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Hard corals form the foundation of the coral caps at Flower Garden Banks National Marine Sanctuary (FGBNMS). They are the building blocks of coral reefs.

Soft corals may also be present in coral reefs, but this is not the case at FGBNMS. However, soft corals are found in the deeper areas of the sanctuary referred to as the mesophotic zone.

A Coral by Any Other Name

Hard corals are just one type of coral, but they are known by many common names:

  • stony corals
  • reef-building corals
  • hexacorals
  • hermatypic (her'-mah-tip-ik) corals
  • scleractinian (sklayr-ak-tin'-ee-an) corals

We use these names interchangeably. For the purposes of this discussion, we will be referring to them as reef-building corals.

Coral Polyps

An individual coral animal is known as a polyp and looks like a miniature sea anemone. A polyp generally ranges in size from one (1) to ten (10) millimeters across, although the polyps of some species may be larger.

Close-up of a single coral polyp

Like an anemone, a coral polyp has a soft, tubular body topped by a ring of tentacles. In reef-building corals, the tentacles and several other body parts occur in multiples of six (6), which is why they are sometimes referred to as hexacorals.

The mouth, its only opening, is located in the center of the tentacles.

Cross-section diagram of a coral polyp

A cross-section of a coral polyp with parts labeled for reference. Image courtesy of NOAA's Ocean Service

To download a copy of this diagram, including an explanation, please visit the Education section of the Document Library.

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Coral Skeletons

Unlike an anemone, a reef-building coral polyp builds a hard (stony) external skeleton that forms a protective cup (calyx or calice) around its base. This skeleton is made of calcium carbonate (CaCO3) in a form known as aragonite.

Skeleton of the ten-ray star coral
Coral skeleton. Each circular indentation was the location of a
living coral polyp.
Photo: FGBNMS/Drinnen

Over time, the polyp continues to lay down additional layers of skeleton (basal plates) beneath itself. These layers form annual growth patterns, much like tree rings.

Underside of a coral skeleton showing growth layers
Layers of growth are visible on the underside of this brain coral (Pseudodiploria strigosa) skeleton.
Photo: FGBNMS/Drinnen

Hundreds to thousands of coral polyps together make up a reef-building coral colony. Each polyp is connected to the next by a thin layer of tissue (coenosarc), creating a living mat over a shared skeleton.

Close up view of about 30 coral polyps with their tentacles extended.
Coral polyps form a living mat over a calcium carbonate skeleton. Photo: FGBNMS/Schmahl

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Polyp Partners

Most people think of reef-building corals as being very colorful, but most of their coral polyps are actually transparent. The color we see comes from symbiotic algae, known as zooxanthellae (zo-o-zan-thel'-ee), that live within the tissues of the polyps.

Closeup of coral polyps with tentacles extended.
A closer look at the polyps reveals both green and orange
coloring provided by the zooxanthellae. The transparency of
the polyps can be seen in the far-reaching tentacles.
Photo: FGBNMS/Schmahl

Through the process of photosynthesis, the algae use sunlight and carbon dioxide to create food and oxygen. This food provides up to 70% of the necessary nutrients for reef-building corals to survive.

Reef-building corals get the remainder of their nutrients by capturing small plankton with their stinging tentacles (raptorial feeding) or through direct absorption of minerals from the surrounding seawater.

For more detailed information about coral biology, including animations and videos, please visit the Coral Tutorial from NOAA Ocean Service Education or Coral 101 from NOAA Coral Reef Conservation Program.

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Asexual Reproduction

Reef-building corals reproduce both sexually and asexually.

Asexual reproduction takes place in the form of budding or splitting polyps. This is how a colony grows.

In splitting, the original polyp divides (splits) in two, then each of those polyps repeats the process, and so on for the rest of their lives. Each new polyp is an identical genetic reproduction of the first, so all of the polyps in a colony are genetically the same.

Black & white drawing of a coral polyp splitting in two.
The polyp on the left is splitting to create a new polyp
on the right. These will eventually become two
separate, but identical, polyps.
Drawing courtesy of Joel Hickerson

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In budding, a new polyp grows out of the side of an existing polyp, much like the bud on a plant. Despite the slight difference in method, the result is the same--a genetically identical polyp.

Sexual Reproduction

Sexual reproduction is a chance for mixing of genetic material and the creation of completely new colonies. About 75% of reef-building corals are broadcast spawners, while the rest are brooders.

In broadcast spawning, coral colonies release gametes (eggs/sperm) into the water in large quantities. The gametes float to the surface where egg and sperm join to form free-floating planula larvae.

The planulae travel with the currents for up to two weeks before settling to the bottom as polyps. Once firmly attached to a hard surface, the polyps will then begin the process of asexual reproduction to create colonies.

For more information about mass spawning events, please visit our Coral Spawning page.

Small bb-like bundles rise from the surface of a spawning coral.
Egg and sperm bundles are released during a mass coral spawning. Photo: FGBNMS/Hickerson

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At Flower Garden Banks National Marine Sanctuary, a majority of the reef-building corals participate in a mass spawning event 7-10 days after the full moon in August each year.

For more information about the annual mass spawning event in the sanctuary, please visit our Coral Spawning at FGBNMS page.

In brooding, only male gametes (sperm) are released into the water. These float along on the currents until they are captured by female polyps with eggs. Fertilization takes place inside the females and results in planula larvae.

These larvae are later released and settle to the bottom soon after. However, they don't tend to settle as far from the parent colonies as the larvae from broadcast spawners, perhaps because of the shorter settlement period.

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Coral Reefs: Past, Present and Future

Nancy Knowlton of the Smithsonian has created a video presentation titled Coral Reefs: Past, Present and Future that is a great reference piece for learning about coral reefs, what threatens them, and what we need to do to protect them. The entire presentation runs about 56 minutes.

Nancy Knowlton also has a second video about biodiversity on coral reefs titled Corals and Coral Reefs: Biodiversity and Why it Matters. This presentation discusses challenges in the taxonomic classification of corals and other reef creatures among other topics. The entire presentation runs about 47 minutes.

Coral Images

Coral Guide (5MB pdf)
- a picture guide to stony corals of the sanctuary within recreational dive limits (0-130 ft, 0-40 m deep). Includes close up images of polyps and skeletons.

Cnidarian Species List
- an online species list of stony corals within recreational dive limits (0-130 ft, 0-40 m deep). Includes photographs from the sanctuary that you may download.


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