Coral Reef are found in the warm ocean waters of the world, what we call tropical or equatorial.  The corals that form the structure of the reefs have a narrow thermal tolerance.  They cannot live in cold waters, but they also cannot live in waters that are too warm. Optimal temperatures for coral reefs are generally between 74oF and 84oF.

Watch this brief introduction to the Australia’s Great Barrier Reef:

 

 

Global Distribution of Coral Reefs

 

The main structure of coral reefs is formed by the accumulation of the calcium carbonate (CaCO3) “skeletons” of generations of corals.  Reef building hard corals are in a group of corals called the hexacorals, meaning their tentacles are in multiples of six.  There are also soft corals in the octocoral group whose tentacles are in multiples of eight, but these don’t build the hard skeletal structures that result in coral reef formation.

Corals are animals that grow in a form called polyps.  Polyps are a round disk with a mouth in the middle and tentacles on the outer rim of the disc.  The mouth leads to a sac like stomach.  Food is taken in through the mouth, and wastes are expelled back out through the mouth (Ewww!).  The tentacles are tipped with nematocysts which are stinging cells.  They are essentially a hollow spear that is connected to a venom sac through a tube.  When something contacts the tentacle, the nematocyst fires, spearing the animal, and venom is pumped through the tube from the venom sac into whatever contacted the tentacle.  The coral polyps use their nematocysts to harvest zooplankton and, in some species, even small fish.  Some corals may also collect organic molecules on mucus.  Corals are colonial, which means they have a large number of individuals living in association with each other. 

                                        Coral Polyps                                                  Nematocyst

 

Living inside the tissues of the coral polyp are single celled algae in a group called Zooxanthellae.  When individuals of two different species live together and benefit each other, we call the interaction type mutualism or say the species have a mutualistic relationship.  The corals provide shelter for the algae and products that the algae need for growth, such as CO2, nitrogen, and phosphorous, produced as waste products from the corals.  In exchange, the algae provide the products of photosynthesis for use by the corals, including oxygen, glucose, and amino acids, and absorb wastes produced by the corals.  The corals use the materials provided by the algae to build proteins, fats, carbohydrates, and their calcium carbonate shells. Zooxanthellae produce almost 90% of the energy used by corals, so without their mutualist symbiont, corals would eventually starve.  Because the zooxanthellae algae in the corals need to photosynthesize, coral reefs develop in waters less than 120 feet deep.  Below this depth there is not enough energy for photosynthesis because the water quickly absorbs the sun’s energy.  80% of the sun’s energy is absorbed in the upper 33 feet of sea water.

 

Zooxanthellae live in a mutualistic relationship with coral polyps.

The coral polyps build hard shells in which they can take refuge.  The corals extend their shells as they grow.  When a polyp dies, a new polyp will build its shell on top of the shell of the dead polyp.  Overtime, the dead "skeletal" material of generations of corals builds up on top of each other forming the structure of the coral reef.  The current generation of polyps forms a thin living layer around the built up calcium carbonate shells of past generations.  Different species build their shells in different patterns resulting in a diversity of structures on the reef within which other organisms can make their homes. 

Coral Growth Forms

 

The corals are a food source for a diversity of fishes and invertebrates.  It is estimated that 25% of marine species use coral reefs at some point in their life.

 

Watch this brief video on coral reef formation:

 

 

 

Coral polyps on a reef in Bonaire.  The disk, mouth, and tentacles of individual polyps are visible here.  The color is a result of their zooxanthellae.

 

A dried piece of coral reef (limestone).  The holes were the homes of individual polyps.

 

A large sponge, a filter feeding organism, on a coral reef in Bonaire.

 

Hawksbill sea turtle on a reef in Tahiti.  Hawksbill sea turtles are critically endangered as a result of human activities.

 

Coral reef are home to the highest diversity of species of any marine ecosystem.  This blenny (about 0.5” wide) is hiding in a small hole in a coral colony.

 

Wall of sharks on a coral reef in Tahiti.