What are the Major Groups of Cnidarians? (with pictures)

Michael Anissimov
Michael Anissimov
Jellyfish are cnidarians.
Jellyfish are cnidarians.

Cnidarians, members of phylum Cnidaria, are among the simplest and most basal animals, splitting off from other phyla even before sponges did, as was discovered by molecular studies in 2008. They are among the earliest animal fossils with recognizable affinity to modern groups. The four main cnidarian groups (or classes) are Anthozoa (anemones, corals, etc.), Scyphozoa (jellyfish), Staurozoa (stalked jellyfish), Cubozoa (box jellies), Hydrozoa (Obelia, Portuguese Man o' War, Aequorea, etc.), and Polypodiozoa, which includes a single species, Polypodium hydriforme, a small parasite.

Anemones belong to the Anthozoa class of cnidarians.
Anemones belong to the Anthozoa class of cnidarians.

Like organisms in the Ctenophora phylum, cnidarians are diploblastic, meaning their body plan consist of just two germ layers, in contrast to all other animal phyla except sponges. This means that they lack true organs and a coelom, or body cavity. Though superficially simple, modern members of this phyla are actually genetically sophisticated and biochemically complex.

The Great Barrier Reef off the northeast coast of Australia is the world's largest coral reef system.
The Great Barrier Reef off the northeast coast of Australia is the world's largest coral reef system.

The phylum gets its name from the Greek word for sting because its members feed and defend themselves using specialized cells called cnidocytes, which contain small harpoon-like stingers. The release of these "harpoons" is among the quickest known processes in nature, occurring in as little as 600 nanoseconds and reaching an acceleration of around five million G's.

Coral is a member of the Anthozoa class of cnidarians.
Coral is a member of the Anthozoa class of cnidarians.

Cnidarians are either stationary, like the Anthozoa, or mobile, like all the other groups. Corals, among the most recognizable of the anthozoans, are known for the huge reef structures they create, which can be 328 ft (100 m) tall and cover areas as large as 133,000 sq mi (344,400 square km), as is the case with the Great Barrier Reef off the northeast coast of Australia. Like other members of this phyla, corals can reproduce either asexually, by budding, or sexually, by releasing sperm into the water.

Hydra are a genus of small multicellular organisms belonging to the phylum Cnidaria.
Hydra are a genus of small multicellular organisms belonging to the phylum Cnidaria.

These organisms have two major body forms: the polyp, which is a stalk with cnidocytes at its top, or the medusa, which is the iconic shape of the jellyfish. While anthozoans stay in the polyp stage their whole life, hydrozoans generally have life stages that go through both, while among jellyfish, the medusa body plan is dominant. A polyp can metamorphisize into a medusa by detaching from its substrate and developing specialized tissues in a process called strobilation, which allows a high throughput of offspring.

Michael Anissimov
Michael Anissimov

Michael is a longtime contributor who specializes in topics relating to paleontology, physics, biology, astronomy, chemistry, and futurism. In addition to being an avid blogger, Michael is particularly passionate about stem cell research, regenerative medicine, and life extension therapies. He has also worked for the Methuselah Foundation, the Singularity Institute for Artificial Intelligence, and the Lifeboat Foundation.

Michael Anissimov
Michael Anissimov

Michael is a longtime contributor who specializes in topics relating to paleontology, physics, biology, astronomy, chemistry, and futurism. In addition to being an avid blogger, Michael is particularly passionate about stem cell research, regenerative medicine, and life extension therapies. He has also worked for the Methuselah Foundation, the Singularity Institute for Artificial Intelligence, and the Lifeboat Foundation.

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    • Jellyfish are cnidarians.
      Jellyfish are cnidarians.
    • Anemones belong to the Anthozoa class of cnidarians.
      Anemones belong to the Anthozoa class of cnidarians.
    • The Great Barrier Reef off the northeast coast of Australia is the world's largest coral reef system.
      The Great Barrier Reef off the northeast coast of Australia is the world's largest coral reef system.
    • Coral is a member of the Anthozoa class of cnidarians.
      Coral is a member of the Anthozoa class of cnidarians.
    • Hydra are a genus of small multicellular organisms belonging to the phylum Cnidaria.
      Hydra are a genus of small multicellular organisms belonging to the phylum Cnidaria.