Cnidaria life cycle

From Palaeos

o Radiata (grade)
|?--Ctenophora
`--+--Cnidaria  
   |  |--Anthozoa
   |  `--+--Hydrozoa
   |     `--+--+--Scyphozoa
   |        |  `--Cubozoa
   |        `?-Conulata (polyphyletic?)
   `--Bilateria

Development of a cnidarian. From "Die Entwicklung der Meduse", in: "Das Meer" by Matthias Jacob Schleiden (1804-1881)

the following material is from Wikipedia:

Asexual reproduction via budding is common among cnidaria, particularly among the Hydrozoa. Asexual larvae bud laterally from the adult polyps, which develop into polyps themselves. The budding is often incomplete, so that colonies of genetically identical polyps physically connected with each other can form.

However, cnidaria can also reproduce sexually. A characteristic here is the alternation of generations, in which asexually reproducing generations alternate with generations that reproduce sexually, which is otherwise not as common among animals as among plants, fungi and protists. This particular form of alternation of generations is known as metagenesis.

For this purpose the adult polyp forms male or female medusae asexually. There are three principal asexual events:

• Budding is particularly common among Hydrozoa.
• Strobilation occurs when a medusa forms on the higher (oral) end of the polyp, and is common among Scyphozoa.
• Finally, complete metamorphosis from polyp to medusa form can also occur.

These then develop to sexual maturity, at which point the male and female gametes are released, which each unite to form a zygote. These develop through cell division into a spherical structure, the blastula, from which the larva (or planula) forms. The larva is flagellate and swims until it encounters a firm substrate, on which it anchors itself and then passes through metamorphosis to the polyp stage.

This process varies significantly between the four classes of cnidaria. Among many Hydrozoa the medusae remain on the polyps in a reduced form, known as gonophores. A few Hydrozoa, such as the hydra, have no medusa stage whatsoever; instead the polyp itself forms male or female gametes. Within group Anthozoa, the medusal stage is virtually non-existent; the larva, once fusing with the substratum and developing into the polyp stage, grows benthic or sessile, meaning it no longer metamorphosises into the medusal stage. Among the Scyphozoa and Cubozoa, the medusae are the dominant form in the life-cycle, while the polyps are in turn reduced or absent.

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