Also known as "rainforests of the sea", coral reefs offer spectacular sights, as well as supporting wildlife, providing food, jobs and coastal protection for an estimated 500 million people.
But human activities are threatening their survival. Scroll through the gallery to see how our actions are putting coral reefs at risk.
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Rising temperatures: Man-made greenhouse gas emissions are making oceans warmer. In hotter water, corals lose their algae coverings and turn white or "bleached". Bleached coral are not yet dead -- but without their algae they eventually starve.
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Overfishing: The coral reef's inhabitants are its first line of defense. Fish eat some of the creatures that eat coral, while crabs and shrimp use their pincers to protect their coral homes. Fish also graze on seaweed and algae so that they don't grow to cover the reef.
Indiscriminate overfishing can disrupt these ecosystems, leaving the coral more vulnerable to predators. Fishing boat anchors can also scar and break the corals below.
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Invasive species: Over the past 25 years, lionfish have colonized Atlantic coastal regions, from their native Indo-Pacific waters. It's thought they were released into the Atlantic by owners who no longer wanted them as aquarium pets.
With fewer natural predators, the species' new Atlantic populations are growing rapidly. They can eat and out-compete native coral reef creatures. Aquarium releases, aquaculture and fishing boats can all unwittingly introduce invasive species to delicate coral environments.