Solving a Mystery

Coral studies from Darwin'sGeology of the Voyage of the Beagle © AMNH Library

Coral studies from Darwin'sGeology of the Voyage of the Beagle

© AMNH Library

Many corals form rocky reefs that rise from hundreds of feet beneath the sea. Before Darwin, how these large reefs formed remained a mystery. The animals that build coral reefs live only near the sunlit surface. They cannot survive in the dark water at the base of the reef—making it impossible to build a large reef from the bottom up. But clearly they could not build from the top down! So how did they do it? Ironically, the answer occurred to Darwin while he was thinking about the rise of the Andes Mountains.

If land could slowly rise over millions of years, as geologist Charles Lyell argued, it could also sink. Suppose a coral reef was growing in shallow water. What if the sea floor began to sink? The reef would grow upward to stay near the sunny surface. The more the sea floor lowered, the taller the reef would grow. Eventually, a huge reef would form, with the bottom anchored in the dark depths—even though the entire reef was built by animals living near the surface.

Reaching for the Sun

Coral reefs are made by living animals. The main reef-builders are colonies of tiny coral polyps, which secrete layer after layer of limestone. Over many years, this limestone can form an immense reef.

Reef-building corals live only in clear, shallow water, because their bodies contain living, photosynthetic algae that need light to grow. The coral cannot survive without these algae. So like plants spreading their leaves, they grow up and out to capture more sunlight.