Lesson Plan

How Do Coral Reefs Form?


Coral reefs provide a variety of habitats, each with its own set of characteristic species. Each species of coral comes in different shapes and forms. Activities implemented will give students an understanding of how coral reefs are formed. Starting from a single “polyp”, to a coral with skeleton attached, finally to a coral reef. Students will identify three different types of corals most commonly found in American Samoa and understand their growth by applying hands on activities.


Students will be able to:

1. Explain how coral reefs are formed.

2. Identify three shapes of corals.

3. Name three threats to coral reefs.



The islands of American Samoa are blessed with an abundance of coral (over 250 species). Corals are animals like ourselves, although that may not be readily apparent because many look like rocks. In a sense, corals are indeed partly rock, because only the outer thin layer of the coral is inhabited by the coral animal itself. In that way, corals are like large trees – the inner part is hard and provides structural support, the outer part is the living, growing organism. And, like trees, most coral animals are permanently attached to one spot on the reef. The coral rubble that Samoans traditionally spread outside their houses, and the coral rocks along our beaches, are old, dead pieces that broke off the reef during a storm, got tumbled around and tossed up on the beach. Living corals grow primarily on the outer reef flat and in deeper water. Although they take varied shapes, the coral animals inhabiting their surfaces are similar. They look somewhat like miniature sea anemones (matamalu, ulumane) or upside-down jellyfish (alualu) with short tentacles that give the coral a slightly fuzzy appearance when the tentacles are extended. Each single coral animal is called a polyp, but the coral branch or block we see on the reef is actually not a single animal but a colony of hundreds or thousands of tiny polyps living side by side, giving the appearance of being a single “coral.” The coral's short tentacles can be pulled back into the hard part of the coral when the animal is disturbed or when the coral is exposed at low tide, so even a live coral can look like a rock at such times.


It seems inconceivable that these tiny coral polyps can build the hard coral “rocks” that we see on the reef. They do this by secreting layers of a hard substance (calcium carbonate) beneath their living cells. It’s as if each tiny polyp built a rock-solid house for itself but then, as it grows bigger, it decides to close off the bottom rooms in its house. Then it grows some more and closes-off another layer of bottom rooms, and so on. In this way, the coral polyp always lives in the outer, top layer which has been built upon layers and layers of rooms below. Each polyp also cements its house to those of its adjacent neighbors which strengthens the whole structure, resembling a solidly built high-rise apartment complex. Adding on these new rooms is a slow process. Growth varies from about 0.5 to3 inches per year depending on the species. Over very long time periods, these corals grow into massively strong reef structures that can bear the brunt of powerful waves that crash upon them day after day. The largest corals on our reefs may be hundreds of years old. Corals are one of the few organisms on earth that continually build on top of their old “houses”, forming such large solid structures. This is not like a bird that might build its nest on top of another nest, because both of these nests decay and disappear in a short time. In fact, most organisms on earth leave little trace after they die as their bones or shells disintegrate (dust to dust). Not corals. They build structures much larger and longer-lasting than the Egyptian pyramids. What other organism can do this (except modern man with his steel and cement)?



1. Microscope or magnifying glass
2. Three different colors of clay
3. 10 Pieces of dead corals

4. Power point presentation
5. Scissors

6. Egg cartons
7. Tape

8. Paper



Introduce Inquiry Question

How do coral reefs form?



Have students watch the power point program about corals. Ask: Why are corals important? Ask: Have you ever heard the terms, “coral polyps,” “coral colonies,” and “coralites?”  Explain the meaning of these terms. (1) A coral polyp is the single living organism of a coral, and the animal responsible for our coral reefs. (2) A coral colony is a group of hundreds to thousands of polyps within the same species living together. Explain to students that the three different types of corals are three different species of corals. (3) A coralite is the skeleton of a single coral polyp. Have each student look through a microscope or a magnifying glass to see a close up view of these coralites. Explain to students that a polyp used to live in each coralite.


Activity 1: Shapes of Corals

Organize the class into four groups or more depending on how many students are in the class. 1) Hand out three different colors of clay to each group. Tell each group that they are going to form a type of coral using clay. 2) Have each group divide the clay into 10 little clay balls. 3) Let each group know that each color represents a certain type of coral. It would either be massive, branch, or table coral. 4) Explain that the growing process of corals takes one year to grow to 0.5 to 3 inches. 5) Have them add on a layer for each year. This will give students an idea of how corals grow Ask:  Name three threats that corals encounter. (Natural Disasters, Human Activities, Global Warming) Ask: What type of coral do you think would stand a better chance when encounter hurricanes? (Massive) Why? Due to their form, they grow wider and more stable on the ocean floor.


Activity 2: Build A Coral

1. Begin by cutting the top half and the closing flap off an egg carton, leaving just the section with the twelve egg cups. Place this upside down on a table and punch a hole in the bottom of each egg cup with scissors. To shorten the activity, cut the egg cup tray into thirds, giving each student a section of four egg cups rather than all twelve.

2. Cut a sheet of paper into three strips horizontally. Each strip will become a coral polyp.

Roll each strip into a tube about the diameter of your finger. Tape the tube to keep it from unrolling and tape the bottom of the tube shut.

3. To make the tentacles of the polyp, make several cuts from the top of the tube, ¾ of the way to the bottom of the tube. Get the tentacles to bend/curl by running each fringe over the blade of a scissor or a metal ruler.

4. Insert one polyp tube in each egg cup, pulling it partway through the hole. Tentacles should be on the top of the egg carton.

After each group builds their coral, ask them to come to the front of the class and place their finished product together. The idea of reef building will be more clear to students. Have students view massive coral reefs on the slide presentation. Relate to the class how these massive reefs can exist from a tiny polyp.


Conclusion with Inquiry Question

How do coral reefs form?


Stewardship Message

Our coral reefs are already affected by global warming, and pollution to name a few. We need to take action now by not littering our ocean, and report any harmful fishing methods such as fish poisoning substances (ava niukini) and dynamite (faga i’a) ect.