Jellyfish, or Medusa (plural: medusae), a water-dwelling animal. It is not a fish but a cnidarian, a spineless, soft-bodied animal. The jellyfish is one of two basic body forms that the animal has during its life cycle; the other is called a polyp. There is no common name for the animal in both its forms.Jellyfish capture food with their tentacles.
The jellyfish has a semitransparent, bell-shaped, sac-like body fringed around the bottom edges by slender, dangling tentacles. The body is composed of a jelly-like material that gives the animal both shape and buoyancy. The name “medusa” comes from the tentacles that hang from the edge of the body, suggesting the snakes that grew from the head of Medusa, a monster in Greek mythology.
Jellyfish range in diameter from less than one inch (2.5 cm) to about 12 feet (3.7 m), depending on the species. They may be pinkish, bluish, brownish, or almost colorless. Some jellyfish are found in freshwater, but most are marine and live mainly in coastal seawaters.
The jellyfish moves vertically through the water, alternately sinking and then rising to the surface by rhythmically contracting its body. In this manner it drifts about, carried by the water current. It usually lives in large groups, or schools. Jellyfish are sometimes washed ashore by tides and storms.
Jellyfish swim to get around. For a jellyfish, this means opening and closing its bell—much as you open and close an umbrella. A jellyfish has a ring of muscle around its bell. When a jellyfish tightens this muscle, its bell closes. This pushes water inside the jellyfish out, shooting the jellyfish forward. As the muscle relaxes, water refills the bell.
Though jellyfish can swim, ocean currents often control where they go. Jellyfish float, drift, and sink with the ocean currents. Ocean currents help jellyfish move around. But currents can also cause a lot of damage. Jellyfish are very fragile. Strong currents during storms can tear a jellyfish’s bell or break off its tentacles. Currents can also leave a jellyfish stranded on a beach.
The jellyfish preys on small aquatic animals, which it captures with its tentacles. Stinging organs on the tentacles and body are used to stun the prey. The mouth, located on the underside, leads to a stomachlike cavity. Some kinds of jellyfish produce light through a process called bioluminescence. The light is probably used to attract prey, confuse predators, and communicate with other jellyfish.
Jellyfish feed on fish and other sea animals. They also eat very small animals. Some of these animals are so small that they are microscopic. That means you need a microscope in order to see them. Scientists call this collection of small animals zooplankton (zoh uh PLANG tuhn). Zooplankton include tiny shrimp, eggs of sea animals, baby sea animals, and even other stingers.
Now, you may be thinking that animals this small are not much of a meal. But to a jellyfish, thousands and thousands of zooplankton do add up.
Jellyfish do not hunt for food as many animals do. Instead, they take in food as they swim through the water. Jellyfish use their oral arms to sweep zooplankton into their mouths.
Jellyfish also use their tentacles to capture larger prey. When a fish brushes against a jellyfish’s tentacles, stinging cells stun or kill the fish. Then the tentacles bring the fish into the jellyfish’s mouth.
Jellyfish reproduce sexually. The female medusae produce eggs, which are fertilized within the stomachlike cavity. The eggs develop into larvae, which are shed into the water. The larvae settle down and attach themselves to a rock or seaweed and develop into the tubular forms called polyps. Polyps reproduce asexually, through budding. A polyp produces two types of buds. One type develops into individuals of the polyp form; the other type, into individuals of the jellyfish form.