Coral crisis! Humans are killing off these bustling underwater cities. Can coral reefs be saved?

Smack in the middle of the Pacific Ocean, 1,600 kilometers (1,000 miles) south of Hawaii, scientist Jeremy Jackson dives beneath the clear, blue water. Rainbow-colored corals--tiny sea animals--sway below. Jackson is exploring the coral reefs that make up the Palmyra Atoll, a group of reef islands that have grown along the donut-shaped rim of a submerged volcano. Jackson, an ecologist from Scripps Institution of Oceanography, is amazed at the vibrant reef around him. "Palmyra is one of the few remaining reefs in the world that is close to pristine," he says.

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.

OCEAN ARCHITECT

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!

FAST FOOD

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.

VANISHING ACT

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.