Jellyfish, also called medusae, are free-swimming, marine invertebrates in the class Scyphozoa (phylum Coelenterata). They have a gelatinous, translucent, dome-shaped body and occur most commonly in warm, tropical seas, although they are found in all the world's oceans. Jellyfish feed on small planktonic animals or fish which they sting and paralyze with special cells called nematocysts located on the tentacles that hang from the edge of their dome-shaped bodies. The body of a jellyfish is 99% water; when washed onto dry land, these animals die and rapidly disappear as the water in their body evaporates.
About 200 species of true jellyfish are known, ranging in size from 0.06 in (1.5 mm) to 6.5 ft (2 m). All jellyfish have a prominent dome; the shape of the dome varies from a shallow saucer to a deep bell. In the subclass Cubomedusae, the dome is cube-shaped. Hanging from the edge of the dome are nematocyst-bearing tentacles; the number and length of these tentacles varies greatly from species to species. On the underside of the dome is a feeding tube (the manubrium or proboscis) with the animal's mouth at its free end. Radial canals (usually four in number or some multiple of four) extend from the jellyfish's four-chambered stomach to the dome's margin where they connect with the ring canal. This system of canals serves to distribute food to the outer parts of the jellyfish's body. Light-sensing organs (eyespots), balance organs (statocysts), and other sensory organs are located at the base of the tentacles. Jellyfish move through the water by pulsating contractions of the muscles on the lower edge of the dome.
Jellyfish release eggs and sperm through the mouth into the water, where fertilization occurs. Fertilization results in free-swimming, ciliated (with tiny, hair-like structures) larvae called planulae; these larvae settle on a surface, such as a rock, and turn into a polyp (a hollow cylinder with tentacles and a mouth at one end) or a strobila (a hollow structure that looks like a stack of upside-down saucers). Strobila develop into adult jellyfish after passing through another free-swimming phase during which they are called ephyrula. Polyps produce adult jellyfish by budding. The average life span of a jellyfish is one to three months; the largest may live for about one year.
The most common jellyfish on the coasts of North America and Europe is the moon jellyfish (Aurelia aurita). This 6–8-in (15.2-20.2 cm) species is found at depths of 0-20 ft (0-6 m). It is whitish, often shaded with pink or blue, and has a saucer-shaped dome with a fringe of numerous, short tentacles around the margin. Its sting is mildly toxic to humans, occasionally producing an itchy rash. In the same order (Semaeostomeae) as the moon jellyfish is the giant pink jellyfish (Cyanea capillata); this species is common in the waters of the Northern Hemisphere where it reaches about 6.5 ft (2 m) across.
The 16 species in the subclass Cubomedusae are commonly called box jellyfish; they live in the warm waters of the continental shelves. The largest species in this subclass, the sea wasp (Carybdea alata), is found in tropical harbors and river mouths. It reaches a diameter of 9.75 in (25 cm) and sometimes eats fish much larger than itself. The sting of this and many other box jellyfish can be highly toxic producing a reaction in humans that may include skin welts, muscle cramps, and breathing difficulty. Two genera, Chiropsalmus and Chironex, found in the Indian Ocean produce a toxin so potent that contact with their nematocysts can kill a person within several minutes.
The 31 species of deep-sea jellyfish (order Coronatae) are heavily pigmented in colors ranging from red and violet to brown and blackish. They are found at extreme depths; for example, Nausithoe has been found at a depth of 23,000 ft (7,100 m). The order Rhizostomeae includes about 80 species commonly known as many-mouthed jellyfish. In these species, the feeding tube has many small pores rather than one large opening. The genus Stromolophus of this order is common on the southeastern coast of the United States where it reaches a diameter of 7 in (18 cm).
Other marine animals in the class Hydrozoa, order Siphonophora, superficially resemble true jellyfish and are often confused with them. The most well-known member of this order is the Portuguese man-of-war (Physalia physalis). Each Portuguese man-of-war (so named because it resembles an eighteenth century warship) is a colony composed of four kinds of polyp. The main polyp is a gas-filled float that measures up to 12 in (30 cm) long. This float has a high crest which serves as a sail to catch the wind and its color varies from blue to purple to red. Hanging below the float in the water are other polyps; some of them are concerned with feeding while other are concerned with reproduction. Also below the float are trailing tentacles (up to 40 ft/12 m long) armed with nematocysts. The animal uses these tentacles to catch the fish and other sea creatures that it eats. The toxin produced by this species is also very potent; humans have been known to die from a Portuguese manof-war sting. Usually when humans are stung, redness, skin welts, and blisters result. When washed ashore, this animal remains toxic for some time.
Cousteau, J. The Ocean World of Jacques Cousteau. Danbury, CT: Grolier, 1975.
Whiteman, Kate. World Encyclopedia of Fish & Shellfish. New York: Lorenz Books, 2000.
Christine B. Jeryan