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Did you know?

• Corals are one of the planet's most complex and diverse ecosystems.

• Corals grow to be massive and complex physical structures that can be seen from space.

• Corals have high economic value (fishing, tourism, etc.).

• Corals help to protect against coastal erosion by buffering wave action.

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

What is a coral reef?

Coral reefs are massive structures made of calcium carbonate (limestone) deposited by coral animals (polyps), forming the base of a complex reef ecosystem. Coral reefs are home to over 4,000 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 housed inside the coral and uses calcium carbonate from seawater to build a hard, cup-shaped skeleton. This limestone skeleton protects the tiny, soft-body polyp in the phylum Cnidaria (same as jellyfish and anemones). It is sessile with an elongated cylindrical body and a ring of tentacles around its mouth. Thousands of polyps grouped to form a coral colony.

A mutualistic relationship

The coral polyp provides shelter for symbionts (zooxanthellae), and in return, the zooxanthellae produce photosynthate (a product of photosynthesis), which can meet up to 90% of a coral’s metabolic needs.

Coral growth

Corals grow at different rates depending on sunlight though they grow very slowly (about 1 to 2 inches per year). Coral reefs are a vulnerable ecosystem significantly impacted by sea surface temperature rise and acidity. Water temperature (70–85°F), salinity, turbulence, and food availability all affect coral growth rate. Corals largely depend on the zooxanthellae that grow inside them, which need sunlight to survive and photosynthesize. Therefore, hard corals rarely develop deeper than 50 meters (164 feet).

Threats to Coral Reefs

As global climate change accelerates, much of the ocean’s ecosystems are increasingly threatened. Coral reefs are an example of a vulnerable ecosystem significantly impacted by sea temperature rise, acidity, pollution, algal abundance, etc. Below are some of the main threats to American Samoa's coral reef.

COTs

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: Chasing Corals 2017, Directed by Jeff Orlowski

Climate Change

Increasing sea temperature causes corals to bleach, and ocean acidification causes coral degradation. 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. 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.’

Sedimentation

The natural sediment flow over land and waterways is important for sustaining coastal habitats. Excess sediment can damage coral habitats by preventing sunlight from reaching the coral reefs (thus preventing photosynthesis).

Overfishing

Removing herbivorous and grazing fish species, such as parrotfish, will make reefs vulnerable to algal growth. The algae will out-compete corals for space and sunlight.

Different Shapes and Growth Forms of Coral Colonies

Branching/Staghorn Coral

the fastest-growing coral and are tree-like. The growth rate is 10-15 cm per year.

Boulder Coral

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

Table/Plate Coral

Designed to collect as much sunlight as possible. Found in deeper areas on the reef slope where the light levels are low.

Brain Coral

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

Foliaceous Coral

Form horizontally flattened plates that may form tiers, whorls, or vases, which can be stacked in a multi-layered arrangement—their flattened shape results from radial (edge) rather than vertical extension.

Mushroom Coral

Different from most hard corals as it is a single polyp that lays free on the reef. It can be round or oblong, up to 9.8'' (25 cm).