Coral Reefs

Coral reefs are extremely complex, productive marine ecosystems that serve as a home to hundreds of thousands of species.  These fragile ecosystems are second only to rain forests in the diversity of organisms they sustain. Reefs represent the largest biological structure on Earth.  The health of reefs can be greatly affected by the slightest natural or manmade environmental stresses. Located between 25 degrees South and 25 degrees North latitude, coral reefs can survive only in a delicate balance of temperature, salinity, and light.  Corals thrive in waters between 18 degrees Celsius and 30 degrees Celsius ("About Coral Reefs", CORIS).

 Image 3:  Florida Coral Reef

Coral Reef Formation

Coral reefs are formed through the accumulation of calcareous exoskeletons of coral animals, calcareous red algae, and mollusks.  Corals are tiny animals that are included in the group cnidaria, which also includes organisms such as hydras, jellyfish, and sea anemones. Corals are sessile animals that live in large colonies, feeding by capturing prey with their tentacles.  They form the foundations of reefs by secreting a calcium carbonate skeleton that provides protection for the coral polyps.  Calcium carbonate is secreted continuously by the coral colony, adding to to the size of the reef.  The hard skeletal secretions build up reefs slowly over time.  Each year, 1 to 20 centimeters, or 0.4 to 7.8 inches, is added to the structure.  There are over 6,000 known species of corals that build different structures of various shapes and sizes.  This contributes to the great diversity of coral reef systems ("About Coral Reefs," EPA, paragraphs 1-4).

Types of Reefs

There are three three different types of reefs: barrier, fringing, and atoll.

Barrier reefs are platforms separated from the shoreline by a channel or a lagoon. The longest barrier reefs are found along the coasts of Belize and Australia.

Image 4:  Barrier Reef

Fringing reefs, the most common type, extend outward from a body of land with no water separating the reef from land.

Image 5:  Fringing Reef in French Polynesia

Atolls are coral islands that usually consist of a narrow, horseshoe shaped reef with a shallow, central lagoon.  Over 300 atolls are found throughout the South Pacific.

Image 6:  Atoll Reef off the coast of Colombia

Symbiotic relationships of corals and algae

The outer layer of a reef is composed of living coral polyps.  Corals polyps depend on an array of symbiotic relationships between consumers and primary producers to survive.  Several species of both fleshy and calcareous photosynthetic algae, including green algae and zooxanthellae, live within and around coral polyps ("About Coral Reefs", CORIS).

Coral Polyp                           

Image 7: Coral Polyp                                                         Image 8:  Coral Polyps  

Single celled, round algae called zooxanthellae live within the gastrodermal cells of the host tissue, providing both food and essential compounds to the coral.  The corals provide a protective environment for zooxanthellae to photosynthesize.  Approximately 90% of photosynthetic products are passed to the corals from zooxanthellae.  These algae also leak amino acids, carbohydrates, and small peptides to the coral.  Coral animals also obtain nutrients, such as phosphorous, by feeding on zooplankton  that they capture at night with their tentacles.  In addition to protection, corals provide zooxanthellae with plant nutrients from the waste metabolism produced by digestion of zooplankton in return.  Microscopic zooxanthellae form the basis of the coral reef food chain.  The mutual exchange of algal photosynthetic products and metabolites from corals is the key to the biological productivity of reefs and the limestone secreting capacity of reef building corals  ("About Coral Reefs", CORIS.)

Image 9:  Zooxanthellae

Importance of Reefs

Corals are critical to the survival of both reef ecosystems and local peoples who live near the reefs.  Eight percent of all of the world's population lives within 100 kilometers of a coral reef.  Reefs provide important sources of income as fishing and nursery areas, and have become valuable as tourist attractions.  Many building materials, biochemicals, and drugs are derived from reefs.  Developing countries obtain 25% of their protein intake through the consumption of fish from coral reefs.  Tourism from reefs generates billions of dollars annually.  It is estimated that the Great Barrier Reef generates $1.5 billion dollars from tourist revenue, while Floridian reefs generate $2.5 billion and Caribbean reefs generate $140 billion.  In addition to the economic importance of reefs, they provide important protection for coastlines by reducing wave action from affecting coastal area.


Coral Reef Vulnerabilities

Coral reefs are very productive although surrounded by unproductive waters.  However, reefs are extremely sensitive to environmental stresses from anthropogenic activities. It is estimated that 50-70% of the world's reefs are at risk.  Eutrophication, increases in sedimentation, over exploitation of marine species, mining, physical destruction, and bleaching pose serious threats to the health of reef systems.