Jellyfish and sea jellies are the informal common names given to the medusa-phase of certain
gelatinous members of subphylum Medusozoa, the majority of phylum Cnidaria. Jellyfish are
mainly free-swimming marine animals with umbrella-shaped bells and trailing tentacles, although
a few are not mobile, being anchored to the seabed by stalks. The bell can pulsate to give
propulsion and highly efficient locomotion. Tentacles are armed with stinging cells and may be
used to capture prey and defend against predators. Jellyfish have a complex life cycle; the medusa
is normally the sexual phase, the planula larva can disperse widely and is followed by a sedentary
polyp phase.
Jellyfish are found worldwide, from surface waters to the deep sea. Scyphozoans ("true jellyfish")
are exclusively marine, but some hydrozoans with a similar appearance live in freshwater.
Large, often colorful, jellyfish are common in coastal zones worldwide. The medusae of most
species are fast growing, mature within a few months and die soon after breeding, but the polyp < br />stage, attached to the seabed, may be much more long-lived. Jellyfish have been in existence for
at least 500 million years, and possibly 700 million years or more, making them the oldest multiorgan
animal group. They are eaten by humans in certain cultures, being considered a delicacy
in some Asian countries, where species in the Rhizostomae order are pressed and salted to remove
excess water. They are also used in research, where the green fluorescent protein, used by
some species to cause bioluminescence, has been adapted as a fluorescent marker for genes inserted
into other cells or organisms. Stinging cells used by jellyfish to subdue their prey can also
injure them. Many thousands of swimmers are stung every year, with effects ranging from mild
discomfort to serious injury or even death; small box jellyfish are responsible for many deaths.