Coral crisis! Humans are killing off these bustling underwater cities. Can coral reefs be saved?
Hundreds of years ago, all of the world's coral reefs may have been as spectacular as Palmyra. But today, healthy reefs are rare due to pollution and poor fishing practices. "It's so depressing to go scuba diving now," says Jackson. "I see the changes that have occurred just in my lifetime."
THE SECRET LIFE OF CORALS
Corals look like rocks or big plants growing on the ocean floor, but they are actually invertebrates (animals without a backbone). Each coral is made up of thousands of individual animals called polyps--cup-shaped creatures with a ring of tentacles on top. "A fist-sized piece of coral may contain 500 to 1,000 tiny polyps," says biologist Selina Ward of Australia's University of Queensland.
Each coral polyp starts out as a tiny larva (immature form of an invertebrate). Larvae result when a pink-colored egg, or female sex cell, from a coral is fertilized (fused) by a male sex cell, or sperm, of the same species. In most coral species, fertilization occurs in the waters surrounding the reef. To make sure a sperm meets an egg of its own species, each type of coral spawns, or releases eggs and sperm into the water, at a different time. "One species may begin to spawn at 6:30 p.m. mid another at 8 p.m. It's like a pink snowstorm," Ward says. Once an egg finds the right sperm mate in the crowd, a larva forms.
The new larva free-floats until it finds a little patch of seafloor to call home. There the larva attaches and builds a tiny cup-shaped house by secreting a hard skeleton of limestone, a rock made of the chemical compound calcium carbonate. Slowly, the polyp grows upward, abandoning its old address and constructing a new home on top. The deserted homes eventually build up into a hard limestone reef. Along the way, the polyp also reproduces asexually--dividing and forming an identical twin. The twin polyp then builds its own home next door. The result: a huge colony of polyps, each in its own tiny cup. But this takes a long time: Corals grow only about 1 centimeter (0.4 inches) per year. Large corals may be up to 500 years old!
Corals are carnivores--they eat other animals. Like all cnidarians (ny-DARE-ee-uhnz), a group of organisms that includes jellyfish and sea anemones, corals have stinging tentacles arranged in a circle around their mouths. These tentacles help catch plankton (microscopic organisms that float in the water) for a tasty treat (see diagram, below).
Corals also rely on another organism for nutrients. Tucked inside a polyp's inner tissue layer are algae (plantlike microorganisms that produce their own food), called zooxanthellae (ZOH-sen-THEL-lee). The two have a mutually beneficial, or symbiotic, relationship. In return for a cozy home, the zooxanthellae provide corals with carbon, an element corals use to build their skeletons. "The algae also give the coral a brownish look. Bright colors, like red and yellow low, come from pigments inside the coral itself," says Ward.
Like many friendships, this relationship works until the coral gets stressed. For instance, if chemicals pollute surrounding seawater, the coral spurts out the zooxanthellae. Scientists call this process coral bleaching because many corals turn bright white after evicting the algae.
A recent increase of bleached corals around the world alarms scientists. It's a sign the reefs aren't healthy. "Many corals will the following a bleaching event," Ward says.
What's killing coral reefs? For starters, fishing nets can accidentally yank corals off the ocean floor. "Shrimp fishing is especially destructive," says Jackson. In some areas, fishermen collect tropical fish for pet stores by stunning the fish with cyanide poison. At the same time, the poison causes coral bleaching.
Scientists predict that more than haft the world's reefs may be gone by the year 2030. In the last 30 years, the area of Caribbean seafloor covered by reefs has fallen by 80 percent.
If coral reefs disappear, many other ocean species will too. Over 25 percent of all sea creatures call the reefs home. And corals are vital to humans: Fish living there provide food, and many medicines come from the reef's plants and animals.
What can be done? Jackson thinks setting aside large marine preserves could help save the reefs. "Today, less than 1 percent of the world's reefs are fully protected," he says. And he urges everyone to carefully choose the types of fish they eat. "Most coral reef fishes, such as groupers and snappers, are dangerously overfished." If everyone pitches in, Jackson thinks the coral reefs can be saved.
ANATOMY OF A CORAL
Corals are individual animals that can build huge reefs in the ocean. Here's how these tiny creatures live.
1 REPRODUCTION: Spawning corals release eggs and sperm into the water. In some species, each polyp releases either sperm or eggs. In others, each polyp releases sperm and eggs together.
2 CORAL BABY: If coral sperm meets an egg of the same species, fertilization forms a new larva. The larva settles on the seafloor and attaches by secreting limestone.
3 NEW HOME: The coral builds a cup-shaped skeleton out of limestone. The larva becomes a polyp and develops a stomach and tentacles. If disturbed, the coral can pull its tentacles inside its skeleton.
4 GROWING UP: Over time, the polyp grows taller. It builds a new limestone floor, closing off its old house below. The polyp always lives on the top floor, leaving behind a hard layer below.
