I.

Introduction

Coral Reef, coastal and oceanic ecosystem renowned for its beautiful life forms and for providing one of the most biologically diverse habitats on Earth. A reef is a ridge or outcrop of rock in the sea that comes close to the surface. A coral reef is a reef that has been built largely or entirely by corals, tiny animals that live together in colonies. Over hundreds or thousands of years the limestone skeletons of coral build up, with new corals growing on the skeletons of the dead ones. This physical structure, with its living surface of corals and other organisms, is a coral reef.

The corals that build reefs are found only in warm tropical waters where sea temperatures rarely fall below 18°C (64°F). They thrive only in clear saltwater where bright sunlight can penetrate. This is because corals cannot exist without the symbiotic algae called zooxanthellae that live in coral tissues and require sunlight for photosynthesis.

Many reefs are found around islands, or quite far from land at the edge of continental shelves. Some of the most extensive areas of coral reefs are found in the Pacific Ocean, around the islands of Southeast Asia and off the coast of Australia. Large areas of coral reef are also found in the Indian Ocean and around the Red Sea and Arabian Sea. Many coral reefs also exist in the Caribbean Sea.

Because their skeletons are made of limestone, corals leave clear and abundant fossils. Some of the earliest forms of life—mound- or pillar-shaped fossilized rocks known as stromatolites—resemble corals. The oldest stromatolites date back 3.5 billion years. They were laid down by some of the simplest organisms on Earth—blue-green algae (also known as cyanobacteria). About 560 million years ago, limestone reefs built by bacteria, algae, and sponges first appeared. The first complex corals, known as tabulate corals, are recorded at about 500 million years ago.

The corals we know today first appeared about 220 million years ago, before the age of the dinosaurs. The first true coral reefs began to appear about 205 million to 210 million years ago. Since this time there have been several periods of major reef-building, but also periods when corals declined and did not build reefs.

In all, shallow coral reefs occupy only about 284,000 sq km (110,000 sq mi), or less than one-tenth of 1 percent (0.1 percent) of the world’s oceans. Yet in this tiny area, coral reefs house a quarter of all marine fish species. Reefs are highly productive resources for human beings. Hundreds of millions of people live within easy reach of coral reefs and rely on them to provide food. Coral reefs also help protect human settlements from large waves during storms.