• Corals are one of the most complex, diverse and colorful tropical ecosystems on the planet.

• Corals are generally found in warm waters.

• Coral build massive & complex physical structures that can be seen from space.

• Coral have high economic value (fishing, tourism, etc)

• Coral help to protect coastal erosion

• Coral provide food and shelter for thousands of species of reef fishes and other marine animals.

• healthy reefs = healthy population

To be Continued

Coral reefs are one of the most complex and colorful tropical ecosystems. Coral reef organisms build massive physical structures that are home to thousands of fascinating plants and animals in the world.

What is a Coral Reef?

Coral reefs are massive structures made of calcium carbonate (limestone) deposited by coral animals (polyps) and this forms the base of a complex reef ecosystem. Coral reefs are home to over 4,000 different species of fish, 700 species of coral and thousands of other plants and animals. Corals are related to jellyfish and sea anemones.

What is a coral polyp?

    • A polyp is a tiny, soft body animal in the phylum cnidaria which is sessile with an elongated cylindrical body and a ring of tentacles around its mouth. Thousands of polyps grouped together to form a coral colony.

    • A polyp has a sac-like body and a single opening or mouth encircled by stinging tentacles.

    • The polyp of a hard coral uses calcium carbonate from seawater to build itself a hard, cup-shaped skeleton. This limestone skeleton protects the soft, delicate body of the polyp.

Mutual Relationship

The coral polyp provides shelter for the zooxanthellae and in return the zooxanthellae provide food for the polyp through the process of photosynthesis.

Coral Growth.

    • In general corals grow very slowly about 1 to 2 inches per year.

    • Corals grow at different rates depending on sunlight. Sunlight plays an important role in coral.

    • Water temperature (70–85°F), salinity, turbulence, and the availability of food are all affecting coral growth rate.

    • Since hard corals depend on the zooxanthellae (algae) that grow inside of them and this algae needs sunlight to survive, corals too need sunlight to survive.

    • Therefore, hard corals rarely develop deeper than 50 meters (164 feet).

Threats to Coral Reefs

  • Crown of Thorns: COTs feed on coral polyps, and an outbreak of COTs can destroy an entire coral reef within a few days or weeks.

Image: Ofu Island - Sabrina Woofter
  • Climate Change: Increasing sea temperature causes corals to bleach. Ocean acidification is also threatening corals. When sea temperatures suddenly increase, it causes the coral polyps to stress and expel their zooxanthellae. The first sign is usually a change in color, from bright pastel hues (blues, pinks, purples and yellow) to pale looking color, indicating that the colony is started to lose its zooxanthellae. If conditions do not improve for the colony, the polyps will die, leaving only the coral’s white skeleton. This phenomenon is commonly described as ‘coral bleaching’. In most cases, corals can recover after the stressful conditions subside. They will slowly regain their zooxanthellae, and hence their color.

Climate change and resulting warming ocean temperatures have been the main cause of recent coral bleaching events in American Samoa.

Image: Chasing Corals 2017, Directed by Jeff Orlowski.
  • Sedimentation: Suspended particles will prevent sunlight from reaching the coral reefs.

Image: Fagaalu - Trevor Kaituu
  • Over-fishing: Removal of the herbivorous fishes will allow algae to grow and compete with the corals for space and sunlight.

Image: Ofu Island
  • Population Pressure: More people on island will put more pressure on our natural resources.

Image: Leone - Trevor Kaituu

When and how do corals feed?

  • Corals feed during day and night time. During day time they utilize the zooxanthellae that lives in their tissues to provide up to 90% of their energy. At night polyps extend their tentacles out to feed on tiny micro-organisms such as plankton.

  • Pigments produced by the zooxanthellae are visible through the clear body of the polyp and give the coral its beautiful color.

  • Zooxanthellae (pronounced zoo-zan-thel-lee) are microscopic unicellular algae that commonly reside within coral tissues also in clam shell tissues. Through the process of photosynthesis, they produce up to 90 % of the coral’s energy intake; in return, the coral provides the zooxanthellae with protection and essential nutrients. This is called a symbiotic relationship because both parties benefit.

Image: www.captivereefs.com

Different Shapes and Growth Forms of Coral Colonies.


Staghorn Coral

Boulder Coral

Table/Plate Coral

Brain Coral

Foliaceous Coral

Mushroom Coral

  • Branching/staghorn coral are the fastest growing coral and are tree-like. Growth rate is 10-15 cm per year.

  • Boulder coral slowest growing coral with a growth rate of 1 – 3 cm per year. Some large colonies can be up to hundreds of years old.

  • Plate coral Are plate shaped and designed to collect as much sunlight as possible. Found in deeper areas on the reef slope where the light levels are low.

  • Brain coral the polyps do not form complete walls around themselves and create lines of mouths separated by skeletal ridges, making it look like a brain.

  • Foliaceous corals form horizontally flattened, unifacial (polyps on one surface facing upward) plates or lobes that are attached to the reef substrate from the basal (ventral) surface. The plates may form tiers, whorls or vases, which can stacked in a complex, multi-layered arrangement. Their flattened shape results from a strong tendency for radial (edge) rather than vertical extension.

  • Mushroom coral is different to most hard corals as it is a single polyp which lays free on the reef. It can round or oblong in shape up to 9.8'' (25 cm).

Importance of Coral Reefs

    • Source of food (fish and shellfish) for people

    • Economic value (fishing, tourism, etc)

    • Storm protection

    • Medicinal benefits - some anti-cancer drugs and painkillers come from reefs

    • Provide food and shelter for various reef fishes and invertebrates

    • A good sign of ocean water quality: Healthy reefs = Healthy water

Image: Manu'a Islands - Mark McDonald

WHAT YOU CAN DO TO HELP (Replace pic with NOAA icons )

Educate yourself about corals and the creatures they support
Abide by fisheries regulations.
Don't damage or remove live coral.
Protect mangroves and wetlands.
Take your fishing gears with you.
Keep chemicals out of waterways