WATCH: Coral Reefs 101

What are coral reefs? Coral can be found in tropical ocean waters around the world. But how much do you know about reefs and the tiny animals—polyps—that build them? Learn all about coral and why warming waters threaten the future of the reef ecosystem.
Tube coral (Cladocora arbuscula) photographed Mote Tropical Research Laboratory in Summerland Key, Florida
Tube coral (Cladocora arbuscula) photographed Mote Tropical Research Laboratory in Summerland Key, Florida
Photograph by Joel Sartore, National Geographic Photo Ark
Common Name:
Corals
Scientific Name:
Anthozoa
Type:
Invertebrates
Diet:
Carnivore
Group Name:
Colony
Average Life Span:
Polyp, 2 years to hundreds of years; colony, 5 years to several centuries
Size:
Polyp: 0.25 to 12 inches

Coral organisms, called polyps, can live on their own, but are primarily associated with the spectacularly diverse limestone communities, or reefs, they construct.

Polyps, Colonies, and Reefs

Coral polyps are tiny, soft-bodied organisms related to sea anemones and jellyfish. At their base is a hard, protective limestone skeleton called a calicle, which forms the structure of coral reefs. Reefs begin when a polyp attaches itself to a rock on the sea floor, then divides, or buds, into thousands of clones. The 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. Some of the coral reefs on the planet today began growing over 50 million years ago.

Color and Bleaching

Coral polyps are actually translucent animals. Reefs get their wild hues from the billions of colorful zooxanthellae (ZOH-oh-ZAN-thell-ee) algae they host. When stressed by such things as temperature change or pollution, corals will evict their boarders, causing coral bleaching that can kill the colony if the stress is not mitigated.

Feeding

Corals live in tropical waters throughout the world, generally close to the surface where the sun's rays can reach the algae. While corals get most of their nutrients from the byproducts of the algae's photosynthesis, they also have barbed, venomous tentacles they can stick out, usually at night, to grab zooplankton and even small fish.

Threats to Survival

Coral reefs teem with life, covering less than one percent of the ocean floor, but supporting about 25 percent of all marine creatures. However, threats to their existence abound, and scientists estimate that human factors—such as pollution, global warming, and sedimentation—are threatening large swaths of the world's reefs.

This photo was submitted to Your Shot, our photo community on Instagram. Follow us on Instagram at @natgeoyourshot or visit us at natgeo.com/yourshot for the latest submissions and news about the community.
This photo was submitted to Your Shot, our photo community on Instagram. Follow us on Instagram at @natgeoyourshot or visit us at natgeo.com/yourshot for the latest submissions and news about the community.
Photograph by Gonzalo Giribet, National Geographic Your Shot

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