Editing Coral (section)

==Reefs==
{{main|Coral reef}}
{{See also|Coral reef fish|List of reefs}}
[[File:Coral reef locations.jpg|thumb|upright=1.5|Locations of coral reefs around the world]]
Many corals in the order [[Scleractinia]] are [[Hermatypic coral|hermatypic]], meaning that they are involved in building reefs. Most such corals obtain some of their energy from [[zooxanthellae]] in the genus ''Symbiodinium''. These are [[symbiosis|symbiotic]] photosynthetic [[dinoflagellate]]s which require sunlight; reef-forming corals are therefore found mainly in shallow water. They secrete calcium carbonate to form hard skeletons that become the framework of the reef. However, not all reef-building corals in shallow water contain zooxanthellae, and some deep water species, living at depths to which light cannot penetrate, form reefs but do not harbour the symbionts.<ref name=Schuhmacher>{{cite journal |author1=Schuhmacher, Helmut |author2=Zibrowius, Helmut |year=1985 |title=What is hermatypic? |journal=Coral Reefs |volume=4 |issue=1 |pages=1–9 |doi=10.1007/BF00302198 |bibcode=1985CorRe...4....1S|s2cid=34909110 }}</ref>

[[File:Hertshoon.jpg|thumb|left|[[Staghorn coral]] (''Acropora cervicornis'') is an important hermatypic coral from the Caribbean]]
There are various types of shallow-water coral reef, including fringing reefs, barrier reefs and atolls; most occur in tropical and subtropical seas. They are very slow-growing, adding perhaps one centimetre (0.4&nbsp;in) in height each year. The [[Great Barrier Reef]] is thought to have been laid down about two million years ago. Over time, corals fragment and die, sand and rubble accumulates between the corals, and the shells of clams and other molluscs decay to form a gradually evolving calcium carbonate structure.<ref>{{cite encyclopedia|publisher=MSN Encarta |year=2006 |title=Great Barrier Reef |url=http://encarta.msn.com/encyclopedia_761575831/Great_Barrier_Reef.html |access-date=April 25, 2015 |archive-url=https://web.archive.org/web/20091028020755/http://encarta.msn.com/encyclopedia_761575831/Great_Barrier_Reef.html |archive-date=October 28, 2009 |url-status=dead }}</ref> Coral reefs are extremely diverse marine [[ecosystem]]s hosting over 4,000 species of fish, massive numbers of cnidarians, [[Mollusca|molluscs]], [[crustacean]]s, and many other animals.<ref name=Spalding>{{cite book |author1=Spalding, Mark |author2=Ravilious, Corinna |author3=Green, Edmund | year = 2001 | title = World Atlas of Coral Reefs |url=https://archive.org/details/worldatlasofcora01spal | pages = [https://archive.org/details/worldatlasofcora01spal/page/205 205–45] | publisher = University of California Press and UNEP/WCMC | location = Berkeley, CA | isbn = 978-0-520-23255-6}}</ref>

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