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# How Do Stony Corals Grow?

## Corals Tutorial

Over the course of many years, stony coral polyps can create massive  reef structures. Reefs form when polyps secrete skeletons of calcium  carbonate (CaCO3). Most stony corals have very  small polyps, averaging 1 to 3 millimeters in diameter, but entire  colonies can grow very large and weigh several tons. As they grow,  these reefs provide structural habitats for hundreds to thousands of  different vertebrate and invertebrate species.

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 in the skeleton. While the colony is alive, CaCO3 is deposited, adding partitions and elevating the coral.

When 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 the 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 against a substrate. Massive corals are ball-shaped or boulder-like and may be small as an egg or as large as a house. Mushroom corals resemble the attached or unattached tops of mushrooms.

Coral species number in the thousands, and stony corals take on several characteristic forms. Get to know eight different stony coral growth patterns.

Stony corals grow when individual polyps lift themselves up from the base of the stony cups in which they reside, and create a new base above it. Learn how stony corals grow.

### Corals Lessons

              - Welcome
              - What are Corals?
              - Zooxanthellae What's That?
              - How Do Stony Corals Grow?
              - How Do Coral Reefs Form?
              - Where Are Coral Reefs Found?
              - How Do Corals Reproduce?
              - Importance of Coral Reefs
              - Natural Threats to Coral Reefs
              - Anthropogenic Threats to Corals
              - Coral Diseases
              - Protecting Coral Reefs
              - References
              - Roadmap to Resources 
              - Subject Review (PDF)

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