Coral

A coral is composed of tiny, fragile animals called coral polyps.   When we say "coral" we are actually referring to the animals and the skeletons they leave behind after they die.  Although there are hundreds of different species of corals, they are generally classified as either "hard coral" or "soft coral". Hard corals grow in colonies and are the architects of coral reefs.   They include such species as brain coral and elkhorn coral.  Their skeletons are made out of calcium carbonate (also known as limestone) which harden and eventually becomes rock.  Hard corals are hermatypes or reef-building corals and need tiny algae called zooxanthellae (pronounced zo-zan-THEL-ee) to survive.  Generally, when we talk about "coral" we are referring to hard corals.Soft corals such as sea fingers and sea whips, are soft and bendable and often resemble plants or trees.  These corals do not have stony skeletons, but instead grow wood-like cores for support and fleshy rinds for protection.  They are referred to as hermatypes or non-reef building corals and they do not always have zooxanthellae.  Soft corals are found in both tropical seas and in cool, dark regions.

Coral Reef

Coral reefs are massive structures made of limestone that is deposited by living things.  Although thousands of species inhabit coral reefs, only a fraction produce the limestone that builds the reef.  The most important reef building organisms are corals.Coral reefs support over twenty five percent of all known marine species.   As one of the most complex ecosystems on the planet, coral reefs are home to over 4,000 different species of fish, 700 species of coral and thousands of other plants and animals.A good way to imagine a coral reef is to think of it as a bustling city or community, with buildings made of coral, and thousands of inhabitants coming and going, carrying out their business.  In this sense, a coral reef is like a metropolis under the sea.

Coral Polyp

A coral polyp is a spineless coral animal.  Coral polyps can be the size of a pinhead while others are larger, sometimes a foot in diameter.  One coral branch or mound is covered by thousands of these animals.  They are invertebrates (spineless animals) and are cousins of anemones and jellyfish.  When thousands of these animals are grouped together, they are referred to as coral colonies.  Each coral "tree" or "mound" is one colony of coral polyps.  A polyp has a sac-like body and an opening or mouth encircled by stinging tentacles called cnidae.   The polyp uses calcium carbonate from seawater to build itself a hard, cup-shaped skeleton.  This limestone skeleton protects the soft, delicate body of the polyp.   Coral polyps are usually nocturnal, meaning that they stay inside their skeletons during the day.  At night polyps extend their tentacles out to feed.