Smithsonian National Museum of Natural History

Constructing a Coral Reef

Constructing a Coral Reef

Tropical coral reefs are amazing underwater habitats. Known as rainforests of the ocean, coral reefs have over 4,000 species of fish and support 25 percent of marine species at  one point in their life cycles. Coral reefs did not suddenly appear in the ocean but were built step-by-step by various animals over millions of years to become the habitats that they are today.

Over time, reef species have changed but the functions and roles of the different species have remained similar. In this video, we will have a general overview of how the ecosystem is built, showing some of the major plant and animal groups that are part of these complex systems.

Scientists from around the world work together to study all the plants and animals that live in these reef systems, so they can better understand changes over time. Here our scientists present step-by-step how the biodiversity of a reef system is built. Let's go over the different groups now.

First, stony corals or hard corals are what create the foundation of the reef. A reef is started when the free swimming coral larvae attach to hard surfaces along the edges of islands or continents and start to grow. These tiny animals are called coral polyps and they create tiny stone skeletons to live in. They take calcium carbonate, the same mineral that makes up chalk and shells, from the water to form the mineral skeleton. These animals live in colonies that form large stony coral structures. As living animals they provide structure and a food source and once the polyps die their hard skeletons still contribute to the reef by adding complex structure and habitat for other plants and animals.

There are over 3,000 species of stony corals, coming in all sorts of shapes and structures. The more complex 3D space a reef has, the more life it will attract.

Once stony corals are established, soft coral and sponges will come in and attach themselves  to the new hard surfaces the stony corals have created. The soft corals are related to stony corals, but do not create a calcium carbonate skeleton. Sponges are animals; they have bodies full of pores and channels that allow water to filter through them to feed.

These flexible animals add complexity and color to a reef. Like the soft coral, different types of algae depend on a hard bottom to land on and provide protection from the waves. They grow here and provide an important food source for animals that live on the reef.

Speaking of animals, small invertebrates, or animals without backbones, are another important group on the reef. They will eat the algae, soft corals, and sponges and will also use the coral reef structure as shelter to hide from predators.

Also attracted to the algae and coral are herbivorous fish, which eat plants, and corallivorous fish which eat coral. These fish will eat the algae and coral, which helps maintain the structure of the reef.  

Then come the mid-level predators: the carnivorous fish that will eat the smaller animals. These carnivorous fish keep the herbivorous 

and coralivorous fish and invertebrates in check so they do not eat all the algae, coral, and sponges.

And finally, top level predators are animals that prey on the carnivorous fish and are attracted to all the life that is in a coral reef. The top predators also contribute in keeping balance in the coral reef food chain.

Coral reefs are complex habitats that are home to many different marine species. Let's recap the different groups of plants and animals that make up a coral reef ecosystem. Can you recall each group's major function?

Let's go over it. To start stony coral make up the reef foundation, and then the soft coral and sponges add to the structure. The algae adds to the structure and is a food source. Small invertebrates eat the coral and algae. The herbivorous fish and coralivorous fish eat the coral and algae to maintain structure. 

Mid-level predator fish eat the smaller animals, and finally the top predators maintain balance in the food chain by feeding on other reef species.  

Well done! Now you know the general structure of tropical reef systems. There are many different groups involved that are in balance with each other, and if there is an imbalance, for example if stony coral die, other organisms like algae or soft corals can take over the space that is left behind. Understanding how these reef systems are constructed helps scientists ask questions and investigate when the reef is out of balance from impacts of climate change, overfishing, pollution, and even disease. By understanding these systems we can help protect them better.

Video Description

Coral reefs are the most diverse of all marine ecosystems. They teem with life, with about one-quarter of all ocean species depending on reefs for food and shelter at some point in their lifecycle.  

Stony coral skeletons are the foundation of these reef systems. They create a three-dimensional structure that provides habitat, food, and protection for other organisms. In this video, we’ll describe some of the other major plant and animal groups and their roles in contributing to a balanced coral reef ecosystem. 

Knowing what organisms live and depend on reef systems and how they interact with each other is important for understanding what will happen to species and reefs when they are threatened or out of balance. Currently, they are threatened by changes in temperature and water chemistry from climate change, destructive and overfishing practices, nutrient run-off and pollution, invasive species, and disease. 

Scientists from around the world are cataloging the biodiversity of reef systems so that we are better equipped to predict how they may change when facing these threats, and ultimately, so we can learn how to conserve and protect them. Some of these scientists are part of the Smithsonian MarineGEO & Tennenbaum Marine Observatories Network.

  • This video is an extension activity for the museum's Reefs Unleashed school program for Grades 6-12.

About Smithsonian MarineGEO 

Smithsonian MarineGEO is a team of scientists from the Smithsonian and our partners all around the world, working together to solve an important mystery of science. The mystery is how the amazing variety of sea life works together to keep the ocean healthy for animals and people, with clean water, lots of fish for us to eat, and safe places for animals to live where they're protected from climate change and other threats. We look for answers by diving into the ocean and identifying the kinds of animals living in different habitats, how many there are, what they're doing, and how all those things are changing. The ocean is massive, so it takes a team of all kinds of people to get the job done. The MarineGEO teams knows that the best way to discover more about the ocean is to do it together.

Learn more about MarineGEO & Tennenbaum Marine Observatories Network.

Related Resources

Resource Type
Videos and Webcasts
Grade Level
6-8, 9-12
Life Science
Sant Ocean Hall