jellyfish

After fertilization, the gametes of a jellyfish may be released into the water as larvae (middle right) or retained in a brood pouch. After settling, the larvae change into a sedentary polyp stage (bottom right) similar to that of a coral. More polyps (bottom left) are produced by simple budding. These polyps may divide again to form the so-called ephyra larvae (middle left). These larvae eventually grow into the free-swimming medusa more familiarly known as a jellyfish (top).

Marine invertebrate, belonging among the coelenterates (subphylum Cnidaria), with an umbrella-shaped body made of a semi-transparent jellylike substance, often tinted with blue, red, or orange colours, and having stinging tentacles that trail in the water. Most adult jellyfish move freely, but during parts of their life cycle many are polyp-like and attached to rocks, the seabed, or another underwater surface. They use the sense of smell in tracking prey, and feed on small animals that are paralysed by stinging cells in the jellyfish tentacles.

Most jellyfish cause no more discomfort to humans than a nettle sting, but contact with the tentacles of the subtropical Portuguese man-of-war (Physalia physalis) or the Australian box jellyfish (Chironex fleckeri) can be life-threatening.

Recent discoveries In 1996 zoologists identified a jellyfish with a self-perpetuating life cycle. When Turritopsis nutricula reaches maturity and spawns, it returns to its juvenile polyp stage and its development cycle begins again. If conditions are unfavourable this reversal of the life cycle may occur before maturity.

A new species of giant jellyfish was discovered in the eastern Pacific in 1998. Chrysaora achlyos has 6-m/20-ft tentacles and a bell that is 1 m/3.3 ft in diameter. The largest jellyfish is the lion's-mane sea jelly Cyanea capillata arctica. One individual had a bell of 2.3 m/7.5 ft diameter and 36.5 m/120 ft tentacles.

Another new species of jellyfish, Stellamedusa ventana, was discovered in 2004. It has four arms instead of the usual tentacles, which it uses to guide food into its mouth. This type of jellyfish is sufficiently different to belong to a new subfamily, which has been named Stellamedusae after the discovery.