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Corals, Jellyfish, Sea Anemones

Compiler and date details

March 2015 (Medusozoa) - Dr Lisa-Ann Gershwin, Hobart


Cnidaria comprises in part two reciprocally monophyletic clades, the Anthozoa (sessile; hard & soft corals, sea, anenomes etc.) and Medusozoa (free swimming; jellyfish, box jellyfish, hydrozoans), which are well-supported by morphological, behavioural and molecular characteristics (Salvini-Plawen 1978, Collins 2002, Daly et al., 2007 and references therein). Although the traditional taxonomic structure of Cnidaria mirrors its phylogenetic structure, the ranks of many groups are incompatible with their hierarchical phylogenetic position. For instance class Anthozoa comprises all members of the clade Anthozoa, while its sister taxon, Medusozoa, comprises four classes. Thus, class and other ranks have different phylogenetic implications across the phylum, and the two clades are sometimes given the rank of Sub-Phylum to better reflect phylogenetic arrangement. Here we follow Daly et al. (2007) in treating the members of the two clades as 5 classes.

We differ to Daly et al. (2007) by including the Myxozoa as a member of the Cnidarians. These minute parasitic Cnidarians posses a greatly reduced body plan and genome but possess cell structures similar to Cnidarian nematocysts and suggest homologous characters. Myxozoans were long treated as Protists until molecular phylogenetics established them to be metazoans (Smothers et al.. 1994) after which they have been treated as a class within Cnidaria, and as a phylum in their own right. However recent work by Chang et al.(2015) has shown that the Myxozoa are the sister group of Polypodium hydriforme, a Cnidarian parasite that does not possess the degenerated body plan and displays conventional Cnidarian features. Furthermore the Myxozoan + P. hydriforme clade were recovered as sister to the medusozoan clade with maximal support (Chang et al. 2015, see also Nesnidal et al. 2013, Feng et al. 2014).

The Anthozoa comprises two monophyletic lineages, Octocorallia and Hexacorallia. This has been well supported by molecular studies, and phylogenetic analyses of morphological data also corroborate this hypothesis, suggesting at least three diagnostic apomorphies (see Daly et al., 2007). All members of Anthozoa are exclusively polypoid, and may be colonial, clonal, or solitary, skeleton-less or with a mineralic and/or proteinaceous skeleton. Anthozoa currently contains approximately 7,500 extant species, with around 1450 species recorded from Australia.

The grouping of Medusozoa was proposed by Petersen (1979) for the extinct class Conulata and the Recent classes Scyphozoa, Cubozoa, and Hydrozoa, which are all united by having a sexual medusa stage, including those that are secondarily reduced or absent. The Medusozoa is thus the sister group to the Anthozoa, in which a medusa is not present and the sexual stage is polypoid. Salvini-Plawen (1978) had proposed a similar grouping a short time earlier under the name Tesserozoa, but this was not widely adopted.

The internal and external relationships of the medusozoan classes has been and continues to be debated. As envisaged by Salvini-Plawen (1978), the Conulata was the forerunner of all Recent medusozoans, whereas Petersen (1979) envisaged them in a more equal relationship. Marques & Collins (2004) demonstrated that the Conulata was the sister group to the Stauromedusae, which they assigned collectively to the class Staurozoa.

Currently four classes of medusozoans are recognised: the Scyphozoa, or the so-called "true jellyfishes", including the Semeaostomeae, Rhizostomeae, and Coronatae; the Cubozoa, or the so-called "box jellyfishes", including the Carybdeida and Chirodropida; the Hydrozoa, or the so-called "water jellies" or hydroids, including the hydromedusae, siphonophores, and hydrocorals; and the Staurozoa, or the so-called "stalked jellyfishes".

The classes of medusozoans are quite easy to tell apart most of the time: the scyphozoans tend to be large, fleshy, and brightly coloured, and often have a hemispherical body with marginal tentacles, or well developed oral arms, or both; the cubozoans are typically box-shaped, with eyes in cavities on the flat sides of the bell, and tentacles separated from the bell by gelatinous leg-like "pedalia"; the hydrozoans typically have a benthic hydroid stage, and the medusae are usually small, transparent, and inconspicuous; and the staurozoans have a goblet-shaped body and are affixed to the substrate by means of a stalk.

