Introduction to Jellyfish
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.
How Do Jellyfish Get Around?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.
What Do Jellyfish Eat?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.
How Do Jellyfish Find Food?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.
The Jellyfish World
What Are the Parts of a Jellyfish?Jellyfish have no gills or lungs. They have no hearts or brains. In fact, jellyfish have no bones or skeletons to give them shape or to support their bodies. The jelly does that.
A jellyfish’s body looks a lot like a bell or an umbrella. Scientists call this body style a medusa (muh DOO suh). A jellyfish’s mouth is a small opening on the underside of the bell. It is the only opening leading into the jellyfish and the only opening leading out.
A jellyfish has many tentacles. Some hang around the bell of the jellyfish. These hold the stinging cells. Other tentacles hang around the mouth. These are called oral arms. They may or may not have stinging cells. The oral arms pass the food into the jellyfish’s mouth. The food then goes into the animal’s stomach.
Are Jellyfish Made of Jelly?If you are thinking of the jelly you eat with peanut butter, the answer is no. That jelly is fruit and fruit juice mixed with a thickener.
But jellyfish are made of another kind of jelly. Jellyfish jelly is an amazing blend of salt, protein, and lots of water. In fact, most jellyfish are over 95 percent water.
A jellyfish has a very simple body. Two layers of cells make up the body walls. A thick layer of jellyfish jelly between these walls helps give the jellyfish its shape.
Jelly is a perfect material for the body of an animal that lives in water. It helps support the jellyfish in the water. It helps jellyfish that live near the surface to float. And it keeps deep-sea jellyfish from being crushed by the pressure of the water.
How Do Jellyfish Sense Their World?Humans learn about their world by using their five senses. But most jellyfish don’t have eyes, ears, a nose, a tongue, or hands. These jellyfish depend on simple sense cells. The jellyfish’s sense cells are located in ridges along the edge of its bell.
These sense cells perform different jobs. One kind acts as eyes and senses light. Sensing light helps the jellyfish know if it is upside down in the water. Another kind acts as a nose and senses chemicals in the water. A third kind helps a jellyfish keep its balance in the water.
Why Do Some Jellyfish Light Up?Some jellyfish, such as sea nettles, make their own light. They glow or give off flashes of light as fireflies do. Some jellyfish use this light to attract prey. But most jellyfish use it as a defense against predators.How do its lights help a jellyfish? A jellyfish may light up to surprise a predator or to frighten it away. Lighted up, a small jellyfish with long tentacles suddenly looks like a large animal.
One jellyfish drops its glowing tentacles when fleeing from a predator. The attacking animal swims after the falling tentacles. This gives the jellyfish time to get away.
Jellyfish may light up for other reasons, too. To find out what those reasons are, scientists dive down in ships called submersibles (sub MER suh buhlz). The ships have windows that let scientists study the jellyfish deep in the ocean where they live.
How Do Jellyfish Grow?Young jellyfish look very different from their parents. In fact, a young jellyfish goes through several stages before it becomes an adult. Other animals go through life cycles, too. Think about the way in which a caterpillar becomes a butterfly or a tadpole becomes a frog.
Most large jellyfish start out as eggs. A jellyfish egg develops into a larva (LAR vuh). The larva drifts along in the ocean currents until it settles and attaches itself to the ocean bottom.
Next, the larva grows into a polyp (PAHL ihp). As the polyp develops, its stem begins to look like a stack of tiny saucers. One by one, the “saucers” break off, and each one develops into a tiny medusa. In this way, a single larva produces many jellyfish.
Why Are Some Jellyfish So Colorful?Many jellyfish that live deep in the ocean are red—like this dunce cap jellyfish. Others are purple, brown, or black. These colors help jellyfish in two ways. They help hide what a jellyfish has eaten. And they make it hard for predators to see the jellyfish.
In the deep ocean, being able to hide a meal can be very important. The reason is that many deep-sea animals use light—much the same way jellyfish do. Imagine what would happen if a clear jellyfish ate a glowing fish. That’s right! The fish would show through the jellyfish’s bell. But a colored bell hides the meal, so predators cannot see the fish.
Very little sunlight reaches deep down into the ocean. A clear jellyfish would reflect this light, almost the same way a mirror would. But dark colors do not reflect light. The dark colors of the jellyfish help camouflage (KAM uh flahzh), or hide, it from its predators.
