EarthLabs > Corals > Lab 3: Building a Reef > 3B: Coral Reproduction and Reef Formation

Building a Reef

Part B: Coral Reproduction and Reef Formation

A coral reef is formed by the skeletons of many coral polyps joining together. Fossil records suggest that some of these structures have been around for hundreds of millions of years. In order to understand how reefs are formed, you need to know how coral polyps grow and reproduce.

  1. Read the short NOAA article How Do Stony Corals Grow? to learn more about how individual coral polyps grow, secrete skeletal material, and what forms they take when they join together. Click on the images in the article for additional information and images.

    Checking In

    Answer the following questions to check your understanding of coral growth.

    • Describe the process by which coral polyps grow upward.
    • Besides contributing to the growth of the polyp, what other purpose does the coral skeleton serve?


    X-ray of coral showing age bands. Photo by Jennifer M. Smith. Image source: Ocean World.

    After corals die, you can learn a lot about how they lived by cutting them open and looking inside, much the way you might dissect an animal in biology class. The inside of a coral reef actually resembles the inside of a tree. The number of annual rings or bands indicates the number of years the coral lived (see image at right). The thickness of the rings tell us how healthy the coral was each year of its life, with thick rings indicating the years in which there were favorable environmental conditions and abundant food, and thin rings indicating times when resources weren't as readily available. Also, as coral skeletons grow, they incorporate traces of the chemicals from the seawater they live in. Scientists can analyze differences in the chemicals in different age bands to determine how ocean and climate conditions changed over the lifetime of the coral.


  2. Read the short NOAA article How Do Corals Reproduce? to learn more about coral reproduction. When you get to the end of the second paragraph, click on the link to watch the short movie of a broadcast spawning event.

    Checking In

    Answer the following questions to check your understanding of coral reproduction.

    • Do corals reproduce sexually or asexually?
    • What are the advantages of mass spawning events in which many corals release their gametes at the same time?

  3. Read the short NOAA article How Do Coral Reefs Form? to learn more about the process by which reefs form and about the different shapes they take.

    Checking In

    Answer the following questions to check your understanding of coral reef formation.

    • What are the three major reef structures?
    • How quickly do coral reefs grow and how long might it take for a reef to form?

  4. Working with a partner or in a small group, come up with a creative way to demonstrate the following coral processes. Use available craft materials to modify, reinvent, or extend the coral polyp model you made in Lab 2. Include as many details as you can to fully describe each process.
    • Demonstrate how a coral polyp grows upward by depositing calcium carbonate.
    • Demonstrate how a single polyp can become a coral colony through asexual reproduction.
    • Put some polyps together and model a coral spawning event. Compare the results of spawning to the results of asexual reproduction.
    • Come up with a way to illustrate the actual size of coral polyps and the number of polyps in a reef. For instance, if a single polyp were as large as your initial model, how large would its coral reef be?
    • Connect multiple polyps together to create a model of a coral head in one of the eight shapes that are characteristic of stony corals.

  5. Paired with another group, or in front of the class, use your models to demonstrate and explain coral reef processes.

    Stop and Think

    1: Discuss the strengths and weaknesses of each model as well as further ideas for improving them.


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