Living Walls of the Sea
1From above, a coral reef can look like a field of colorful rocks. But a reef is not made of plants or ordinary stone. It is built by animals. Coral reefs are home to an amazing variety of life, and they help people in ways that many of us never notice.
2The builders of a reef are tiny animals called coral polyps. Each polyp has a soft, tube-shaped body with a ring of tentacles around its mouth. Many kinds of polyps build hard cups of limestone around their bodies, using minerals they take from seawater. When polyps die, their stony cups stay behind, and new polyps grow on top of them. Over hundreds or even thousands of years, layer upon layer of these cups forms a reef.
3Polyps do not live alone inside their cups. Tiny algae live inside the bodies of many reef-building corals. Like plants, these algae use sunlight to make food, and they share much of that food with the coral. In return, the coral gives the algae a safe place to live. The algae also give many corals much of their color. Because the algae need sunlight, most reef-building corals grow in shallow, clear water where light can reach them. These corals also need warm water, so most reefs are found in tropical oceans.
4A healthy reef is crowded with life. Fish dart in and out of cracks, crabs and shrimp hide in small holes, and sea turtles glide past in search of food. Some fish nibble on the algae that grow on the reef, while larger fish hunt the smaller ones. Coral reefs cover only a tiny part of the ocean floor, yet scientists say they are home to a huge share of all the kinds of animals that live in the sea.
5Reefs help people, too. A reef acts like a wall that breaks up large ocean waves before they reach the shore. This helps protect beaches and the towns behind them during storms. Many families who live near reefs catch fish there for food. Visitors travel from far away to swim and snorkel above the coral, which brings money to towns along the coast. Scientists have even studied reef animals while searching for new medicines.
6Today, many reefs are in trouble. When ocean water stays too warm for too long, corals become stressed and push the algae out of their bodies. Without the algae, the corals turn white, which is called coral bleaching. Bleached corals are still alive, but they are weak and hungry. If the water cools down soon, the algae can return and the coral can recover. If the water stays hot, the coral may die. Pollution and careless fishing can also harm reefs. Around the world, people are working to protect reefs by keeping the water clean and setting aside areas where fishing is limited.