Blog: An Introduction to Coral Reefs

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May 29

If you ask people about their bucket-list highlights, many of them will tell you they dream of seeing the Great Barrier Reef. Despite this, a lot of people don’t realise exactly what they’re dreaming of. So, what are coral reefs?

Animal, Vegetable or Mineral?

It’s easy to mistake coral reefs as beautiful, colourful rocks or mystical underwater plants. But what you’re actually looking at is a colony of animals all living and working together - a bit like the ants or bees of the underwater world.

If you look up ‘coral reefs’, the dictionary will tell you they are “a hard stony substance secreted by certain marine coelenterates as an external skeleton, typically forming large reefs in warm seas.” But what does that MEAN?! Reading that, it’s no wonder why coral is one of the most misunderstood creatures in the animal kingdom!

So put away your dictionary because here’s a clear and simple introduction to coral reefs.

What are coral reefs?

So let’s take a journey into a day in the life of coral. First of all, If I asked you to imagine a coral animal, would you know what it looked like? Trying to picture one coral from this:

Picture Credit: The Ocean Agency / XL Catlin Seaview Survey

…is about as easy as trying to see one person in this:

"Just Around The Corner | HSBC" by Staudinger + Franke, mladen penev is licensed under CC BY-NC 4.0

Picture sourced from Wikimedia Commons

Coral up close

Zooming right in, an individual coral animal is called a coral polyp. It looks a bit like an upside down jellyfish. Like most animals, it can move, feed and reproduce.

Corals are part of a group of animals called Cnidarians, which live underwater and have special stinging cells.

For corals, these stinging cells are in their tentacles and help them catch their prey.

Polyps live together in large groups, similar to how you live in a house with your family. This is called a coral colony.

Picture Credit: Shaun Wolfe / Coral Reef Image Bank

The coral colony lives in a group of many other colonies, like how many families make up a neighbourhood.

These neighbourhoods of colonies are the coral reefs we know and love.

Types of coral

Hard Coral

Corals come in all shapes, sizes and colours. Some corals, like massive coral, have hard skeletons and grow into giant structures. These act like building blocks of entire reef ecosystems. They are accurately named “hard coral” and are often mistaken for rocks. Generally speaking, they are very slow growing and take a long time to recover from damage.

Picture Credit: Brook Peterson / Coral Reef Image Bank

Soft Coral

Other species of coral, with no hard skeleton, appear like beautiful feathers that flow with the current. These soft-bodied animals often look more like plants and are called (you guessed it) “soft coral”.

How does coral feed?

Now we’ve worked out what coral is, how about a bite to eat? Corals use their special tentacles to get food using two very different strategies. During the night they stretch out their tentacles and use their stinging cells to catch tiny animals called zooplankton floating in the water. The tentacles pull the plankton into the mouth of the coral for its stomach to digest. This accounts for 20% of a coral’s food.

The rest of its food requires a close relationship with a special, tiny algae called zooxanthellae. This algae is so small it can live inside the cells of coral tissues. Zooxanthellae is the reason corals are such striking colours. Like most plants, zooxanthellae feed by turning sunlight into food through photosynthesis. Lucky for the coral, there’s plenty of food to go around!

This type of relationship - when two organisms both benefit from having a relationship with each other - is called “symbiosis.” Corals get food and the zooxanthellae can shelter within the coral skeleton. It’s a bit like letting your friend live with you rent-free as long as they cook you dinner!

How do coral reefs bleach?

Now, as you might imagine, living with your rent-free friend is not always easy. Imagine if they demanded you live in a certain area, that the house is clean and kept at a certain temperature or they’ll leave - and then you’ll run out of food.

Zooxanthellae are very sensitive - they need direct access to sunlight to produce food and can only survive at certain temperatures. This is why corals are so vulnerable to climate change. As climate change alters the ocean environment, it can directly affect zooxanthellae’s ability to survive. Increases in temperature, changes in sea level or changes in the levels of nutrients and minerals in the ocean can stress these tiny plants. This may trigger them to leave their coral host to find a better home. This is called “coral bleaching.” Without the zooxanthellae, the coral will lose most of its food source and, if they do not return, the coral will eventually die.

If current trends continue, and the world fails to reduce its greenhouse gas production, severe bleaching will take place every year on 99% of the world's coral reefs within the next 80 years.