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.
    Image source: NOAA. Click image to enlarge.

    From NOAA:
    Polyps are made of an outer cell layer called epidermis ("ep-ih-DERM-iss") and an inner cell layer called gastrodermis ("gas-tro-DERM-iss"), with a jelly-like substance called mesoglea ("mez-oh-GLEE-uh") in between. Each polyp makes its own cup-shaped skeleton called a calyx ("KAY-lix)" from limestone (calcium carbonate). The base of the calyx is called the basal plate, and the outer walls of the calyx are called the theca ("THEE-kuh"). Vertical partitions called septa (plural form of septum) extend part-way into the cup from the inner surface of the theca. The outer surface of the theca is covered by the soft tissues of the coral. Polyps have a mouth surrounded by a ring of arms called tentacles. The tentacles have stinging cells called nematocysts ("nee-MAT-oh-sists") that polyps use to capture food. Most corals are carnivorous, and feed on small floating animals or even fish. Many corals also feed by collecting very small bits of floating material on strings of mucous, which they pull into their mouths. Food is digested by digestive filaments in the stomach. Waste is expelled through the mouth. Most reef-building corals have very small polyps, about one to three millimeters in diameter. Individual polyps in a coral colony are connected by a thin band of living tissue called a coenosarc ("SEE-no-sark").

    Checking In

    • What physical characteristics does the hydra share with a coral polyp? Identify any common body parts.
      Hydras and coral polyps both have: symmetric tube-like structure, tentacles, nematocysts, a single mouth/waste opening, a gastrovascular cavity (stomach), epidermis, mesoglea, gastrodermis, and a basal disc/plate for attaching to hard surfaces.
    • What are the key differences between the anatomy of coral polyps and hydra.
      The calyx, theca, and coenosarc, which are not found in hydra, are responsible for connect individual polyps and creating skeletal reef-building material.

    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)

    Here's how to build your polyp model

    roll the sticky note around a pencil Place the note on a tabletop so the sticky edge is facing up and away from you. Place a pencil on the edge of the note nearest you and roll the paper loosely around the pencil. Press on the sticky portion to make the tube.
    tape the bottom closed

    Use tape to close off one end of the tube and hold the note in its new shape.
    cut strips halfway down

    Use scissors to make 4 evenly spaced cuts about halfway down from the top of the cylinder.
    curl the strips outward

    One at a time, roll the cut strips back around your finger to give them a slight curl, opening the top of the cylinder.
    punch a hole through the egg cup

    Use a pencil to poke a hole in the bottom of the egg cup. Enlarge the hole slightly by working and rotating the pencil through the material.
    insert the tube in the egg cup

    Insert the bottom of the tube into the egg cup.
    admire your model coral polyp

    Admire your model coral polyp.

    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.