What are Coral Reefs?

In this video, we take a look at what coral reefs are made of including their symbiotic association with zooxanthellae.

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Underneath the crystal clear waters of the tropics lie a complex community with an abundance of life. This community that I speak of is called the coral reef.

Coral reefs are the tropical rainforests of the seas where you’ll find a large diversity and abundance of organisms living there in one community. Nowhere else on earth can you find living organisms with such spectacular colors and fantastic shapes as you do on the coral reefs.

Stony corals are also known as true corals, and those are the corals that you will find living in the shallow tropical waters. They produce large reefs and they deposit a material called calcium carbonate that makes up the structure of coral reefs.

Tropical coral reefs will not develop in waters where the average annual temperature is below 18 degrees Celsius. Coral reefs love and thrive and warm water, so you’ll find them growing in places where the average annual water temperature is between 23 degrees Celsius and 25 degrees Celsius.

Almost all of the world’s coral reefs are confined to tropical seas. But what are coral reefs? Are they plants? Are they animals?

If you take a close look at any reef, you’ll notice that there’s a large colony of animals called coral polyps. These coral polyps are a part of the phylum that is called Cnidaria. In the Cnidaria phylum, you’ll also find sea anemones and jellyfish.

If we zoom in a little further and take a look at the tissues of each individual coral polyp, you’ll notice that there’s a type of algae called a dinoflagellate that lives within the tissues of the coral polyps. These dinoflagellates are called zooxanthellae.

In fact, many marine organisms have zooxanthellae living within the tissues, including corals, jellyfish and mollusks.

The zooxanthellae and the coral polyps have a working symbiotic relationship. This means there is a win-win relationship between the two organisms. The zooxanthellae provides nutrients like glucose and amino acids, along with oxygen and energy for the coral polyp. Meanwhile, the coral polyp provides a safe habitat for the zooxanthellae.

The relationship between the zooxanthellae and the coral polyps works so well that reef building corals do not grow any deeper than where the sunlight can penetrate in order to make sure that the zooxanthellae can receive sunlight for their photosynthetic needs.