They have no brains and no spines. You can
often see right through them. But they can cause painful, sometimes deadly
stings. Some can even glow in the dark! Scientists know them as medusas, but
most people call them jellyfish.
WHAT ARE JELLYFISH?
Jellyfish are not really fish. They are
invertebrate animals. This means that unlike fish or people, they have no
backbones. In fact, they have no bones at all.
Jellyfish have stomachs and mouths, but no heads. They
have nervous systems for sensing the world around them, but no brains. They are
made almost entirely of water, which is why you can look through them.
Most jellyfish live in salt water, apart from a few
types that live in fresh water. Jellyfish are found in oceans and seas all over
the world. They live in warm, tropical seas and in icy waters near the North
and South poles.
BELL-SHAPED BODIES
Adult jellyfish are shaped like an open umbrella or
a bell. Tentacles dangle down from the edges of a jellyfish’s body. At the
center of a jellyfish’s body, where it is widest, is the mouth.
The ends of the tentacles have stinging cells
on them, which jellyfish use to paralyze and catch their prey. The sting from a
jellyfish can be very painful. Some jellyfish, such as the sea wasp, have
powerful enough stings to kill people.
WHAT DO JELLYFISH EAT?
Most jellyfish eat fish, tiny creatures called
plankton, and even other jellyfish. Once a jellyfish stings and paralyzes its
prey, it takes the prey to its mouth and swallows it.
MANY SIZES AND COLORS
Some jellyfish are very small, no larger than a
marble. Others are quite large. The largest jellyfish can grow to about 7 feet
(about 2 meters) across and weigh hundreds of pounds! Jellyfish may be blue,
orange, pink, yellow, purple, and other colors.
HOW DO JELLYFISH MOVE?
Jellyfish use their bell-shaped bodies to move
themselves through the water. They expand their bodies and then squish their
sides together. This pushes water out behind them, propelling them along. Most
jellyfish swim very slowly. They do most of their moving by riding on ocean
currents.
THE LIFE OF A JELLYFISH
Life for a jellyfish begins as an egg. Adult
jellyfish release thousands of eggs into the water. The eggs develop into
larvae. These larvae attach to rocks or other objects on the ocean floor and
grow into a polyp. Polyps usually grow in colonies, or groups. They look like
plants because they attach to the ground and grow upwards.
The polyps grow taller until they look like tall
stacks of saucers. The saucers gradually detach themselves from the polyps and
swim free. These free-swimming saucers are young jellyfish.
The jellyfish then grow into adults, lay eggs, and
the life cycle begins anew.
JELLYFISH THAT GLOW
Some jellyfish can glow in darkness by making their
own light. The light is made by a chemical reaction inside the jellyfish.
Scientists believe jellyfish glow for several reasons. For example, they may
glow to scare away predators or to attract animals they like to eat.