Coral Reefs 101 | National Geographic

(Gentle music) - [Narrator] Coral reefs, their bright, vivid colors can be seen in tropical ocean waters around the globe. Beyond their brilliant appearance lies a hidden significance.

Coral are animals. Though they may look like colorful plants, coral are, in fact, made up of tiny animals called polyps. These invertebrates can range from the size of a pinhead to a bit larger than a basketball. Each polyp consists of a soft, saclike body topped by a mouth covered in stinging tentacles. To protect their soft bodies and add support, the polyps secrete limestone skeletons, or calicles.

Corals are mega builders. Polyp calicles connect to one another, creating a colony that acts as a single organism. As colonies grow over hundreds and thousands of years, they join with other colonies and become reefs that can grow to hundreds of miles long. The largest coral reef is Australia's Great Barrier Reef, which began growing about 20,000 years ago.

Coral reefs are some of the most diverse ecosystems on Earth. Though they cover less than 1% of the ocean floor, coral reefs are home to 25% of all marine creatures. It's been estimated that up to two million species inhabit coral reefs, rivaling the biodiversity of the rain forest. The reefs provide rich habitat that helps protect young fish as they grow.

Coral are translucent. Coral reefs get their rainbow of colors from algae, or zooxanthellae, that live in their tissue. Though corals use their tentacles to capture some food, most of their food comes from the algae they house. When coral become stressed by pollution or other factors, they evict their algae. Coral bleaching results, revealing corals' white skeletons.

Coral provide a window to the past. As coral grow, their limestone skeletons form layers, similar to tree rings, that vary in composition and thickness based on ocean conditions at the time. With some coral reefs growing for thousands or even millions of years, scientists can study these layers to reveal what the Earth's climate may have been like in the ancient past.

Unfortunately, climate change is putting coral's future in danger, along with the millions of species that inhabit the reefs and the half-billion people that rely on reef fish for food. Warming waters result in prolonged coral bleaching that kill coral reefs or leave them vulnerable to other threats. Without significant action on climate change, our oceans could lose many of their colorful reefs by the end of the century.