Coral Reef Ecology and Facts

Background

Coral reefs are the most diverse, productive marine communities and create the biggest, most spectacular structures made by living organisms. Local communities exploit their high productivity and tourists admire their beauty and uniqueness - for these very reasons they need to be used sustainably.

What are coral reefs?

A coral reef is a living system comprising of a symbiosis between the animal (polyp) and plant (zooxanthellae). Many polyps aggregate forming colonies that secrete calcium carbonate, creating a collective limestone "skeleton". Successive generations of polyps build on top of previous generations "skeletons" leading to reef formation. Through symbiosis with zooxanthellae, reef-building (hermatypic) corals are the basis of life in reef communities.

Where do coral reefs live?

Coral reefs are present in the waters of over 100 countries. These are warm (18-29'C), shallow, sunny regions primarily between the Tropics of Cancer and Capricorn. They support over 25% of all known marine fish species whilst only representing 0.25% of the marine environment.

Critical environmental variables affecting Coral Reefs

Reef survival requires temperature, light, depth, water clarity, salinity and water movement to remain within narrow limits.

Temperature:

Ideal reef-building temperature 25-29 'C
Within these limits primary reef construction occurs
Hermatypic corals can survive within the range of 18-36 'C and are confined to tropical waters
Ahermatypic (non reef-building) corals can survive down to 1 'C

Light:

Abundant light is required to enable the zooxanthellae to photosynthesise
Some deep sea corals have adapted to receive virtually no sunlight

Depth:

Light intensity diminishes with depth
Zooxanthellae must photosynthesise so few hermatypic corals are found at depths greater than 50 metres
Ahermatypic corals sustain growth with organic matter from deep sea currents and thus can live to far greater depths

Salinity:

Hermatypic corals tolerate a salinity range of 27-40 ppt
Preferred salinity is 36 ppt.

Water movement:

Refreshes oxygen and plankton supply
Assists the removal of waste products

How long have coral reefs been around for?

Reef structures have been around for 450 million years. The major reef building species has changed from blue-green algae to sponges and corals.Coral reefs as we know them date back 195 million years. Reefs have survived many mass extinctions, the most recent 136 million years ago, prior to which many more hermatypic corals were present (of 7500 known coral species, around 5000 are now extinct).

For more information see:

The Coral Reef Alliance (CORAL)

The International Coral Reef Action Network (ICRAN)

National Oceanic and Atmospheric Administration (NOAA)