Additional information about the Medusozoa can be found in Mayer (1910), Russell (1953, 1970), Gwilliam (1956), Totton (1965), Bouillon & Boero (2000) and Gershwin (2005).


General References

Bouillon, J. & Boero, F. 2000. Phylogeny and classification of the Hydroidomedusae. Thalassia Salentina 24: 1–296

Chang, S.E., Neuhof, M., Rubinstein, N.D., Diamante, A., Philippe, H., Huchon, D. & Cartwright, P. 2015. Genomic insights into the evolutionary origin of Myxozoa within Cnidaria. Proceedings of the National Academy of Sciences 112(48): 14912-14917

Collins, A.G. 2002. Phylogeny of Medusozoa and the evolution of cnidarian life cycles. Journal of Evolutionary Biology 15: 418–432

Daly, M., Brugler, M.R., Cartwright, P., Collins, A.G., Dawson, M.N., Fautin, D.G., France, S.C., McFadden, C.S., Opresko, D.M., Rodriguez, E., Romano, S. & Stake, J. 2007. The phylum Cnidaria: A review of phylogenetic patterns and diversity three hundred years after Linneaeus. (pp. 127–182). In: Zhang, Z.-Q. & Shear, W.A. (Eds) Linnaeus Tercentenary: Progress in Invertebrate Taxonomy. Zootaxa 1668: 1–766

Feng, J-M., Xiong, J., Zhang, J-Y., Yang, Y-L., Yao, B., Zhou, Z-G. & Miao, W. 2014. New phylogenomic and comparative analyses provide corroborating evidence that Myxozoa is Cnidaria. Molecular Phylogenetics and Evolution 81: 10-18

Gershwin, L. 2005. Taxonomy and phylogeny of Australian Cubozoa. Unpublished PhD thesis. School of Marine Biology and Aquaculture, James Cook University, Townsville, QLD. 221 pp. 49 pls

Gwilliam, G.F. 1956. Studies on west coast Stauromedusae. Ph.D. Dissertation, University of California, Berkeley. 191 pp.

Kramp, P.L. 1961. Synopsis of the medusae of the world. Journal of the Marine Biological Association of the United Kingdom 40: 1-469

Marques, A.C. & Collins, A.G. 2004. Cladistic analysis of Medusozoa and cnidarian evolution. Invertebrate Biology 123(1): 23–42

Mayer, A.G. 1910. Medusae of the World. Vol. 1 and 2, The Hydromedusae. Vol. 3, The Scyphomedusae. Washington, D.C. : Carnegie Institution 735 pp., 76 pls. [reprinted by A. Asher & Co., 1977]

Nesnidal, M.P., Helmkampf, M., Bruchhaus, I., El-Matbouli, M. & Hausdorf, B. 2013. Agent of whirling disease meets orphan worm: Phylogenomic analyses firmly place Myxozoa in Cnidaria. PLoS ONE (Public Library of Science) 8(1): e54576

Petersen, K.W. 1979. Development of coloniality in the Hydrozoa. pp. 105-109 in Larwood, G. & Rosen, B.R. (eds). Biology and Systematics of Colonial Organisms. Proceedings of an International Symposium held at the University of Durham. (The Systematics Association, Special Vol. No. 11). London : Academic Press.

Russell, F.S. 1953. The Medusae of the British Isles. Anthomedusae, Leptomedusae, Limnomedusae, Trachymedusae and Narcomedusae. London : Cambridge University Press 530 pp., pls 1-35.

Russell, F.S. 1970. Medusae of the British Isles. II. Pelagic Scyphozoa with a Supplement to the First Volume on Hydromedusae. Cambridge : Cambridge University Press 284 pp.

Salvini-Plawen, L. 1978. On the origin and evolution of the Lower Metazoa. Zeitschrift für Zoologische Systematik und Evolutionsforschung 16: 40–88

Smothers, J.F., von Dohlen, C.D., Smith, L.H.,Jr. & Spall, R.D. 1994. Molecular evidence that the myxozoan protists are metazoans. Science 265(5179): 1719-1721

Totton, A.K. 1965. A Synopsis of the Siphonophora. London : British Museum of Natural History.


History of changes

Note that this list may be incomplete for dates prior to September 2013.
Published As part of group Action Date Action Type Compiler(s)
05-Aug-2022 CNIDARIA 23-May-2022 MODIFIED
10-Feb-2015 MODIFIED