Jellyfish - Classification and Evolution

Classification and Evolution

Medusa jellyfish are a life stage exhibited in some species of the phylum cnidaria. Medusa jellyfish belong exclusively to medusozoa, the clade of cnidarians which excludes anthozoa (e.g., corals and anemones). (This suggests that the medusa form evolved after the polyps.)

There are four major classes of medusozoan cnidaria:

• Scyphozoa are often called true jellyfish. They have tetra-radial symmetry. They have tentacles around the outer margin of the bowl-shaped bell, and oral arms around the mouth.
• Cubozoa (box jellyfish) have a box-shaped bell, and their velarium assists them to swim more quickly. Box jellyfish may be related more closely to "true jellyfish" than either are to hydrozoa.
• Hydrozoa may form medusa which resemble scyphozoans (but generally with a velum) and are distinguished by an absence of cells in the mesoglea. However, many hydrozoa species do not form medusa at all (such as hydra, which is hence not considered a jellyfish).
• Staurozoa (stalked jellyfish) do not have a polyp stage, however the medusa is generally sessile, oriented upside down and with a stalk from the "bell" planted to the substrate. Until recently, staurozoa was classified within scyphozoa.

Some other animals are frequently associated with or mistaken for medusa jellyfish.

• Siphonophorae are an order of hydrozoa which generally live as colonies (for example, free swimming chains of repeated units, some units similar to polyps or to medusa). They are not considered medusa jellyfish. A well known example is the Portuguese Man o' War.
• Ctenophores (comb jellies) are a separate phylum from cnideria. Their method of propolsion is cillia paddles rather than a pulsating bell.
• Salps are transparent, gelatinous marine organisms that form pelagic colonies like siphonophores. Salps are chordates, and as such are actually more closely related to humans than they are to cnidarians and comb jellies.

There are over 200 species of Scyphozoa, about 50 species of Staurozoa, about 20 species of Cubozoa, and in Hydrozoa there are about 1000-1500 species that produce medusa (and many more hydrozoa species that do not).