Jellyfish look like blobs when washed up on the beach. But in the water jellies are graceful. They range in size from about 1 inch (2 1/2 centimeters) to 200 feet (61 meters) long. They have been drifting through the world's oceans for more than 650 million years. Jellyfish are members of the phylum Cnidaria, a structurally simple marine group of both fixed and mobile animals: sea anemones, sea whips, corals and hydroids are polyps that grow attached to rocks or other hard surfaces; jellyfish and colonial siphonophores like the Portuguese man-of-war are mobile. Inherent to both types of life histories is their radial symmetry (body parts radiating from a central axis). This symmetry allows jellyfish to detect and respond to food or danger from any direction. Jellyfish come in a wide variety of sizes, shapes and colors. Most are semi-transparent or glassy and bell-shaped, measuring less than an inch to over a foot across the bell, although some may reach 7 feet in diameter. The tentacles of some jellyfish can reach lengths greater than 100 feet. Regardless of their size or shape, most jellyfish are very fragile, often containing less than 5% solid organic matter.
Jellyfish swim by contracting and expanding their bodies. They do not have scales or shells. They "swim" by the action of comblike paddles, composed of rows of fused cilia, that beat in sequence to propel the comb jelly through the water. Locomotion for true jellies is less dynamic. As planktonic animals, they have only limited control over movement, so their mobility is partly a matter of passive drifting on waves and currents. However, they can regulate vertical movement to some extent, employing a kind of jet propulsion. The tissue on the underside of the umbrella contracts, pushing water out of the hollow bell in one direction to propel the jelly in the opposite direction. Because jellyfish are sensitive to light, this vertical movement can be important. Some jellyfish, like the sea wasp, descend to deeper waters during the bright sun of the midday and surface during early morning, late afternoon and evenings. Despite this ability to move vertically, jellyfish largely depend upon ocean currents, tides and wind for horizontal movement. Jellyfish have a defense mechanism of oral arms or tentacles which are covered with organelles called nematocysts. These nematocysts are paired with a capsule which contains a coiled filament that stings. The filament unwinds and launches into the target, thereby injecting toxins upon contact by foreign bodies.
Jellyfish lifespans typically range from a few hours (in the case of some very small hydromedusae) to several months. The life span and maximum size of each species is unique. One unusual species is reported to live as long as 30 years and another species, Turritopsis dohrnii as T. nutricula, is said to be effectively immortal because of its ability to transform between medusa and polyp, thereby escaping death. Most of the large coastal jellyfish live about 2 to 6 months, during which they grow from a millimeter or two to many centimeters in diameter. They feed continuously and grow to adult size fairly rapidly. After reaching adult size (which varies by species), jellyfish spawn daily if there is enough food in the ecosystem. In most jellyfish species, spawning is controlled by light, so the entire population spawns at about the same time of day, often at either dusk or dawn.