NOAA National Ocean Service Education: Corals



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" "** Corals **

*How Do Coral Reefs Form?*

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Diagram of coral atoll formation
Corals usually develop into one of three characteristic structures:
fringing reefs, barrier reefs or atolls. /Click the image to see an

Coral reefs begin to form when free-swimming coral larvae attach to
submerged rocks or other hard surfaces along the edges of islands or
continents. As the corals grow and expand, reefs take on one of three major
characteristic structures —fringing, barrier or atoll. Fringing reefs,
which are the most common, project seaward directly from the shore, forming
borders along the shoreline and surrounding islands. Barrier reefs also
border shorelines, but at a greater distance. They are separated from their
adjacent land mass by a lagoon of open, often deep water. If a fringing
reef forms around a volcanic island that subsides completely below sea
level while the coral continues to grow upward, an atoll forms. Atolls are
usually circular or oval, with a central lagoon. Parts of the reef platform
may emerge as one or more islands, and gaps in the reef provide access to
the central lagoon


how are coral reefs formed

Coral reef - Wikipedia, the free encyclopedia


** Coral reef **

From Wikipedia, the free encyclopedia
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Marine habitats
Blue Linckia Starfish.JPGBiodiversity of a coral reef
· Littoral zone
· Intertidal zone
· Estuaries
· Kelp forests
· *Coral reefs*
· Ocean banks
· Continental shelf
· Neritic zone
· Straits
· Pelagic zone
· Oceanic zone
· Seamounts
· Hydrothermal vents
· Cold seeps
· Demersal zone
· Benthic zone

· v
· t
· e

*Coral reefs* are underwater structures made from calcium carbonate
secreted by corals. Coral reefs are colonies of tiny animals found in
marine waters that contain few nutrients. Most coral reefs are built from
stony corals, which in turn consist of polyps that cluster in groups. The
polyps belong to a group of animals known as Cnidaria, which also includes
sea anemones and jellyfish. Unlike sea anemones, coral polyps secrete hard
carbonate exoskeletons which support and protect their bodies. Reefs grow
best in warm, shallow, clear, sunny and agitated waters.

Often called "rainforests of the sea", coral reefs form some of the most
diverse ecosystems on Earth. They occupy less than 0.1% of the world's
ocean surface, about half the area of France, yet they provide a home for
25% of all marine species,^[1]^[2]^[3] including fish, mollusks, worms,
crustaceans, echinoderms, sponges, tunicates and other
cnidarians.^[4]Paradoxically, coral reefs flourish even though they are
surrounded by ocean waters that provide few nutrients. They are most
commonly found at shallow depths in tropical waters, but deep water and
cold water corals also exist on smaller scales in other areas.

Coral reefs deliver ecosystem services to tourism, fisheries and shoreline
protection. The annual global economic value of coral reefs was estimated
at US$ 375 billion in 2002. However, coral reefs


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