in the body spaces of various marine invertebrates including tissue spaces, gonads, genitorespiratory bursae. This pathogen causes host castration of different species.^[7] Their actual phylogenetic position is uncertain, suggestions include as sister group to bilateral animals, as a relative of platyhelminthes and roundworms, as parasitic cnidarians and even as member of kingdom Protista, outside the animal kingdom.^[2]

The best known of Orthonectida is the parasite of brittle stars. The multinucleate syncytial stage lives within tissues and spaces of the gonad but can spread into arms. It causes the destruction of starfish ovary and eggs to cause castration (the male gonads are usually unaffected). The stages of the plasmodium develop into more plasmodia by simple fragmentation; at some point, they decide to go sexual. The syncytia are monoecious (either male or female), but young syncytia can fuse to produce both male and female. The males are ciliated and smaller than the females. The females and the males leave the starfish and mate in the sea. Tailed sperm enters the female and fertilizes the numerous oocytes. Each oocyst produces a small ciliated larva which makes its way to another star.

References

^ Pawlowski, J.; J. I. MontoyaBurgos; J. F. Fahrni; J. Wuest; and L. Zaninetti (1996). "Origin of the Mesozoa inferred from 18S rRNA gene sequences". Molecular Biology and Evolution 13 (8): 1128–32. doi:10.1093/oxfordjournals.molbev.a025675. PMID 8865666.
^ ^a ^b Hanelt, B.; Van Schyndel, D., Adema, C.M., Lewis, L.A. & Loker, E.S. (1996). "The Phylogenetic Position of Rhopaluva ophiocomae (Orthonectida) Based on 18s Ribosomal DNA Sequence Analysis" (PDF). Molecular Biology and Evolution 13 (9): 1187–1191. doi:10.1093/oxfordjournals.molbev.a025683. PMID 8896370. Retrieved 14 March 2013.
^ T. Syed and B. Schierwater (2002). "Trichoplax adhaerens: discovered as a missing link, forgotten as a hydrozoan, re-discovered as a key to metazoan evolution" (PDF). Vie et Milieu 52 (4): 177–187.
^ Salinella: Monoblastozoa
^ Meeûs, Thierry de; and Renaud, François (2002). "Parasite within the new phylogeny of eukaryotes" (PDF). TRENDS in Parasitology 18 (6): 247–51. doi:10.1016/S1471-4922(02)02269-9.
^ Hochberg, F.G. (1983). "The parasites of cephalopods: A review". Memoirs National Museum Victoria 44: 109–145.
^ Robert D. Barnes (1982). Invertebrate Zoology. Philadelphia, PA: Holt-Saunders International. pp. 247–248. ISBN 0-03-056747-5.

English Journal

Using the giant Australian cuttlefish (Sepia apama) mass breeding aggregation to explore the life cycle of dicyemid parasites.

Catalano SR1, Whittington ID, Donnellan SC, Gillanders BM.
Acta parasitologica / Witold Stefański Institute of Parasitology, Warszawa, Poland.Acta Parasitol.2013 Dec;58(4):599-602. doi: 10.2478/s11686-013-0186-y. Epub 2013 Dec 13.
Dicyemid mesozoan parasites, microscopic organisms found with high intensities in the renal appendages of benthic cephalopods, have a complex, partially unknown life cycle. It is uncertain at which host life cycle stage (i.e. eggs, juvenile, adult) new infection by the dispersive infusoriform embryo
PMID 24338325

A new dicyemid from Octopus hubbsorum (Mollusca:Cephalopoda:Octopoda).

Castellanos-Martinez S1, Gómez MC, Hochberg FG, Gestal C, Furuya H.
The Journal of parasitology.J Parasitol.2011 Apr;97(2):265-9. doi: 10.1645/GE-2577.1. Epub 2010 Oct 21.
A new species of dicyemid mesozoan is described from Octopus hubbsorum Berry, 1953, collected in the south of Bahia de La Paz, Baja California Sur, México. Dicyema guaycurense n. sp. is a medium-size species that reaches about 1,600 µm in length. It occurs in folds of the renal appendages. The ver
PMID 21506788

A new dicyemid from Benthoctopus sibiricus (Mollusca: Cephalopoda: Octopoda).

Furuya H1.
The Journal of parasitology.J Parasitol.2010 Dec;96(6):1123-7. doi: 10.1645/GE-2411.1. Epub 2010 Jul 23.
A new species of dicyemid mesozoan is described from Benthoctopus sibiricus Loyning, 1930, collected in the eastern Chukchi Sea. Dicyemennea chukchiense n. sp. is a medium species that reaches about 2,000 µm in length; it lives in folds of the renal appendages. The vermiform stages are characterize
PMID 21158621

Japanese Journal

中生動物ニハイチュウの寄生適応戦略--発生中のDNA増幅と削減,そして細胞分化後の反復配列の特異的増幅 (第39回日本原生動物学会大会講演要旨)

原生動物学雑誌 40(1), 68-70, 2007-04
NAID 40015440041

Two New Species of Dicyema (Dicyemida: Dicyemidae) from Amphioctopus areolatus (Mollusca: Cephalopoda: Octopodidae)

Species diversity : an international journal for taxonomy, systematics, speciation, biogeography, and life history research of animals 11(3), 257-269, 2006-08-31
NAID 110006794412

Three New Species of Dicyema (Dicyemida: Dicyemidae) from Octopus kagoshimensis (Mollusca: Cephalopoda: Octopodidae)

Species diversity : an international journal for taxonomy, systematics, speciation, biogeography, and life history research of animals 10(4), 231-247, 2005-11-30
NAID 110006794390

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リンク元	「Mesozoa」「中生動物」

「Mesozoa」

Mesozoa
	Dicyema macrocephalum
Scientific classification
Domain:	Eukaryota
(unranked):	Opisthokonta
Kingdom:	Animalia
Subkingdom:	Eumetazoa?
(unranked)	Bilateria?
(unranked):	Mesozoa
Phyla
	Dicyemida; Monoblastozoa; Orthonectida;