The Mesozoa (singular: mesozoan) are minuscule, worm-like parasites of marine invertebrates. 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 monophyletic, and emerged in the Lophotrochozoa as sister of the Rouphozoa.[1]

Dicyema macrocephalum
Scientific classification
Kingdom: Animalia
Clade: ParaHoxozoa
Clade: Bilateria
Clade: Nephrozoa
(unranked): Protostomia
(unranked): Spiralia
Clade: Platytrochozoa
(unranked): Mesozoa
van Beneden, 1876

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]


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 sporocysts 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.


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][5] As described, the animal had only a single layer of tissue. [6]

Rhombozoan mesozoans

Rhombozoa, or dicyemid mesozoans, are found in the nephrid tracts of squid and octopuses.[7] 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.

Molecular evidence suggests that this phylum are derived from the Lophotrochozoa.[8][9]

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.[10]

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 dioecious (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.

The genome of one of these species – Intoshia linei – has been sequenced.[11] The Orthonectids may be degenerate annelid worms.[5][12]


  1. Lu, Tsai-Ming; Kanda, Miyuki; Satoh, Noriyuki; Furuya, Hidetaka (2017-05-29). "The phylogenetic position of dicyemid mesozoans offers insights into spiralian evolution". Zoological Letters. 3: 6. doi:10.1186/s40851-017-0068-5. ISSN 2056-306X. PMC 5447306. PMID 28560048.
  2. Hanelt, B.; Van Schyndel, D.; Adema, C.M.; Lewis, L.A.; Loker, E.S. (November 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 2013-03-14.
  3. Syed, Tareq; Schierwater, Bernd (December 2002). "Trichoplax adhaerens: discovered as a missing link, forgotten as a hydrozoan, re-discovered as a key to metazoan evolution". Vie et Milieu. 52 (4): 177–187.
  4. "Salinella: Monoblastozoa". Animal Life Resource. JRank.
  5. Philipp, Schiffer; Helen, Robertson; Maximilian, Telford (2018-04-03). "Orthonectids are highly degenerate annelid worms". Zenodo. doi:10.5281/zenodo.1206208.
  6. de Meeûs, Thierry; Renaud, François (July 2002). "Parasite within the new phylogeny of eukaryotes". Trends in Parasitology. 18 (6): 247–51. doi:10.1016/S1471-4922(02)02269-9. PMID 12036736.
  7. Hochberg, F. G. (30 June 1983). "THE PARASITES OF CEPHALOPODS: A REVIEW". Memoirs of the National Museum of Victoria. 44: 109–145. doi:10.24199/j.mmv.1983.44.10.
  8. Kobayash, M.; Furuya, H.; Wada, H. (September–October 2009). "Molecular markers comparing the extremely simple body plan of dicyemids to that of lophotrochozoans: insight from the expression patterns of Hox, Otx, and brachyury". Evol Dev. 11 (5): 582–589. doi:10.1111/j.1525-142X.2009.00364.x. PMID 19754714.
  9. Suzuki, T. G.; Ogino, K.; Tsuneki, K.; Furuya, H. (June 2010). "Molecular markers comparing the extremely simple body plan of dicyemids to that of lophotrochozoans: insight from the expression patterns of Hox, Otx, and brachyury". Journal of Parasitology. 96 (3): 614–625. doi:10.1645/GE-2305.1. PMID 20557208.
  10. Barnes, Robert D. (1982). Invertebrate Zoology. Philadelphia, PA: Holt-Saunders International. pp. 247–248. ISBN 978-0-03-056747-6.
  11. Mikhailov, K. V.; Slyusarev, G. S.; Nikitin, M. A.; Penin, A. A.; Aleoshin, V. V.; Panchin, Y. V. (11 July 2016). "The Genome of Intoshia linei Affirms Orthonectids as Highly Simplified Spiralians". Curr Biol. 26 (13): 1768–74. doi:10.1016/j.cub.2016.05.007. PMID 27374341.
  12. Telford, Maximilian J.; Robertson, Helen E.; Schiffer, Philipp H. (2018-06-18). "Orthonectids Are Highly Degenerate Annelid Worms". Current Biology. 28 (12): 1970–1974.e3. doi:10.1016/j.cub.2018.04.088. ISSN 0960-9822. PMID 29861137.
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