Imagine you are snorkeling in the warm ocean water
around a tropical coral reef. You put on your mask and swim fins. You float
around looking at the reef below you. The coral reef looks like strangely
shaped rocks sticking up from the ocean floor. Crooked coral that looks like
tree limbs twists up from cracks and caves in the reef. A big piece of coral that
looks like a brain lies on the bottom. Orange and pink coral shaped like fans
wave in the currents. Fish of all different shades and varieties of color swim
near the reef.
Coral reefs play an important role in ocean
life. Many kinds of plants, fish, and other animals live on and around a coral
reef. Even the reef itself is made mostly of tiny coral animals, both living
and dead.
HOW DO REEFS FORM?
Coral animals do not move around. These tiny
creatures live together in groups called colonies. One single coral animal is
called a polyp. It has a body shaped like a tube. Its mouth is on the top of
the tube. Little tentacles around the mouth help the coral polyp catch food
that floats by. Coral animals eat microscopic sea creatures called zooplankton.
The reef-building polyp makes a hard outer shell for
itself from materials found in seawater. When the polyp dies, the shell is left
behind. The shells from colonies of polyps build up over time to form a rock
called limestone. This limestone becomes the inner part of the reef. The living
coral animals form the top part of the reef. As each layer of polyps dies,
their stony skeletons get added to the reef. A new layer then grows on top of
the old. This is how a reef gets bigger.
Tiny single-celled algae called zooxanthellae live in
coral polyps. The polyp and the algae make food for one another. The polyps
could not live without the algae.
KINDS OF CORAL
There are hundreds of different kinds of coral.
Hard coral or stony coral is the kind of coral that builds reefs. These corals
have hard outer skeletons. Other corals do not have hard outer skeletons and
look like fans or flowers. Some kinds of coral look like the branches of a
tree. Other kinds can sting you if you touch them.
THREE KINDS OF REEFS
There are three kinds of coral reefs: fringing
reefs, barrier reefs, and atolls. All three kinds grow in warm places. Coral
reefs cannot form in water colder than about 68° Fahrenheit (about 20°
Celsius). Reefs form in clear, shallow water. They need sunlight in order for
plants and animals to live there.
Fringing reefs are close to shore. There is no
lagoon or other body of water between a fringing reef and the shore. Barrier
reefs are farther out in the ocean. There is usually a lagoon between a barrier
reef and the shore. Barrier reefs can be huge. The largest group of reefs in
the world is Australia’s Great Barrier Reef. It covers thousands of square
kilometers. Atolls are really the tops of volcanoes that sank below the sea.
The reef grows on the rim of the volcano. Inside the reef is a lagoon.
REEFS IN DANGER
Scientists fear that many coral reefs are in
danger. Many reefs have had a problem called bleaching. This occurs when the
zooxanthellae, the algae that live in polyps, die off and the reef changes
color. Then the coral polyps die. No one knows what causes bleaching.
A starfish called the Crown of Thorns starfish is
also a threat to reefs. Sometimes too many of these starfish suddenly show up
on a reef. The starfish kill the coral. No one knows why there are so many
starfish.
Pollution is also a threat to coral reefs.
Conservationists and governments are working to protect the Great Barrier Reef
and other coral reefs.
