There are about 6,000 species of coral around the world, with some species growing in warm shallow waters near coastlines and others thriving on the dark, cold seafloor of the open ocean.

Though coral may look like a colorful plant growing from roots in the seafloor, it is actually an animal. Corals are known as colonial organisms, because many individual creatures live and grow while connected to each other. They are also dependent on one another for survival. The tiny, individual organisms that make up large coral colonies are called coral polyps. The polyps use ions in seawater to make limestone exoskeletons—skeletons outside the body—for themselves.

A coral polyp is shaped like a cylinder, with a mouth at one end, surrounded by tentacles. The arm-like tentacles gather food and sting creatures that threaten the coral. After food is digested, the polyp’s waste products exit through the mouth.

Coral polyp bodies are usually clear. The bright colors that characterize many corals are actually various types of algae growing in the polyp’s tissue. The presence of the algae, specifically a type of algae called zooxanthellae, helps the coral in several ways. For one, the algae remove waste from the coral. The algae also use the coral’s waste products for photosynthesis, which is how a plant makes its own food. Byproducts of photosynthesis include oxygen and carbohydrates, which the coral consumes and uses to build reefs. The mutually beneficial relationship between coral and algae is called symbiosis.

Coral reefs are among the most complex and fascinating marine ecosystems in the sea, and they include a wide range of symbiotic relationships. Coral reefs are sometimes known as the “rainforests of the sea.” Nearly a quarter of all the fish in the sea rely on healthy coral reefs. Corals provide habitats for fish and other organisms in the ocean. The Northwest Hawaiian Island coral reefs are home to about 7,000 species of plants and animals.

Coral reefs not only provide marine species with a rich habitat, but they also assist people as well. Millions of people around the world rely on fish caught in and around coral reefs. The reefs also draw in tourists, which helps the local economies. Coral reefs protect people and land from storms as well, serving as a barrier that reduces the impact of large waves on shore.

The protection of coral reefs is a high priority for many oceanographers and other marine conservationists. National Geographic Explorer Tracy Allie Turner is a coral reef scientist who hopes her research on reefs around the South Maldives will convince government leaders to designate more coral reefs as marine protected areas.

Further research and protection of coral reefs is important, in large part because the world has lost nearly 20 percent of its coral reefs, with many more reefs under threat today. Pollution and increasing ocean temperatures are two major causes of coral reef loss—a problem that can have long-reaching effects on humans and marine species alike. Both pollution and warming waters can cause coral bleaching, a process where stressed corals expel their symbiotic algae causing them to turn white. The absence of the zooxanthellae causes the coral to have a more limited access to food and this increases their susceptibility to diseases.