Lesson Plan

Coral Bleaching: Turning Up The Heat

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Grade Level:
Fifth Grade-Eighth Grade
Biology: Animals, Biology: Plants, Climate Change, Conservation, Environment, Marine Biology, Wildlife Biology, Wildlife Management
45 minutes
Group Size:
Up to 24 (4-8 breakout groups)
National/State Standards:
Standard 6: Students assess the interrelated cycles and forces that shape Earth’s surface, including human interaction with Earth. (ASDOE Elementary Science Standards: Grade 5-8, pp. 42- 73)


Coral reefs in American Samoa have turned pure white on several occasions in recent years. They look freshly bleached, quite pretty, but that's a clear sign that they are in trouble. Two very different kinds of stress cause corals to turn white: (1)clorox bleach, and (2)warm water temperatures. Clorox bleaching happens from time to time when a foolish fisherman dumps clorox onto the reef to kill fish. This is very short-sighted because it also kills everything else in the vicinity.


Students will be able to:

1. Define the vocabulary terms global warming and adaptations.

2. Identify several coral reef species in American Samoa and explain their role.

3. Describe coral reef adaptations and how global warming threatens the health of coral reefs.


American Samoa has unusual coral reefs in some ways. First, many of the reefs are dominated by a marine plant that does not even look like it is alive: coralline algae. This is a plant that usually grows as a smooth pink coating (it looks like pink paint) that slowly spreads a thin layer across reef surfaces. It can grow over pieces of dead coral rubble and eventually cement them together and stabilize a field of loose rubble that was rolling around with the waves.

Young corals (larvae) that attach to rolling coral rubble get knocked off or smothered, so it is hard for coral to get established on loose rubble. Some coralline algae also release a chemical that attracts coral larvae to settle on it. Coral larvae are tiny ovals, about half the size of a grain of rice that swim around and then settle down and grow into corals. So the pink coralline algae help corals to get re-established after hurricane damage to the reefs. American Samoa has an unusually large amount of coralline algae for unknown reasons.

A second unusual thing about our reefs is the abundance of encrusting corals. These are corals that are fairly flat and also cover the bottom like a thick layer of paint. Corals compete for space and light (so their zooxanthellae can use the sunlight to produce food), and it seems like encrusting corals have a strategy of trying to claim as much space as possible before anyone else does. Encrusting corals can be found on coral reefs anywhere, but they are more common in American Samoa than on most reefs.

Coral communities are made up of different species in different places. There are two main types of corals in the pools, "finger coral" and “staghorns”. Finger corals have branches that look like fingers with round tips, and staghorns look like deer antlers with many branches and sharp tips.

A second coral community lives on the “reef crest” where the waves crash. Here, corals tend to be sturdy, yet even on the reef crest one common species has many small branches about the thickness of a pencil. These corals get hit the hardest by waves, yet they have some of the thinnest and most delicate branches which seem puzzling. Perhaps the branches’ being close together reduces the waves’ force.

A third set of coral communities live on the reef slopes, where the reef drops away into deeper water. Here encrusting corals and coralline algae are common, but there are some places where other communities of corals can be found. In some places, reef slopes at medium depths are dominated by a “flower coral” (Lobophyllia hemprichii) where the coral polyps are relatively large, up to 2-3 inches in diameter. Deeper on some slopes, the most common corals (Mycedium sp.) form overlapping plates almost like shingles on the slope.

Some corals grow in a shape rather like a table — they have a single stalk, often in the center of the coral, and their top is a big flat disk or table-top. Some may even have multiple layers of table-tops. These “table corals” are actually related to the staghorns (in the genus Acropora; a common species of table coral is Acropora hyacinthus).

A few corals live to become giants. Some of these are called boulder corals, often yellow or brown in color. These slow-growing corals can be found in all sizes on our reefs. One of the world’s largest is at Ta’u Island and is over 15 feet tall—it may be hundreds of years old. Boulder corals are in the genus Porites, along with the finger corals.

Most corals are firmly attached to the reef, but a few are not. One common type that isn’t attached is the “mushroom coral”. These corals, in the genus Fungia, look like a mushroom cap that has been turned over, with radiating ridges that look like the “gills” on a mushroom. Their larvae settle anywhere on the slope, but wave action can move the adults around when it is rough. On a slope, they tend to slide downwards when the waves move them, and end up at the bottom of the slope where they accumulate. Some tiny young mushroom corals have even been observed to “walk” on their tentacles.

Coral reefs look like they are the same day after day, but over a longer time span of decades to centuries or longer, they can change considerably. Because most corals live only in warm tropical waters, it seems odd that corals will die when the water gets slightly warmer. The reason is that corals live close to the hottest temperature that they can tolerate, so it doesn't take much to push them over the limit. To explain what is happening, recall that corals are animals with colorful plantlike cells (Zooxanthellae) living in their tissues. These cells use the sun's light to produce food which is also used by the coral animal. Many coral animals receive much of their food this way, so this relationship is quite important to the coral animal. The coral, in turn, provides the zooxanthellae with nutrients and a secure place to live. Both the coral and the zooxanthellae benefit from this arrangement.

When the coral is stressed by warmer than usual temperatures, the zooxanthellae are released from the coral, for reasons known only to them. What's left is a rather colorless coral animal overlying a bright white coral skeleton. The animal portion of the coral may eventually recover its zooxanthellae and continue living, or it may die, depending on how stressed it gets. It's easy to tell when portions of the coral die because they become covered with fuzzy green algae.



1. Dead coral pieces
2. Blank poster boards
3. Markers and/or crayons
4 Recycled magazines
5. Straws
6. White paper plates
7. Synthetic feathers
8. Volleyball net
9. Clothes pins
10. Power point program

Handouts & Worksheets

1. Coral Reef Illustration Key


Last updated: February 28, 2015