Cities Beneath the Waves

A shallow tropical coral reef looks like a colorful rocky garden, but living animals build it. A coral colony contains tiny soft-bodied polyps. Many reef-building polyps make hard calcium-carbonate skeletons. As generations grow on older skeletons, the reef structure rises.

Inside many shallow reef-building corals live microscopic algae. The algae use sunlight to make food and share some products with the coral. In return, they receive shelter and materials they need. This partnership helps explain why these corals thrive in clear, sunlit water.

Not all corals live this way. Deep-sea corals grow far beyond sunlight and lack the same photosynthetic algae. They capture food carried by currents. Some form large habitats, although their shapes and growth differ from familiar tropical reefs.

When shallow tropical water stays too warm, a coral may expel its algae and turn pale, an event called bleaching. A bleached coral is alive, but it has lost an important energy source and may starve if stressful conditions continue. By naming which corals depend on sunlight, scientists avoid turning one important partnership into a rule for every coral in the sea.