Corals: Natures Greatest Builders

By Richard Ling (Coral Garden) [CC-BY-SA-2.0 (], via Wikimedia Commons

Photograph by Richard Ling via Wikimedia Commons

Coral reefs are the largest structures on earth that are built by animals.  There are reefs that are more massive than the hover dam, the great wall of china and the great pyramids.  And what creatures build them?  You guessed it: corals.

To understand how they do it, it’s first important to understand what a coral is.  Many people mistakenly believe them to be a plants (like plants, they grow in one place and can’t move from that spot on their own), but corals are actually animals.  There are two kinds of coral, hard(or stony) and soft corals.  Hard corals build reefs, so those are the ones that I am going to write about now.

Hard corals have limestone (calcium carbonate) skeletons that are covered by tiny polyps.  I’ll describe polyps in more detail in my next post, but for now you can picture them as miniature sea anemones.  The polyps live on the surface of the skeleton, and as they grow, they add more limestone to the skeleton.  On hard corals, only the polyps on the surface are alive and the structure beneath the polyps is stone.

When a coral dies, it’s polyps decay, leaving its skeleton, which becomes a place where a new corals can attach and grow.  Over time, new corals pile atop old corals and that’s how the reef grows.  Over thousands of years, corals build their skeletons, and then die, their skeletons becoming part of the reef structure, and over time an underwater mountain of limestone of formed: a coral reef.

By Hannes Grobe, Alfred Wegener Institute for Polar and Marine Research, Bremerhaven, Germany (Own work) [CC-BY-SA-2.5 (], via Wikimedia Commons

This photo shows the cross section of a coral. The white center is limestone, and the outer edge is where the polyps grow. It has been stained purple by scientists studying its growth rate. Photo by Hannes Grobe, via wikimedia commons.

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