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Coral reefs are underwater structures formed from the calcium carbonate secreted by corals.

en.wikipedia.org/wiki/Coral_reef

Coral reefs are diverse underwater ecosystems held together by calcium carbonate structures .... As the name implies, the bulk of coral reefs is made up of coral skeletons from mostly intact coral colonies. As other chemical elements present in ...

oceanservice.noaa.gov/facts/coralmadeof.html

A coral reef is made of thin layers of calcium carbonate.

oceanservice.noaa.gov/facts/coral.html

Coral, a sessile animal, relies on its relationship with plant--like algae to build the ... in fact, made up of hundreds to thousands of tiny coral creatures called polyps. ... slowly laying the limestone foundation for coral reefs and giving shape to the ...

coral.org/coral-reefs-101/coral-reef-ecology/how-reefs-are-made

Reefs are built by coral polyps as they secrete layers of calcium carbonate. The corals that build reefs are known as “hard” or “reef-building” corals.

www.livescience.com/40276-coral-reefs.html

Oct 8, 2013 ... Coral reefs are large underwater structures composed of the skeletons of coral, which are marine invertebrate animals.

ocean.si.edu/corals-and-coral-reefs

In the so-called true stony corals, which compose most tropical reefs, each polyp sits in a cup made of calcium carbonate. Stony corals are the most important ...

www.teachoceanscience.net/teaching_resources/education_modules/coral_reefs_and_climate_change/what_is_a_coral

It is made out of the same substance that shellfish use to make their shells–– calcium carbonate. coral heads and coral reefs. Corals can grow into structures as ...

www.defenders.org/coral-reef/basic-facts

Coral reefs are some of the most diverse ecosystems in the world, housing tens of thousands of marine species. About one-third of all marine fish species live ...

www.ducksters.com/science/ecosystems/coral_reef_biome.php

At first glance, you may think that coral reefs are made up of rocks, but they are actually live organisms. These organisms are tiny little animals called polyps.