Jellyfish belong to a group of sea creatures, including coral, that attach to rocks and other hard surfaces. However, jellyfish are mobile, either swimming by their own power or moved by currents and wind. Jellyfish don't have brains, but their nervous systems detect smells, light and other stimuli, and they coordinate their physical responses.
Jellyfish have a gel-like, colorful and round body from which other body parts stretch outward. This body structure allows jellyfish to sense and react to danger or food sources in any direction. The animals capture prey with tentacles and move the food into their mouths with as many as eight "oral arms."
Jellyfish are common in all the world's oceans at varying depths, and a few live in fresh bodies of water. They can withstand a range of temperatures and various levels of salt in water. They have lived in the oceans for as many as 700 million years. Since jellyfish are not actually fish, some aquariums use the terms "jellies" or "sea jellies" for the animals. Many jellyfish together are known as a "bloom," "swarm" or "smack," with "bloom" referring to a very large group of the animals gathering in a small area. Jellyfish often appear suddenly in large numbers. This typically occurs during spring, which is when there are more plankton available as a food source.