Jellyfish or jellies are the major non-polyp form of individuals of the phylum Cnidaria. They are typified as free-swimming marine animals consisting of a gelatinous umbrella-shaped bell and trailing tentacles. The bell can pulsate for locomotion, while stinging tentacles can be used to capture prey. Jellyfish are found in every ocean, from the surface to the deep sea. Scyphozoans are exclusively marine, but some hydrozoans live in freshwater. Large, often colorful, jellyfish are common in coastal zones worldwide. Jellyfish have roamed the seas for at least 500 million years, and possibly 700 million years or more, making them the oldest multi-organ animal.
The English popular name jellyfish has been in use since 1796. It has traditionally also been applied to other animals sharing a superficial resemblance, for example ctenophores (members from another phylum of common, gelatinous and generally transparent or translucent, free-swimming planktonic carnivores now known as comb jellies) were included as "jellyfishes". Even some scientists include the phylum ctenophora when they are referring to jellyfish. Other scientists prefer to use the more all-encompassing term gelatinous zooplankton, when referring to these, together with other soft-bodied animals in the water column.
Most jellyfish do not have specialized digestive, osmoregulatory, central nervous, respiratory, or circulatory systems. The manubrium is a stalk-like structure hanging down from the centre of the underside, often surrounded by oral arms, which connects with the mouth/anus at the base of the bell. This opens into the gastrovascular cavity, where digestion takes place and nutrients are absorbed. It is joined to the radial canals which extend to the margin of the bell, where tentacles are attached. Nematocysts, which deliver the sting, are located mostly on the tentacles; Scyphozoans also have them around the mouth and stomach. Jellyfish do not need a respiratory system since their skin is thin enough that the body is oxygenated by diffusion. They have limited control over movement, but can use their hydrostatic skeleton to navigate through contraction-pulsations of the bell-like body; some species actively swim most of the time, while others are mostly passive. Depending on the species, the body contains between 95 and 98% water. Most of the umbrella mass is a gelatinous material — the jelly — called mesoglea which is surrounded by two layers of protective skin. The top layer is called the epidermis, and the inner layer is referred to as gastrodermis, which lines the gut.
Jellyfish employ a loose network of nerves, located in the epidermis, which is called a "nerve net". Although traditionally thought not to have a central nervous system, nerve net concentration and ganglion-like structures could be considered to constitute one in most species. A jellyfish detects various stimuli including the touch of other animals via this nerve net, which then transmits impulses both throughout the nerve net and around a circular nerve ring, through the rhopalial lappet, located at the rim of the jellyfish body, to other nerve cells.
Some jellyfish have ocelli: light-sensitive organs that do not form images but which can detect light and are used to determine up from down, responding to sunlight shining on the water's surface. These are generally pigment spot ocelli, which have some cells (not all) pigmented. Certain species of jellyfish, such as the box jellyfish, have more advanced vision than their counterparts. The box jellyfish has 24 eyes, two of which are capable of seeing color, and four parallel information processing areas or rhopalia that act in competition, supposedly making it one of the few creatures to have a 360-degree view of its environment. The eyes are suspended on stalks with heavy crystals on one end, acting like a gyroscope to orient the eyes skyward. They look upward to navigate from roots in mangrove swamps to the open lagoon and back, watching for the mangrove canopy, where they feed.
Jellyfish range from about one millimeter in bell height and diameter to nearly 2 metres (6.6 ft) in bell height and diameter; the tentacles and mouth parts usually extend beyond this bell dimension. The smallest jellyfish are the peculiar creeping jellyfish in the genera Staurocladia and Eleutheria, which have bell disks from 0.5 mm to a few millimeters in diameter, with short tentacles that extend out beyond this, which these jellyfish use to move across the surface of seaweed or the bottoms of rocky pools. Many of these tiny creeping jellyfish cannot be seen in the field without a hand lens or microscope; they can reproduce asexually by splitting in half (called fission). Other very small jellyfish, which have bells about one millimeter, are the hydromedusae of many species that have just been released from their parent polyps; some of these live only a few minutes before shedding their gametes in the plankton and then dying, while others will grow in the plankton for weeks or months. The hydromedusae Cladonema radiatum and Cladonema californicum are also very small, living for months, yet never growing beyond a few mm in bell height and diameter. Another small species of jellyfish is the Australian Irukandji, which is about the size of a fingernail.
Jellyfish belong to Medusozoa, the clade of cnidarians which excludes Anthozoa (e.g., corals and anemones). This suggests that the medusa form evolved after the polyps. The phylogenetics of this group are complex and evolving. The Medusozoa and Octocorallia are proposed as sister groups according to research published in 2012. That research also proposes coronate Scyphozoa and Cubozoa as a sister clade to Hydrozoa and discomedusan Scyphozoa, which are themselves sister groups. The hydroidolinans are a sister group to Limnomedusae, also called Trachylina. Semaeostomae is paraphyletic with Rhizostomeae. The class Storozoa was the earliest group of Medusozoa to diverge and the Limnomedusae were the earliest Hydrozoa to diverge.
Jellyfish lifespans typically range from a few hours (in the case of some very small hydromedusae) to several months; there are some indications that deep sea species may live on the order of years. Life span varies by species. Most large coastal jellyfish live 2 to 6 months, during which they grow from a millimeter or two to many centimeters in diameter. Aquarium jellyfish that are carefully tended, fed daily even when food might be seasonally rare in the wild, and sometimes treated with antibiotics if they develop infections, may live several years, though this would be very unusual in the wild.
An unusual species, Turritopsis dohrnii, formerly classified as T. nutricula, might be effectively immortal because of its ability under certain circumstances to transform from medusa back to the polyp stage, thereby escaping the death that typically awaits medusae post-reproduction if they have not otherwise been eaten by some other ocean organism. So far this reversal has been observed only in the laboratory. At least one professor at the Seto Marine Biological Laboratory at Kyoto University in Japan has concluded that there are three species of jellyfish that are immortal, and says their immortality may hold the key to immortality for human beings, as he says that genetically they are not that much different from humans.