Chapter 10: Coral Reefs

What Makes a Coral Reef?

Coral reefs are made of calcium carbonate, or limestone, that is deposited by coral polyps, the living animals inside of a coral reef. Although many corals look like plants, they are in fact animals belonging to phylum Cnidaria (the same phylum as jelly fish.)

Corals that build reefs are called hermatypic corals, while those that do not build reefs are called ahermatypic corals. Most hermatypic corals have zooxanthellae algae living symbiotically inside of the coral polyps. Examples of ahermatypic corals are soft corals, black corals, gorgonians, precious corals, most have no symbiotic relationship with algae. Alaskan’s cold waters have many species of ahermatypic corals.

Zooxanthellae algae are unicellular, yellow-brown (dinoflagellate) algae which live symbiotically in the gastrodermis of reef-building corals. The photosynthetic power of zooxanthellae can provide a large input of nutrients that make it possible for the corals to grow and reproduce in waters that are low in food availability. In exchange for a large input of carbohydrates, the coral polyp provides protection to the algae and access to light. Coral polyps can also obtain nutrition through eating. Coral polyps have long coiled tubes attached to the walls of their gut which can be extruded to grab and absorb food. coral polyp gif

Conditions Needed for Reef Formation

Although corals are found throughout the oceans of the world in polar and temperate waters, as well as in the tropics, it is only in the tropics that reefs are developed (i.e., hermatypic corals are only found in the tropics). Coral reefs grow best in clear, shallow waters that remain above 18 degrees C. Reefs are, therefore, only built in shallow tropical waters. Reef building is favored by clear water, and can be adversely affected by sedimentation. Corals have a ciliary-mucus mechanism which traps sediment and removes it, but that mechanism can be overwhelmed resulting in smothering of the coral. In general, coral reef development is greater in areas that are subject to strong wave action which provides a constant source of fresh, oxygenated water and prevents sediment from settling on the colony.

Limits on Reef Formation

Coral reef formation can be limited by temperature, depth, salinity, or air exposure. Coral reefs are only found in waters bounded by the 20 degrees C isotherm, and cannot develop below 18 degrees C or above 30 degrees C. Coral reefs are not normally found in water deeper than 50-70 m, and usually grow in 25 m or less; this is why reefs are restricted to the margin of the continents or islands; due to light restrictions, as sufficient light must be available to the symbiotic zooxanthellae in the coral tissue for photosynthesis. Coral reefs need salinities between 32 and 35 parts per thousand, and are not found near large influxes of freshwater from rivers. Coral reefs can survive limited air exposure but killed by longer periods of air exposure.

Types of Reefs

Image from:

Image from:

Fringing Reef – A fringing reef is either attached to, or closely borders, adjacent land. There is no lagoon between the reef and the land.

Barrier Reef – A barrier reef is separated from the land by a lagoon that may be of considerable length and width. Lagoons provided a greater variety of habitat types. The sheltered sides of lagoons are often nurseries for juvenile fish and other marine life.

Atolls – Atolls are roughly circular reefs enclosing a shallow lagoon. Some atolls have a central island. Atolls are formed as fringing reefs around a central island. As the island is eroded, the reef becomes a barrier reef, and the central lagoon gets broader, until the island is eroded to sea level, forming the atoll.

Atoll Formation

The modern theory of coral atoll formation was originated by Charles Darwin as a result of the observations he made during the voyage of the Beagle around the world in the 1830’s.

Darwin believed that the fringing reefs, barrier reefs, and atolls of volcanic islands represented a successional series through geologic time. He hypothesized that the transition from fringing to barrier reef to atoll could result from the upward growth of coral on the edge of a gradually sinking volcano. Gradual subsidence and continuous reef growth were fundamental to his theory. He believed that barrier reefs represented an intermediate stage between fringing reefs and atolls, and that the ring-like appearance of an atoll with a central lagoon resulted from the total submergence of the summit of a volcano.


An animation of how coral reefs form around volcanic islands.

Modern knowledge of plate tectonics has confirmed Darwin’s theory. When an undersea volcano emerges from the ocean to form and island, it will be colonized by reef building corals. As the reef grows it forms a fringing reef around the island. As the volcanic island goes dormant and ages it will be eroded by weather. The island will also begin to sink back into the crust. But coral reefs are living things and they will continue to grow upwards, building upon older coral skeletons. Over time, a barrier reef is formed with a lagoon between the island and the reef. Eventually, the volcanic island completely disappears below the water, leaving an atoll composed of low coral islets in a ring with a lagoon in the center. To learn more check out the explanation of atoll formation – Wikipedia.

The Biology of Coral

Although corals may resemble plants, they are animals. They are members of phylum Cnidaria (the same phylum as jelly fish). Corals can be solitary or colonial. Click to learn more about The Coral Animal.

Threats to Coral Reefs

Coral reefs are ecologically important ecosystems that have a high biodiversity. Coral Reefs are home to over 25 percent of all marine life and are among the world’s most fragile and endangered ecosystems. In the last few decades, millions acres of coral reefs have been damaged by human activities. However, that damage pales to what has recently been lost to large scale coral bleaching events. Since 2016, over half of the Great Barrier Reef, the world’s largest, has died.

A fire coral in Bermuda. The one on the left is a healthy fire coral, while the one on the right is completely bleached.

A fire coral in Bermuda. The one on the left is a healthy fire coral, while the one on the right is completely bleached. Image from Gizmodo.

Coral bleaching occurs when corals are stressed by high temperature, ultraviolet light, or other environmental changes. When a bleaching event occurs, they lose their symbiotic algae and may appear white. In brief events, the coral animal may be able to re-uptake their algae. However, in events that are longer or more severe the coral will die.

Gif: Nick DeSantis, Forbes Staff

Coral Bleaching, Click on it to animate. GIF by Nick DeSantis, Forbes

Deep Sea Corals

Increases in exploration of the deep sea, particularly by ROV’s, has made oceanographers aware that corals can exist in huge coral gardens in very deep, very cold waters, far beyond the reach of sunlight.

Like reef building corals, these corals are animals that are often living in large colonies. Unlike reef building corals, they do not depend upon a symbiotic relationship with algae to provide them with nutrition. Deep sea corals often position themselves in currents that are rich in food sources. Also unlike reef building corals, deep sea corals form groves of tree, feather, column, or fan shapes, that might be hundreds or even thousands of years old and may reach dozens of feet tall. Because these corals are fragile and slow growing they are very susceptible to potential damage from fish trawling and other human developments such as laying cables or oil pipelines. Check out images of deep sea coral communities, from the Okeanos Explorer.

bamboo coral

This deep sea bamboo coral was seen during Dive 4 at Blake Deep by the NOAA ROV Jason

Questions to Research:

  1. Describe the difference between hermatypic and ahermatypic corals.
  2. Using the same link as above, describe where in the world reef building (hermatypic) corals can be found.
  3. What are zooxanthellae? How much of a coral’s nutrition is provided by the zooxanthellae?
  4. The coral animal can also eat. Describe how coral eats.
  5. Briefly describe the differences between a fringing reef, barrier reef, and an atoll. You may instead, choose to sketch a picture.
  6. Describe three things that reef forming corals need to survive.
  7. Explain how coral’s reproduce, and manage to spread their offspring over a wide geographic area.
  8. Read a bit about deep sea corals. Describe one thing you learn about deep sea octocorals.
  9. Coral reef bleaching has become a terrible problem in the world tropical oceans. What is it?  What causes it?
  10. Go take a virtual dive around the Great Barrier Reef in Google Earth.  What kinds of things do you see?