Jellyfish can be found in oceans throughout the world, and some even live in saltwater lakes. In general, jellyfish prefer the warm temperatures of tropical waters, although there are a few species that dwell in cold or even frigid waters in the Arctic and Antarctic.
Jellyfish can live in many different zones in the ocean, and they range from the open ocean to the water near the coast. They can survive at different depths, depending on the species, with some living in the deeper, dark depths of the ocean and others living closer to the surface.
No true jellyfish dwell in freshwater. Any species that is referred to as a freshwater jellyfish are actually hydrozoans.
Most jellyfish exist well above the ocean floor, floating with the plankton, although there are species that spend their lives at the bottom of the ocean. Some are not able to swim and use their tentacles to "walk" across the ocean floor. Others migrate back and forth to the surface during the day, and then retreat back to the depths at night. If a jellyfish is removed from the water, it collapses and perishes, as the water provides its body with necessary structural support.
Some jellyfish species are kept in aquariums. Although the colored lighting and contrasted backgrounds make the jellyfish appear to glow, many of them are actually so transparent that they would be difficult to see in the wild. Because jellyfish depend on water currents to move them, professional jellyfish exhibits have to use precise pumps to keep the water moving, preventing jellyfish from becoming trapped in corners.