Cnidarians are an entirely aquatic phylum of animals. They include jellyfish and corals, along with some lesser known creatures. Most species are marine, but a few burrow under the seafloor or live in freshwater lakes and rivers.
Cnidarians live in waters ranging from polar to tropical. They are a very ancient phylum and have survived environmental changes in the world's waterways from the late Precambrian onward. Although there are species which live in very cold water, such as in the Arctic Ocean, global warming has been blamed for increasing numbers of jellyfish blooms in more temperate waters. Jellyfish are important to oceanic ecosystems as both predator and prey, but other cnidarians play an even more vital environmental role.
The largest and most extensive collection of coral is found in the tropical waters off the coast of northeastern Australia. The Great Barrier Reef is an ecosystem unto itself, supporting 1,500 kinds of fish and 4,000 kinds of mollusks, in addition to the 400 species of coral which make up the reef and many species of sea anemone, another kind of cnidarian. Other coral reefs exist in warm, shallow waters around the world, especially off the eastern edge of small islands. They prefer temperatures between 68 and 82 degrees Fahrenheit and locations with plenty of breaking waves to sweep in nutrients.