Nature Diary
Coral reefs

A coral reef is an underwater ecosystem characterized by reef-building corals. Reefs are formed of colonies of coral polyps held together by calcium carbonate. They look like rocks, but most coral reefs are built from stony corals, whose polyps cluster in groups. The picture shows a close up of polyps arrayed on a coral, waving their tentacles. There can be thousands of polyps on a single coral branch.
Coral reefs can be found all over the world. However, most coral reefs grow in shallow, clean ocean waters on either side of the Equator, because they need sunlight and warm temperatures all year to survive.
Some famous coral reefs include the Great Barrier Reef in Australia (see picture), the Amazon Reef in Brazil and French Guiana, the Tubbataha Reef in the Philippines and the Miami Terrace Reef in the United States. Most coral reefs today are 5,000–10,000 years old.
There are many kinds of reef-building corals: table coral, mushroom coral, staghorn coral, pillar coral, brain coral, etc., all named after the shapes they resemble. There are even fluorescent corals!
Corals are animals. They can appear like plants because they are sessile (that means, fixed in one place) and take root on the ocean floor. But unlike plants, corals do not make their own food. Coral polyps do not photosynthesize, but have a symbiotic relationship with microscopic algae, commonly referred to as zooxanthellae. These microscopic algae that live inside coral, gives it colour and provides it with food through photosynthesis. Because of this relationship, coral reefs grow much faster in clear water, which admits more sunlight. Coral that loses a large fraction of its zooxanthellae becomes white and is said to be bleached, a condition which, unless corrected, can kill the coral.
Corals also absorb nutrients, including inorganic nitrogen and phosphorus, directly from water. Many corals extend their tentacles at night to catch and feed on plankton which drift with the tide. In fact, plankton are marine drifters — organisms carried along by tides and currents. They are food for fish and whales too.

Life in coral reefs
Lots of different types of sea creatures call coral reefs their home – giant clams, starfish, sea turtles, seahorses, eels, cuttlefish, and more. Coral reefs support over one-quarter of all marine species. This diversity results in complex food webs, with large predator fish eating smaller forage fish that eat yet smaller zooplankton and so on. However, all food webs eventually depend on plants, which are the primary producers.
The reefs provide this diverse range of animals with everything they need, including food and shelter. A lot of animals who live on the reef camouflage themselves to blend amongst the coral, either to stay safe from other animals or to hide as they hunt.
Plants also live on reefs. Algae live inside soft coral, using sunlight to provide food and help the coral grow. In other areas, sea grass provides food for sea animals like dugongs and turtles.

Endangered species
The biggest threats to coral reefs are from pollution spilling into the oceans, damage from ships and boats, and climate change, which makes the water warmer and more acidic.
Pollution is a big threat to all sea wildlife, including coral reefs. To save our coral reefs, many people are working hard to clean up the pollution on land and in the sea. Some countries have even protected their coral reefs. Coral reefs have existed for nearly 500 million years. Let us not destroy them through our careless activities.
Sources: Wikipedia, https:www.natgeokids.com