Lesson Plan

How Do Coral Reefs Form?

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Grade Level:
Sixth Grade-Eighth Grade
Biology: Animals, Biology: Plants, Earth Science, Education, Environment, Marine Biology
45 minutes
Group Size:
Up to 24 (4-8 breakout groups)
National/State Standards:
Standard 8: Students inquire how organisms and populations of organisms obtain resources from their environment.
Standard 7: Students examine organisms’ structures and functions for life processes, including growth and reproduction.


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



Last updated: February 28, 2015