There are three major classes of jellyfish under Cnidaria, namely Scyphozoa (true jellyfish), Hydrozoa (water jellyfish) and Cubozoa (box jellyfish). Amongst the three, jellyfish under the class Hydrozoa records the most speciose of sea jellies, with over 3,500 described species.
Jellyfish are the simplest swimming animals on Earth. They are composed of about 95 percent water. The high-water content of jellyfish explains why they immediately collapse into defeated, deflated blobs when removed from water.
They glow in the dark, in many different colors such as green, blue, and purple. They make their own light through bioluminescence. This process is common for animals living in the deep ocean, as it is too deep for sunlight to reach.
Jellyfish have a bell-shaped body (umbrella) of different sizes, with a varying number of tentacles, depending on the species. The tentacles reach from a few millimeters up to 40 m in length, depending on the species, with their color ranging from transparent to whitish, yellowish, purple or bluish.
Jellyfish are found in all aquatic environments on Earth, from freshwater to seawater, from coastal to open ocean, and from surface waters to the deep sea. This is possible because jellyfish have a life cycle that adjusts to its environment and they can adapt to environmental changes.
All true jellyfish and some species of jellyfish-like creatures sting; a single stinging tentacle may be studded with thousands of stingers. Stinging gelatinous creatures cause various reactions in people, ranging from no noticeable sensation to rashes, and in relatively rare cases, death.
Jellyfish
