Stoney Corals

Header Photo: Courtesy of Malcolm Browne (flickr)


The Great Barrier Reef, located just off the coast of Australia, earns its name, with the word “great” meaning both large and extraordinary. Here are some reasons why:

  • The Great Barrier Reef is the worlds most immense coral reef system
  • It is the world’s largest single structure built by living organisms
  • It is listed as one of the seven wonders of the natural world
  • It is the only living thing that can be seen from outer space
  • It covers an area of 133,000 square miles

So what is it that has built and continues to bolster such massive formations? It turns out to be billions upon billions of tiny organisms, called coral polyps.

Most reefs are built by stony corals (order scleractinia). The reef building animals within this order are referred to as hermatypic or zooxanthellate. While alive, polyps cluster together, and secrete calcium carbonate which forms skeletal cups for their protection, slightly enlarging the overall size of the reef. Over many years, individuals are added to the colony. Those that die off are replaced by others, which create their own protective exoskeletons. The resulting reefs become some of the most diverse ecosystems on the planet. And perhaps some of the most captivating and beautiful as well.

Although reefs are impressively large and abundant, it is important to note that they only cover about one percent of the ocean’s surface. But the true importance of reefs become apparent when we realize that they provide a dwelling place for twenty five percent of all marine species.

It is amazing how such small creatures can erect these monumental structures. But what is more amazing still is the help they receive from even tinier organisms.

Like their relatives, the jellyfish, coral polyps have stinging tentacles which they use to catch prey. However, the polyps receive around 90 percent of their nutrients through photosynthesis. But wait! Aren’t coral polyps, animals? How do they photosynthesize?

This is where the help from other organisms comes in. The polyps have a mutualistic relationship with microscopic algae from the genus Symbiodinium. The coral polyps ingest the algae. The algae then reside in the body tissue of the polyps where they turn sunlight into energy.

Without this assistance, coral reef growth would be significantly weakened and large structures would most likely seldom form. Instead of the Great Barrier Reef we might have the Mediocre Barrier Reef.

Another thing the algae do for the polyps is give them their coloration. The color is determined by the variety of algae. This fills some reefs with rich luscious hues, but it is unclear whether or not the polyps are helped in any way by this, or if it is just a side effect of the association.

Reef MutualismFor their efforts, the algae are rewarded with protection from enemies as well as a steady supply of the carbon dioxide they need in order to photosynthesize.

As the reef growth is accelerated, the reef reaches toward the ocean surface, allowing better penetration of sunlight for future generations.

Through the cooperative efforts between two tiny organisms, both prosper and create massive structures, which not only meet their needs, but also become home to a vast number of marine species.