Cnidarians, including jellyfish, corals, and sea wasps, are a phylum of animals with stinging cells called cnidocytes. They are among the most basal animals and have over 10,000 species divided into four main classes. Cnidarians can be beautiful but also painful or deadly, with some species being the most venomous animals on the planet. Coral reefs, made up of cnidarians, are the world’s largest superorganism and home to many marine animals.
Cnidarians are a relatively simple phylum of animals (one in 38) that includes jellyfish, corals, sea pansies, sea pens, box jellies, and sea wasps. The phylum takes the name Cnidaria, from the Greek “cnidos”, which means “stinging needle”.
All cnidarians, including stationary corals (the builders of famous and beautiful coral reefs) have stinging cells called cnidocytes, which receive their sting from organelles called nematocysts (also called cnidocytes or cnidoblasts). Cnidarians are thought to be among the most basal animals, with the exception of sponges and fossils dating to the Ediacaran period, 580 million years ago. The Cnidarians left behind some of the earliest clearly identifiable animal fossils.
There are over 10,000 species of cnidarians, divided into four main classes: Anthozoa (anemones, corals), Scyphozoa (true jellyfish), Cubozoa (box jellies), and Hydrozoa (Obelia, Aequorea, Portuguese Man o’ War, others). minor groups include Polypodium (strange parasitic cnidarians, one of the few animals to live in the cells of other animals) and Myxozoa, tiny fish/worm parasites. The classification of these last two groups within Cnidaria is relatively recent and was made possible by genetic analysis.
Cnidarians have a reputation for being beautiful but painful or even deadly. Every year, thousands of people are stung by jellyfish, resulting in extreme pain for many and even death for some. In Australia and other high-risk locations, selected beaches are fenced off with netting to protect swimmers from painful contact by cnidarians. Jelly boxes are especially dangerous, and one species, the Irukandji jellyfish, is considered one of the most venomous animals on the planet. One victim said: “I didn’t think it was possible for anyone to endure that level of pain without turning into a vegetable.”
Cnidarians (coral) make up the world’s largest superorganism, the Great Barrier Reef in Australia, covering an area of approximately 344,400 square km (132,974 sq mi). This reef, and many others around the world, have grown little by little over thousands of years, as coral polyps die, leaving their skeletons behind, then sprout to form new reef tracts. These coral reefs are home to many marine animals, including the majestic green sea turtle.
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