Coral Reefs: Under Attack
- Biology: Animals, Biology: Plants, Climate Change, Conservation, Ecology, Environment, Marine Biology
- 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)
OverviewAmerican Samoa’s reefs have several kinds of starfish (aveau, fetu). Most have five “arms,” like the brilliant blue starfish (Lynkia laevegata), but the crown-of-thorns starfish (alamea, Acanthaster planci) has about 15 arms. They’re a big starfish, with adults commonly over a foot in diameter. They can be a beautiful dark red, or a dark green, often with some red markings. This starfish is one to look at, but not touch.
Students will be able to:
1. Define the vocabulary terms crown-of-thorn and conservation.
2. Identify how crown-of-thorns has spread in American Samoa.
3. Describe how coral reefs are threatened by crown-of-thorns.
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.
Crown-of-thorn starfish have a upper surface that is covered with nasty spines. Anything but the very lightest touch, and the spines will stick into you, and you will be very sorry. There are toxic chemicals in the spines, and once you are stuck, it will hurt, and continue to hurt for hours if not days. If you get stuck by several spines, the area may go numb and stay that way for a week or more, and if you get stuck by a lot of spines you could become weak or paralyzed in that part of your body or even have more trouble.
The underside of a starfish has little suckers at the ends of their tube feet. They “walk” on them and hold on to the substrate with these suckers. It is said that if you have been stung by a crown-of-thorns, have the tube feet touch or walk on the area where you have been stung, and it will reduce the pain. A much better idea is not to get stung in the first place. Crown-of-thorns, like most starfish, have an unusual way of eating. They push their stomach out their mouth to cover what they want to eat, then their stomach digests their food outside their mouth, and then they pull their stomach back inside them. If you think about trying that on your dinner, you’ll realize just how amazing that is. Crown-of-thorns are a bit unusual in what they eat: coral. They eat the outer living part of the coral, leaving the dead white skeleton. Sometimes little patches of live coral tissue are left and the coral survives and can slowly re-grow, other times the coral is completely killed.
Most of the time, crown-of-thorns are pretty rare on coral reefs. But back in the 1950’s huge numbers of them appeared on coral reefs in Japan and ate the corals killing most of the reefs there. Then in the 1960’s the same thing happened in Australia’s Great Barrier Reef. In 1978-1979, huge numbers of them appeared in American Samoa and killed most of the corals there. When such outbreaks happen, people may try to save the reef by killing as many crown-of-thorns as they can, but cutting them up doesn’t kill them, they just grow the missing parts back. It’s best to spear them and take them out of the water, where they die quickly.
One idea about why such huge outbreaks of millions of these starfish occur is that humans have killed the predators that feed on them, like trumpet triton mollusks and hump-head wrasse. So it is best to leave those predators on the reef. Another idea that has more evidence to support it is that unusually heavy rainstorms wash nutrients from the land into the water, which feed the tiny plants and animals that are the food of the young starfish, so more juveniles survive and they grow up into the invading hordes of coral-killers. From this viewpoint, it makes sense to reduce the amount of soil, piggery sewage, and high phosphate laundry detergents that flush down the streams into the ocean. These nutrients might help more crown-of-thorn juveniles survive and grow up to eat the corals.
National Park Service divers have killed almost 90 crown-of-thorn starfish (Acanthaster planci, alamea) in an effort to protect our reef from these voracious predators. A single alamea can consume up to 107 square feet of living coral in a year, and will eat almost every coral species on the reef. The last time there was an alamea outbreak in American Samoa was 1977, when hungry alamea devoured over 80% of the territories coral reefs. The National Park Service is collaborating with the Department of Marine and Wildlife Resources (DMWR), the National Marine Sanctuary, and the governor’s Coral Reef Advisory Group to combat alamea outbreaks.
1. Dead coral pieces
2. Clay (red and gray)
4. Chalk or whiteboard
6. Paint brushes
Introduce Inquiry Questions?
What is a crown-of-thorn starfish? How does it negatively impact coral reefs?
Ask: Have you ever heard the term crown-of-thorn? If yes, explain where or how they heard the term? If not, what do you think it is? Explain that crown-of-thorn is a type of starfish that feeds on corals.
Ask: Have you ever heard the term conservation? If yes, what and how did you hear about it? If not, what do you think it means? Explain that conservation means “to protect something important from being destroyed or overused.”
Before the video, ask students to pay close attention as to why corals are so worried about the spread of the crown-of-thorns. After the video, check student’s comprehension by asking the following questions:
1. What is a crown-of-thorn starfish?
2. What else is affected when corals are consumed by the crown-of-thorns?
3. What purpose does the crown-of-thorns have in the reef ecosystem?
Activity 1: Make your own Coral and Crown-of-thorn
Have students arrange their desks and provide materials as indicated so that they may create their own corals along with a replica of a crown-of-thorn. First, distribute dead coral pieces to each student. Explain that the dead coral pieces represent their corals and they have to use materials like paint and paint brushes to decorate one of the dead coral pieces as if they were still alive. Tell students to be creative in that all corals come in different colors, shapes, and sizes.
Distribute clay and instruct students create a replica of the crown-of-thorns starfish using the toothpicks as the thorns sticking out of the starfish. Show students images of the crown-of-thorn in the video to give them an idea of what it looks like. Place replica next to dead coral piece and decorated coral piece.
Have students discuss their creations. Have student take turns sharing about their creations and describe the role it plays in the ocean ecosystem. To check for understanding, have students discuss what a healthy coral look like and after their consumed by the crown-of-thorns. After each student has shared, discuss with students how people are affecting the spread of the crown-of-thorns. Then revisit some of the same questions students discussed after the video. Ask: What purpose does the crown-of-thorns have in the reef ecosystem?
Conclusion with Inquiry Question
What is a crown-of-thorn starfish? How does it negatively impact coral reefs?
The ocean and all its inhabitants are special; please do your part to conserve, preserve, and protect this precious resource by preventing illegal dumping and overfishing.