Which Is the Largest Jellyfish?The lion’s mane is the largest kind of jellyfish. Its bell can grow to widths of more than 7 feet (2.1 meters). And its long tentacles can extend more than 130 feet (40 meters). This jellyfish can be found in the cold waters of the Atlantic and Pacific oceans. The largest lion’s manes are found in the Arctic Ocean.
The lion’s mane jellyfish is named for its shaggy oral arms and hairlike tentacles. Its stings are toxic, or poisonous. To catch prey, this jellyfish sinks down in the water. It spreads its tentacles out around the prey. The tentacles act like a huge net to catch the prey.
Which Jellyfish Spends Its Life Upside Down?Most jellyfish swim through the water with their tentacles and oral arms hanging below them. But not Cassiopeia (kas ee uh PEE uh), the upside-down jellyfish. This stinger spends most of its life with its tentacles floating above it.
Cassiopeia is a bottom-feeder. It lives in the shallow water of swamps near the seacoast. It sinks under the water and uses its bell as a suction cup to hold onto the bottom. Then Cassiopeia waves its oral arms to catch passing zooplankton.
Stingers
Why Do Jellyfish Sting?Jellyfish sting in order to catch their prey. They also sting to keep other animals from eating them.
Jellyfish tentacles and oral arms are covered with thousands of stinging cells. Inside each stinging cell is a hollow tube. This tube looks like a coiled thread. When an animal touches the stinging cell, the tube fires out to sting the prey.All cnidarians have stinging cells. But the cells may work in different ways to deliver their stings. Some stings pierce an animal’s skin. These stings release a poison. The poison paralyzes the prey so that it cannot move. Other stings are very long. They wrap around the prey and trap it.
A jellyfish’s sting is a good defense. But it does not protect a jellyfish from all sea animals. Some animals that eat jellyfish aren’t affected by the stings. They include sea slugs, ocean sunfish, and sea turtles.
What is a Stinger?Jellyfish are not fish—although they do spend their entire lives in water. Rather, these soft-bodied animals and their relatives are called cnidarians (ny DAIR ee uhnz). Cnidarians all have one thing in common: stinging cells. Some people call these creatures stingers.
One kind of stinger is a compass jellyfish. Its body is shaped like an umbrella. There are also cnidarians that don’t look at all like the compass jellyfish. Some have square shapes, while others look like flowers or plants.
Where in the World’s Oceans Do Stingers Live?Stingers live in all the oceans in the world. Some live in warm tropical waters. Others live in the icy waters of the Arctic. A few stingers even live in fresh water.
You may have seen jellyfish floating near the surface of the water. In fact, most jellyfish live in the upper waters of the ocean—the sunlit zone. At night, these jellyfish may sink down into deeper waters. Some stingers live their entire lives in deeper waters. Still others—such as sea anemones and corals—attach themselves to the ocean floor.
Jellyfish may swim or drift along with the ocean currents. Some swim alone. Others are found in huge groups, or shoals (SHOHLZ). Wherever they are, these unusual creatures are always moving.
Which Jellyfish Has the Most Powerful Sting?Sea wasps have powerful stings that can cause severe burns. But the Australian sea wasp is the most feared jellyfish of all. Its poison is deadlier than any snake venom. And it can kill large animals, including humans, in as little as three minutes.
The Australian sea wasp may grow as large as a basketball. And its threadlike tentacles can be as long as 15 feet (4.6 meters). But the bell is clear and very hard to see in the water. Australian sea wasps pose a special threat to swimmers. The reason is that these jellyfish often feed close to the shore where people swim.
Even Australian sea wasps, however, have enemies. Large sea turtles feed on them. The sea turtle’s tough skin and stomach may protect it from the jellyfish’s sting. But scientists aren’t really sure how the sea turtle can swallow the jellyfish without being poisoned.
Can Sea Wasps See in the Sea?Sea wasps are not like most other jellyfish. Sea wasps are the only stingers that have eyes. But scientists aren’t sure what sea wasps can see. Like other stingers, sea wasps don’t have brains. So, it is hard to figure out how these stingers use their eyes. But they probably use them to find prey and keep away from their predators.
Sea wasps are different from most jellyfish in other ways, too. Sea wasps don’t have oral arms. Instead, they have four groups of tentacles. The tentacles are found at each corner of their box-shaped medusa. Because of their unusual shape, these stingers are also known as box jellyfish.
Some jellyfish belong to the class Scyphozoa; these jellyfish are usually larger than the polyps from which they develop. Other jellyfish belong to the class Hydrozoa; these jellyfish are usually smaller than their polyps. Jellyfish belong to the phylum Cnidaria.