How do corals grow?

This animation demonstrates how stony corals grow vertically. Learn more about how stony corals grow.

Both hard and soft corals grow in somewhat similar ways, but it’s important to distinguish between these two types of corals and the forms they take as they grow.

Hard, or stony, corals have very small polyps, averaging 1 to 3 millimeters in diameter. Reefs form when stony coral polyps secrete skeletons of calcium carbonate (CaCO₃). Over many years, stony coral polyps can create massive reef structures weighing several tons. As they grow, these reefs provide habitat for hundreds to thousands of different vertebrates (like fish) and invertebrates (like lobsters).

The skeletons of stony corals are secreted by the lower portion of the polyp. This process produces a cup, or calyx, in which the polyp sits. The walls surrounding the cup are called the theca, and the floor is called the basal plate. Periodically, a polyp will lift off its base and secrete a new basal plate above the old one, creating a small chamber or partition in the skeleton. With each added partition, the polyp is elevated and colonial coral structure grows.

There are approximately 800 stony coral species globally that vary in form and function. Learn more about types of stony coral. Learn more about types of stony coral.

When stony coral polyps are physically stressed, they contract into their calyx so that virtually no part is exposed above their skeleton. This protects the polyp from predators and the elements. At other times, polyps extend out of their calyx. Most polyps extend the farthest when they feed.

Reef-building corals exhibit a wide range of shapes. For instance, branching corals have primary and secondary branches. Digitate corals look like fingers or clumps of cigars and have no secondary branches. Table corals form table-like structures and often have fused branches. Elkhorn coral has large, flattened branches. Foliose corals have broad plate-like portions rising in whorl-like patterns. Encrusting corals grow as a thin layer on a substrate - like a rock. Massive corals are ball-shaped or boulder-like. They may be as small as an egg or as large as a house. Mushroom corals resemble the attached or unattached tops of mushrooms.

Like stony corals, soft corals have specific forms and functions. Learn more about types of soft coral.

Soft corals do not secrete rigid skeletons of calcium carbonate or produce massive reef structures. Instead, their polyps produce a flexible skeleton-like structure made of a protein called gorgi, which is embedded with spiny structures of calcium carbonate called sclerites. This flexible skeleton allows them to bend with ocean currents. The sclerites support their unique vertical structures which grow to resemble a wide range of shapes including trees, bushes, fans, whips, and grasses. Soft coral colonies can grow two to four centimeters per year. Soft corals don’t produce massive reef structures, but are often found on reefs, adding to the entire reef ecosystem. Like hard corals, soft corals are cnidarians (stinging animals).