They are sumptuous environments, home to hundreds of species of plants and animals, more per acre than almost anywhere else on the planet. They are among the Earth's last great unexplored natural frontiers. And they are endangered: Many are already ruined while others hover on the brink of ecological destruction.

Rain forests? No, coral reefs - colonies of marine organisms that look like rocks but are really plant-animal hybrids. Reefs are attracting the attention of conservation groups, scientists and government agencies just as rain forests first did a decade ago."Coral reefs are the rain forests of the '90s," said Christopher Andrews, senior director of biological programs for the National Aquarium in Baltimore. "The concerns are parallel. All hard corals are now endangered. America's reefs are dying. There's all this going on below the sea and no one has been looking there."

In April, the National Oceanic and Atmospheric Administration released its draft management plan for the Florida Keys, a controversial blueprint for balancing the interests of local fishermen, pleasure boaters, tourists, treasure salvagers, sugar-cane farmers and others whose livelihood either depends on or has an impact on Florida's reef system, the third largest on the planet.

This summer, in the Philippine city of Dumaguete, coral reefs got international attention as representatives from more than 40 countries met for the first time to inaugurate the International Coral Reef Initiative. The goal is to foster global cooperation for the sustainable use of these rich biological resources.

"Coral reefs are stunning ecosystems by any measure," said James Porter, a marine ecologist at the University of Georgia in Athens. With coral reefs, he said, "it's not jobs versus the environment. It's jobs because of the environment."

Reefs have all the right stuff to reach superstar status on the environmental stage: colorful characters (coral reefs occupy a fraction of 1 percent of the Earth's surface but are home to one-quarter of all marine fish species); kinky cohabitation (corals are part plant, part animal); unusual sexual behavior (male coral polyps ejaculate clouds of sperm into the midnight ocean); and tragedy (many of the world's finest reefs are dying).

But reefs are more than just the latest cause for environmental activists. They are finicky ecosystems that scientists can use as sensitive indicators of water-quality changes. Pesticides, silt and fertilizer runoff all take a toll on reefs' health.

Reefs also are economically important. They protect coastlines from wave erosion, help create sandy beaches and harbors, attract tourists, and provide food and breeding grounds for one-tenth of all the fish caught for human consumption.

Yet coral reefs are in decline.

Ten percent of the world's reefs are dead or damaged beyond the capacity to recover, according to U.S. government reports. At current rates of loss, an additional 30 percent will disappear in the next 20 years. As the reefs die, many of the fish, sponges, sea urchins and algae that add to the reef's life, color and economic value also die or leave.

Some reefs are dying because the energy-providing algae that live inside coral cells themselves are dying of disease. A bacterium that began killing reef algae in the Cook Islands of the South Pacific in 1993 has spread more than 3,700 miles. Scientists recently reported that reefs surrounding Fiji that were completely free of disease in 1992 are now 100 percent infected, and similar diseases have cropped up elsewhere.

No one knows why these coral diseases have emerged in recent years, but the consensus among marine scientists is that most of the loss in coral-reef vitality can be traced to human activities.

In Florida, for example, Porter has linked declining reef health to increased salinity of the water that flows from Florida Bay to the Keys. The bay has grown saltier in the past decade from diversion of fresh water for agriculture and urban development north of the Everglades, he said. Andrews noted that the Midwest floods of 1993 dumped tons of silt on Florida's reefs in part because of poor erosion control in America's heartland. As far as reefs are concerned, he said, "Kansas is a coastal state."

Florida's reefs gained partial protection in 1990 when Congress passed the Florida Keys National Marine Sanctuary Act. NOAA is now gathering feedback on a management plan it created for the sanctuary in collaboration with various interest groups. It includes 10 action plans, some non-controversial (such as deployment of mooring buoys so boats don't have to drop anchors, and continued monitoring of coral reef health) and some hotly debated (including "zoning" ordinances that would prohibit some activities, such as fishing, in certain areas).

State and federal agencies also are trying to agree on a plan to reduce Florida's fresh-water consumption and restore normal salinity levels to the Everglades, Florida Bay and the Keys, although efforts in Congress to rewrite the Clean Water Act may complicate such planning.

Internationally, the United Nations Environment Programme and a half-dozen other conservation organizations recently produced a report outlining the plight of the world's reefs and highlighting the most damaging human activities, including the growing use of dynamite and poisons to catch coral-dwelling fish.

The meeting in the Philippines aimed to develop coordinated strategies that would help countries profit from their reefs without destroying them. "People are realizing they need a public-private partnership to make this work," said Kathleen Sullivan, a marine ecologist at the University of Miami.