5 MAKING FRIENDS: The coral's inner tissue layer houses zooxanthellae, which provide carbon to the coral. Cells in the coral's outer layer catch and digest food.
6 SNACK TIME: Coral hold a coiled harpoon inside their tissue, ready to catch prey. If plankton float by, the coral shoots out the harpoon and snags the snack. It stuns the organism with a poisonous filament. Then the plankton is pulled inside and digested.
IT'S YOUR CHOICE
After reading the article, choose the correct answer to these questions.
1 Corals are:
A. plants growing on the seafloor.
B. made up of thousands of tiny individual invertebrates.
C. rock ledges in the ocean.
D. brightly colored fish.
2 Which of the following is not a reason that coral reefs are dying?
A. poor fishing practices
B. coral bleaching
D. fish eating too many corals
3 How do zooxanthellae help corals build skeletons?
A. They provide corals with the element carbon.
B. They help corals catch plankton.
C. Corals use them to poison fish.
D. The zooxanthellae fertilize coral eggs.
Did You Know?
* Coral reefs cover 0.2 percent of the ocean floor. The Great Barrier Reef, located off northeastern Australia, is 2,027 kilometers (1,260 miles) long-the largest coral reef in the world. It's the largest structure ever built by living creatures, and can be seen from space.
* Coral reefs keep shorelines from eroding away with the force of pounding waves. "If the reefs disappear, a lot of islands will eventually disappear too," says biologist Selina Ward.
* Global warming is a serious threat to corals. Scientists predict that sea temperatures will rise between 1 and 4 degrees Fahrenheit in the next 20 years. When the ocean water gets too warm, corals expel their zooxanthellae and eventually die.
Geography: Research the three different types of coral reefs: barrier, fringing, and atoll. Then use a world map to locate areas where atoll reefs commonly occur.
Research five threats to coral reefs. What are some changes you can make in your life to help save the corals?
How much do you know about corals?
Directions: Answer the following questions in complete sentences.
1. What are corals made of? What are the young of these individuals called?
2. What is a coral atoll?
3. What is an invertebrate? Give one example.
4. How does coral grow on the seafloor?
5. What do corals eat? How?
6. What is a symbiotic relationship? How does it relate to corals?
7. What causes coral bleaching?
8. Why are corals disappearing?
9. What will happen if coral reefs disappear? Name two consequences.
10. What are two things that could be done to help save corals? Why?
1. A coral is made up of thousands of individual animals called polyps. A premature polyp is called a larva.
2. A coral atoll is a group of reef islands that have grown along the donut-shaped rim of a submerged volcano.
3. An invertebrate is an animal without a backbone. For example, coral is an invertebrate.
4. A new larva free-floats until it finds a patch of seafloor to attach to. It secretes a hard skeleton made of limestone to form a tiny cup-shaped house. It grows upward by abandoning its old address and constructing a new home on top. The deserted homes eventually build up into a hard limestone reef. Along the way, the polyp also reproduces asexually-dividing and forming an identical twin. The twin polyp then builds its own home next door.
5. Corals are carnivores. They eat other animals. Corals have stinging tentacles arranged in a circle around their mouths. They use them to help catch food, such as plankton.
6. A symbiotic relationship is one that is mutually beneficial. Besides food, corals rely on another organism for nutrients. Tucked inside a polyp's inner tissue layer are zooxanthellae, a type of algae. In return for a home, zooxanthellae provide corals with carbon, an element corals use to build their skeletons.
7. When coral gets stressed--for instance, if chemicals pollute surrounding seawater--the coral spurts out zooxanthetlae. Many corals turn bright white after evicting the algae. Scientists call this coral bleaching.
8. Fishing nets can yank corals off the ocean floor. Pollution also harms corals. For example, fishermen collect tropical fish for pet stores by stunning the fish with cyanide poison--the poison causes coral bleaching.
9. Over 25 percent of all sea creatures call the reefs home. So, if corals disappear, many other ocean species will disappear too. Without corals, humans will lose many fish species that provide food, and many medicines that come from the reefs' plants and animals.
10. Scientists think setting aside large marine preserves could help save coral reefs. Today, less than 1 percent of the world's reefs are fully protected. Also, people should choose carefully the types of fish they eat. That's because many coral reef fishes, such as groupers and snappers, are dangerously overfished.
1.b 2. d 3. a
To learn more about coral reefs and the creatures that live there, check out this site from the Monterey Bay Aquarium: www.mbayaq.org/efc/efc_se/sz_colorful_coral.asp
Discover 25 things you can do to help save coral reefs: www.yoto98.noaa.gov/books/reefs/reef1.htm
Find out what the U.S. Environmental Protection Agency is doing to protect coral reefs: www.epa.gov/owow/oceans/coral
A team of scientist with the organization Shifting Baselines is educating the public about overfishing and the ocean crisis. Check out their public service announcement starring Jack Black: www.shiftingbaselines.org
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|Title Annotation:||Life science: corals|
|Date:||Dec 8, 2003|
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