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Medusozoa. Red Medusa. Medusozoa is a clade in the phylum Cnidaria, and is often considered a subphylum.[2][3] It includes the classes Hydrozoa, Scyphozoa, Staurozoa and Cubozoa, and possibly the parasitic Polypodiozoa. Medusozoans are distinguished by having a medusa stage in their often complex life cycle, a medusa typically being an umbrella-shaped body with stinging tentacles around the edge.[4] With the exception of some Hydrozoa (and Polypodiozoa), all are called jellyfish in their free-swimming medusa phase.[3][5]

Medusozoa

Temporal range: Cambrian–Recent 

PreꞒ

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S

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C

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Pacific sea nettles, Chrysaora fuscescensScientific classificationKingdom:AnimaliaPhylum:CnidariaSubphylum:MedusozoaClasses[1]

Hydrozoa – hydrozoans

Acraspeda

Cubozoa – box jellyfish

Scyphozoa – true jellyfish

Staurozoa – stalked jellyfish

Polypodiozoa – a parasite

EvolutionEdit

The phylum Cnidaria is widely accepted as being monophyletic and consisting of two clades, Anthozoa and Medusozoa. Anthozoa includes the classes Hexacorallia, the hard corals, and Octocorallia, the soft corals, as well as Ceriantharia, the tube-dwelling anemones. There is strong support for this group having been the first to branch off from the ancestral line.[6]

Medusozoa includes the classes Staurozoa, Cubozoa, Scyphozoa and Hydrozoa, but the relationships between these are unclear. Analysis using ribosomal RNA subunits suggests that within Medusozoa, Staurozoa was the first group to diverge, with Cubozoa and Scyphozoa forming a clade, a sister group to Hydrozoa. Further study involving the order of mitochondrial genes supports this view,[6] and their possession of linear mitochondrial genomes is striking evidence of the monophyly of medusozoans.[7] The stem group of Medusozoa also includes Auroralumina attenboroughii, the earliest known animal predator.[8]

The affinities of the class Polypodiozoa, containing the single species Polypodium hydriforme, have long been unclear. This species is an endoparasite of fish eggs and has a peculiar life cycle. It has traditionally been considered to be a cnidarian because of its possession of nematocysts, but molecular studies using 18S rDNA sequences have placed it closer to Myxozoa. Further studies involving 28S rDNA sequences suggest that it is either part of the hydrozoan clade Leptothecata, or a sister taxon to Hydrozoa, and does not group with myxozoans.[9]

Animalia

Ctenophora 

Cnidaria

Anthozoa 

stem‑group Medusozoa



crown‑group Medusozoa

Staurozoa 

Hydrozoa 

Polypodiozoa ? 

Cubozoa 

Scyphozoa 

Bilateria 

CharacteristicsEdit

Medusozoans differ from anthozoans in having a medusa stage in their life cycle. The basic pattern is medusa (usually the adult or sexual phase), planula larva, polyp, medusa. Symmetry is tetramerous, with parts in fours or multiples of four.[10] The mitochondrial DNA molecules are linear rather than circular as in anthozoans and almost all other animals.[11] The cnidae, the explosive cells characteristic of the Cnidaria and used in prey capture and defence, are of a single type, there being nematocysts but no spirocysts or ptychocysts.[4] In contrast, the anthozoan life cycle involves a planula larva which settles and becomes a sessile polyp, which is the adult or sexual phase.[10