Coral reefs
Coral reefs are formed by the calcium carbonate skeletons of coral organisms, which are marine invertebrates in the phylum Cnidaria.
Source
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"OpenStax College, Aquatic Biomes. October 17, 2013."
http://cnx.org/content/m44859/latest/Figure_44_04_03.jpg
OpenStax CNX
CC BY 3.0.
