Coral reefs
Coral reefs are warm, clear, shallow ocean habitats that are rich in life. The reef's massive structure is formed from coral polyps, tiny animals that live in colonies; when coral polyps die, they leave behind a hard, stony, branching structure made of limestone.

The coral provides shelter for many animals in this complex habitat, including sponges, nudibranchs, fish (like Blacktip Reef Sharks, groupers, clown fish, eels, parrotfish, snapper, and scorpion fish), jellyfish, anemones, sea stars (including the destructive Crown of Thorns), crustaceans (like crabs, shrimp, and lobsters), turtles, sea snakes, snails, and mollusks (like octopuses, nautilus, and clams). Birds also feast on coral reef animals.
Types of Corals: There are two types of coral, hard coral and soft coral. Hard corals (like brain coral and elkhorn coral) have hard, limestone skeletons which form the basis of coral reefs. Soft corals (like sea fingers and sea whips) do not build reefs.

Where are Coral Reefs?: Coral reefs develop in shallow, warm water, usually near land, and mostly in the tropics; coral prefer temperatures between 70 and 85 ° F (21 - 30 °C). There are coral reefs off the eastern coast of Africa, off the southern coast of India, in the Red Sea, and off the coasts of northeast and northwest Australia and on to Polynesia. There are also coral reefs off the coast of Florida, USA, to the Caribbean, and down to Brazil.

The Great Barrier Reef (off the coast of NE Australia) is the largest coral reef in the world. It is over 1,257 miles (2000 km) long.

Types of Reefs: The different types of reefs include:

• Fringing reefs are reefs that form along a coastline. They grow on the continental shelf in shallow water.
• Barrier reefs grow parallel to shorelines, but farther out, usually separated from the land by a deep lagoon. They are called barrier reefs because they form a barrier between the lagoon and the seas, impeding navigation.
• Coral Atolls are rings of coral that grow on top of old, sunken volcanoes in the ocean. They begin as fringe reefs surrounding a volcanic island; then, as the volcano sinks, the reef continues to grow, and eventually only the reef remains.

Coral Reefs in Danger: Many coral reefs are dying. Major threats to coral reefs are water pollution (from sewage and agricultural runoff), dredging off the coast, careless collecting of coral specimens, and sedimentation (when silt or sand from construction or mining projects muddies the waters of a reef and kills coral, which needs light to live).