C is for Coral

Corals have an alarmingly slow growth rate, many expanding mere millimeters or centimeters per year. It can take nearly 10,000 years for a fringing coral reef to form.

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1 year. For the Earth to complete one revolution around the sun.

10 years. Since Charlie bit his brother's finger.

100 years. How long ago the Girl Scouts sold their first cookie.

1,000 years. The vikings first sight North America.

10,000 years. For a fringing coral reef to form.

Illustration for C is for Coral with Several Different Corals and Seaweed on the Sea Floor

Corals have an alarmingly slow growth rate, many expanding mere millimeters or centimeters per year. The largest barrier reefs and atolls can grow for thousands or even millions of years, if the conditions are right.

Fringing reefs extend directly from the shore, whereas barrier reefs are separated from the land by a lagoon. An atoll circles a volcanic island that has sunk below the water's surface, creating a round reef with an expanse of water at its center.

Believe it or not, corals are animals, and many of them are colonial. So, what we think of as a single coral actually comprises hundreds to thousands of individual polyps.

Corals begin life as free-floating larvae that fall and attach to hard, submerged substrates where they grow into polyps. Each polyp has a "mouth" where food enters and waste exits. The mouth is surrounded by tentacles containing stinging cells, called nematocysts, that help capture prey.

Acropora Coral Close-Up

Many reef-building stony corals also have a symbiotic relationship with zooxanthellae, photosynthetic algae. Coral offers algae shelter and resources for photosynthesis. The coral utilizes byproducts of photosynthesis to secrete a calcium carbonate skeleton, which is how reefs grow. Zooxanthellae also give stony corals one additional bounty: their incredible colors.

The reef-building stony corals of the sunlit shallows may be the most familiar, but they aren't the only corals in the sea. Soft corals are found throughout the tropics and deep-sea corals thrive in cold, dark waters far beneath the surface.

Oceans A to Z More in this Series

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