Phylum CNIDARIA

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Phylum CNIDARIA

Corals, Jellyfish, Sea Anemones

Compiler and date details

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

Introduction

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).