Anatomy of Coral

Part B: Coral Polyps

  1. Now that you are familiar with the basic structure of hydra polyps, let's take a look at how closely they actually compare to coral polyps.



    Checking In

    • What physical characteristics does the hydra share with a coral polyp? Identify any common body parts.
    • What are the key differences between the anatomy of coral polyps and hydra.


    There are two basic types of coral: hard corals and soft corals. Hard corals have an outer skeleton made of limestone, also known as calcium carbonate (CaCO3), and soft corals have bits of calcium carbonate embedded inside their bodies. The CaCO3 in soft corals is in the form of little spikes that help bind many individual polyps together in fan- or whip-like structures. In hard corals, polyps sit inside little cups built from calcium carbonate. Many cups connected together make up a coral colony, and when hundreds of hard coral colonies grow next to and on top of one another, coral reefscoral reef: aragonite (calcium carbonate) structure produced by corals and found in shallow, tropical marine waters. are formed.

    Hard corals (left): elkhorn coral and a clubtip finger coral in the foreground. Photo courtesy of NOAA Photo Library. Soft coral (right) with polyps extended. Photo courtesy of NOAA Photo Library.

    Most reef-building corals have a symbiotic relationship with photosynthetic algae, called zooxanthellaezooxanthellae: any of various yellow-green algae that live symbiotically within the cells of other organisms, such as reef-building coral polyps. ("zo-ah-zan-THELL-ee"), which live in their gastrodermis tissue. The zooxanthellae, like all plants, convert sunlight, carbon dioxide, and water into oxygen and carbohydrates that feed the coral polyps and help them produce reef-building calcium carbonate. In return, the corals provide the zooxanthellae with protection and the compounds they need for photosynthesis. Pigments in zooxanthellae also give corals their beautiful orange, red, purple, and yellow colors.

    Zooxanthellae in a coral polyp. Image source: Maricopa Community College.

    Stop and Think

    1: Why do you think corals need both zooxanthellae and nematocysts?
  2. Build a simple physical model of a single coral polyp using the following everyday materials:
    • One 3" x 3" sticky note (i.e. Post-It© notes)
    • pencil or pen
    • Transparent tape
    • Scissors
    • 1 egg carton cup (1/12 of an egg carton)

    Stop and Think

    2: What does each part of your model represent?
    3: What part of the model represents the animal? Where would you find zooxanthellae?
    4: What improvements can you think of to make the model more realistic?
  3. Working with a partner or in a small group, come up with a creative way to model a full day in the life of a polyp. Demonstrate the details of how it gets nutrients in the daytime and in the nighttime. Use available craft materials to modify, reinvent, or extend your coral polyp model. Include as many details as you can to fully describe each process.
  4. Keep your model in a safe place so you can use it again later in the unit.