What is a coral?

 

What may look like one coral is actually a colony of thousands of coral polyps living very close together.

 

Inside the tissue of a coral polyp, zooxanthellae (plant-like cells) make their home. The coral and zooxanthellae work together to survive.

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Hard corals have a skeleton that supports them. It is made out of the same substance that shellfish use to make their shells––calcium carbonate.

 

Corals can grow into structures as small as a coin and as large as an entire island. There are thousands of coral species.

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Are corals plants, animals, or rocks?

Well, corals are animals. However, corals make a calcium carbonate skeleton that looks similar to a rock and have a symbiotic relationship with plant-like cells called zooxanthellae.
The zooxanthellae living in the soft tissue of a coral polyp use sunlight to produce food through photosynthesis and create a byproduct that the coral can use as food. Thus, zooxanthellae provide corals with food; in return, the coral provides the zooxanthellae with shelter and nutrients.
Corals also capture food. At night, they stretch out their stinging tentacles and catch the microscopic organisms that float in the water and digest them in their stomachs. So, corals have two ways of getting food—through zooxanthellae and capturing microscopic organisms.

Corals grow in warm shallow waters that receive plenty of light. Thus, there are many places in the Pacific Ocean where corals can live, but corals can also be found around the world including the Indian Ocean, Caribbean Sea, Red Sea, and Arabian Gulf, and in places influenced by warm currents such as the Florida coast.
A few species can tolerate cooler and deeper waters; however, most coral species live in waters close to the warmest temperature they can tolerate (about 85° F or 29° C). This means that slight increases in ocean temperature can harm corals. Already climate change and the warmer ocean waters associated with it have decreased the health of corals and triggered bleaching events in certain areas. As climate change continues, corals will be placed under more stress.

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How do hard corals build skeletons and reefs?

For protection and support, hard corals build skeletons made of calcium carbonate. To do this, a coral polyp secretes layer upon layer of calcium carbonate underneath its body. As time goes by, the skeleton grows larger and larger, and the polyp lives on its outside edge. As long as a polyp can get the right building material from the water, it can build a strong skeleton.

 

Hard corals, often called the reef-builders, produce a calcium carbonate skeleton.

 

Soft corals do not build calcium carbonate skeletons. Instead, they are supported by tiny spines.

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Hard corals build reefs that support a diversity of life.

Hard corals are often called the reef builders because their skeletons provide support for other corals and other organisms. Soft corals do not build calcium carbonate skeletons. Instead they have spines that support them. They are not considered reef-builders.
When hundreds or thousands of coral polyps build their skeletons close together, they create a calcium carbonate structure that provides habitat and food for a variety of organisms. This is known as a coral reef.