| Medusozoa | |
|---|---|
 | |
| Pacific sea nettles,Chrysaora fuscescens | |
Scientific classification | |
| Domain: | Eukaryota |
| Kingdom: | Animalia |
| Phylum: | Cnidaria |
| Subphylum: | Medusozoa |
| Classes[1] | |
| |
Medusozoa is aclade in thephylumCnidaria, and is often considered a subphylum.[2][3] It includes the classesHydrozoa,Scyphozoa,Staurozoa andCubozoa, and possibly the parasiticPolypodiozoa. 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 calledjellyfish in their free-swimmingmedusa phase.[3][5]
The phylumCnidaria is widely accepted as beingmonophyletic and consisting of two clades,Anthozoa and Medusozoa. Anthozoa includes theclassesHexacorallia, the hard corals, andOctocorallia, the soft corals, as well asCeriantharia, 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 classesStaurozoa,Cubozoa,Scyphozoa andHydrozoa, but the relationships between these are unclear. Analysis usingribosomal 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 includesAuroralumina attenboroughii, the earliest known animalpredator from the lateEdiacaran.[8]Burgessomedusa from the mid-Cambrian Burgess Shale is the oldest known free-living medusa (commonly known as jellyfish).[9]
The affinities of the classPolypodiozoa, containing the single speciesPolypodium hydriforme, have long been unclear. This species is anendoparasite 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 using18S rDNA sequences have placed it closer toMyxozoa. Further studies involving28S rDNA sequences suggest that it is either part of the hydrozoan cladeLeptothecata, or a sister taxon to Hydrozoa, and does not group with myxozoans.[10]
Medusozoans differ from anthozoans in having amedusa 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.[11] Themitochondrial DNA molecules are linear rather than circular as in anthozoans and almost all other animals.[12] The cnidae, the explosive cells characteristic of the Cnidaria and used in prey capture and defence, are of a single type, there beingnematocysts but no spirocysts or ptychocysts.[4] In contrast, the anthozoan life cycle involves a planula larva which settles and becomes asessile polyp, which is the adult or sexual phase.[11]
There is considerable divergence from the basic life cycle pattern among medusozoans.[11]
Scyphozoa is the group commonly known as "true jellyfish" and occur in tropical, temperate and polar seas worldwide. Scyphozoans generally have planula larvae that develop into sessile polyps. These reproduce asexually, producing similar polyps by budding, and then either transform into medusae, or repeatedly bud medusae from their upper surface in a process known asstrobilation.[4]
Cubozoa is a group commonly known as box jellyfish, that occur in tropical and warm temperate seas. They have cube-shaped, transparent medusae and are heavily-armed with venomous nematocysts. Cubozoans have planula larvae, which settle and develop into sessile polyps, which subsequently metamorphose into sexual medusae,[11] the oral end of each polyp changing into a medusa which separates and swims away.[4]
Staurozoa is a small group commonly known as stalked jellyfish. The animals remain attached to thesubstrate by a stalk at the opposite end from the mouth. Staurozoans can be regarded as large polyps that have partially differentiated into sexually mature medusae. These spawn gametes which develop into non-swimming planulae that crawl away to new locations.[4]
Hydrozoa is a large group of solitary and colonial cnidarians from both marine and freshwater environments worldwide. Hydrozoans exhibit the greatest variety of life cycles among medusozoans, with either the polyp or the medusa stage being missing in some groups.[4] In general, medusae are budded laterally from polyps, become mature and spawn, releasinggametes into the water. The planulae may settle to become polyps or continue living in the water column as medusae.[11]
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