Orthonetida dict flat and cylinder.png
Two different female Orthonectids
Scientific classification e
Kingdom: Animalia
(unranked): Mesozoa
Phylum: Orthonectida
Giard, 1877 [1][2]

See text.

Orthonectida (/ˌɔːrθəˈnɛktɪdə, -θ-/[3]) is a small phylum of poorly known parasites of marine invertebrates[4] that are among the simplest of multi-cellular organisms. Members of this phylum are known as orthonectids.


The adults are microscopic wormlike animals, consisting of a single layer of ciliated outer cells surrounding a mass of sex cells. They swim freely within the bodies of their hosts, which include flatworms, polychaete worms, bivalve molluscs, and echinoderms. They are gonochoristic, with separate male and female individuals.[5]

When they are ready to reproduce, the adults leave the host, and sperm from the males penetrate the bodies of the females to achieve internal fertilisation. The resulting zygote develops into a ciliated larva that escapes from the mother to seek out new hosts. Once it finds a host, the larva loses its cilia and develops into a syncytial plasmodium larva. This, in turn, breaks up into numerous individual cells that become the next generation of adults.[5][6]


The phylum consists of about 20 known species, of which Rhopalura ophiocomae is the best-known.[4] The phylum is not divided into classes or orders, and contains just two families.

Although originally described in 1877 as a class,[7] and sometimes characterized as an order of the phylum Mesozoa, recent study shows that orthonectids are quite different from the rhombozoans, the other group in Mesozoa.[4] The genome of one species, , has been sequenced.[8] These animals are simplified spiralians. Their position in the phylogenetic tree has yet to be determined. The genome data confirms the earlier proposal that these organisms are spiralians based on their morphology.[9]

They appear to be related to the Annelida.[10][6] Of the Annelida this taxon appears to be most closely related to the Clitellata.[11]

Known species

Phylum Orthonectida

  • Family Stunkard, 1937
    • Tajika, 1979 – Hokkaido, Japan; in flatworms (Turbellaria)
    • (Caullery and Mesnil, 1899) – E North Atlantic, in polychaetes
    • Kozloff, 1965 – San Juan Islands, WA (USA); in polychaete (Neosabellaria cementarium)
    • Giard, 1877 – E North Atlantic, in flatworms ()
    • Giard, 1877 – E North Atlantic, in nemertines (Lineus) = Rhopalura linei
    • Shtein, 1953 – Arctic Ocean; in gastropods (Lepeta, Natica, Solariella) = Rhopalura major
    • (Caullery & Mesnil, 1899) – E North Atlantic, in polychaetes and nemertines
    • (Westblad, 1942) – E North Atlantic, in flatworms (Acoela)
    • (Alexandrov & Sljusarev, 1992) – Arctic Ocean, in flatworms (Macrorhynchus)
    • Shtein, 1953 – Arctic Ocean, in bivalves (Astarte)
    • (Giard, 1877)
    • Atkins, 1933 – E North Atlantic, in bivalves ()
    • Metchnikoff – Mediterranean, in nemertines
    • Shtein, 1953 – Arctic Ocean, in gastropods (Lepeta, Natica, Solariella)
    • Shtein, 1953
    • Shtein, 1953 – Arctic Ocean, in gastropods (Rissoa, Columbella)
    • Giard, 1877 – E North Atlantic, in ophiuroids (usually Amphipholis)
    • Caullery & Mesnil, 1901 – E North Atlantic, polychaetes and nemertines
    • Lang, 1954 – E North Atlantic, in gastropods
    • de Saint-Joseph, 1896 – E North Atlantic, in polychaetes
    • Kozloff, 1993
    • Kozloff, 1993
    • Caullery & Mesnil, 1899 – E North Atlantic, in polychaetes
    • Kozloff, 1993 – E North Atlantic, in molluscs
  • Family Stunkard, 1937
    • Caullery and Mesnil, 1904 – E North Atlantic, in polychaetes and nemertines


  1. ^ H. Furuya & J. van der Land (2010). "Orthonectida". World Register of Marine Species. Retrieved January 12, 2011.
  2. ^ "Orthonectida Giard, 1877". Integrated Taxonomic Information System. Retrieved January 12, 2011.
  3. ^ "Orthonectida". Oxford Dictionaries UK Dictionary. Oxford University Press. Retrieved 2016-01-21.
  4. ^ a b c Hanelt, B; Van Schyndel, D; Adema, C. M; Lewis, L. A; Loker, E. S (1996). "The phylogenetic position of Rhopalura ophiocomae (Orthonectida) based on 18S ribosomal DNA sequence analysis". Molecular Biology and Evolution. 13 (9): 1187–91. doi:10.1093/oxfordjournals.molbev.a025683. PMID 8896370.
  5. ^ a b Robert D. Barnes (1982). Invertebrate Zoology. Philadelphia, PA: Holt-Saunders International. pp. 247–248. ISBN 0-03-056747-5.
  6. ^ a b Zverkov, Oleg A.; Mikhailov, Kirill V.; Isaev, Sergey V.; Rusin, Leonid Y.; Popova, Olga V.; Logacheva, Maria D.; Penin, Alexey A.; Moroz, Leonid L.; Panchin, Yuri V.; Lyubetsky, Vassily A.; Aleoshin, Vladimir V. (24 May 2019). "Dicyemida and Orthonectida: Two Stories of Body Plan Simplification". Frontiers in Genetics. 10: 443. doi:10.3389/fgene.2019.00443. PMC 6543705. PMID 31178892.
  7. ^ Alfred Mathieu Giard (1877). "Sur les Orthonectida, classe nouvelle d'animaux parasites des Échinodermes et des Turbellariés" [On Orthonectida, a new class of parasitic animals of Echinoderms and Turbellarians]. Comptes Rendus (in French). 85 (18): 812–814.
  8. ^ Mikhailov, Kirill V; Slyusarev, Georgy S; Nikitin, Mikhail A; Logacheva, Maria D; Penin, Aleksey A; Aleoshin, Vladimir V; Panchin, Yuri V (2016). "The Genome of Intoshia linei Affirms Orthonectids as Highly Simplified Spiralians". Current Biology. 26 (13): 1768–74. doi:10.1016/j.cub.2016.05.007. PMID 27374341.
  9. ^ Sliusarev, G. S (2008). "Тип ортонектида (Orthonectida): строение, биология, положение в системе многоклеточных животных" [Phylum Orthonectida: Morphology, biology, and relationships to other multicellular animals]. Zhurnal Obshchei Biologii (in Russian). 69 (6): 403–27. PMID 19140332.
  10. ^ Bondarenko, N.; Bondarenko, A.; Starunov, V.; Slyusarev, G. (8 March 2019). "Comparative analysis of the mitochondrial genomes of Orthonectida: insights into the evolution of an invertebrate parasite species". Molecular Genetics and Genomics. 294 (3): 715–727. doi:10.1007/s00438-019-01543-1. PMID 30848356.
  11. ^ Slyusarev, George S.; Starunov, Viktor V.; Bondarenko, Anton S.; Zorina, Natalia A.; Bondarenko, Natalya I. (April 2020). "Extreme Genome and Nervous System Streamlining in the Invertebrate Parasite Intoshia variabili". Current Biology. 30 (7): 1292–1298.e3. doi:10.1016/j.cub.2020.01.061. PMID 32084405.

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