Coral Reefs

“Coral” is general term for several different types of cnidarians (phylum Cnidaria) that produce calcium carbonate skeletons (CaCO3, a.k.a limestone).

These corals and their skeletons provide the framework for a coral reef to grow on – similar to how large trees provide the framework for tropical rainforests.

How a Reef is Built

1. A planula (coral larva) settles on a hard surface.

2. The larva metamorphoses into a polyp.

3. This “founder” polyp divides over and over to form a colony. The digestive systems and nervous systems usually remain connected.

4. Coral polyps secrete calcium carbonate skeletons that their soft body sits in.

5. The polyps continually secrete new layers, building up the skeleton beneath them so that it grows upward and outward to form a reef.

Where a Reef is Built

1. Corals only grow in clear shallow water. (The zooxanthellae need light, and cloudy sediments can settle on the corals and smother them.)

2. Corals only grow in warm water from about 68°F to 86°F.

3. Corals do not grow near river mouths – they are sensitive to reduced salinity.

4. Corals (especially larvae) are very sensitive to pollution.

5. Corals often live in naturally nutrient-low areas – allowing them to compete with seaweeds and other creatures.

Types of Reefs

Fringing Reefs• Simplest, most common reef

• Develop near the coast

• The longest reef in the world is a fringing reef along the coast of the Red Sea

• Consist of a reef flat and reef slope

Barrier Reefs• Separated from shore by a lagoon

• Consist of a back-reef slope, reef flat, and a fore-reef slope

• Often have spur-and-groove formations

• Includes the Great Barrier Reef in Australia (225,000 sq. km / 80,000 sq. mi)

Atolls

• A ring of reef• Often includes islands or sand cays• Range in size from 1–20 miles in

diameter• Form on top of underwater volcanic

islands

Reef Biology

Zooxanthellae• Single-celled, photosynthetic algae

• Zooxanthellae feed the coral “from the inside.”

• In return, the zooxanthellae receive nutrients from the coral’s waste products.

• They are the most important source of nutrition for corals.

• Ahermatypic corals (ones that do not build reefs) often lack zooxanthellae.

Encrusting Coralline Algae

• Red algae that deposit calcium carbonate from their tissues.

• Coralline algae form rock-hard sheets over the reef surface. This algal ridge protects the reefs from waves and erosion.

• “The glue that holds the reef together”

Underwater Recycling• Zooxanthellae use sunlight to turn the corals’ waste

products into organic compounds.

• These organic compounds are then passed on to the coral, which breaks them down, releases the nutrients, and turns them back into waste products.

• This is how corals can survive so well in nutrient-poor areas.

Nitrogen Fixing

• Corals need nitrogen to grow and thrive.

• Coral reefs have among the highest rates of nitrogen fixing of any natural community.

• The main nitrogen fixers are cyanobacteria.

Coral Competition• There is limited space on a coral reef, so all species (fish, snails, coral,

algae) must compete for nutrients and space.

• Some corals develop long sweeper tentacles to sting neighboring corals and fish.

• Some corals contain spicules, sharp calcium carbonate needles.

• Many also contain poison chemicals that discourage predators.

• Butterflyfish and parrot fish are the two main coral-eating fish.

• The crown-of-thorns sea star is another important coral predator.

Ciguatera• Ciguatera is the Spanish name for tropical fish poisoning.

• It is caused by toxic dinoflagellates (single-celled algae). Because these algae bloom and fade, ciguatera usually comes in irregular cycles.

• Herbivorous fishes eat the dinoflagellates, and are then eaten by predatory fishes, who develop a bioaccumulation of the toxin.

• Humans then eat these fishes when they go to the tropics, and develop ciguatera.

• Chemical tests for ciguatera poison are available, but not widespread.