Corals and Coral Reefs
Corals - plants or animals?
Corals belong to the animal kingdom, and are members of the same group of animals as jellyfish and sea anemones (Phylum: Cnidaria). The actual coral animal or “polyp” is soft bodied, with tentacles like a sea anemone. The main difference is that corals secrete an external calcium carbonate skeleton and sea anemones do not. This hard skeleton forms the framework of coral reefs. The tiny coral polyps occupy little cups or corallites in the massive skeleton. Corals can be colonial or solitary and there are several hundred species, some are large and branching and grow rapidly at a rate of up to 10cm per year, while others are mound shaped, growing slowly at only 1cm per year.
The second largest Brain Coral in the world, at Tobago.
Reef building corals live in symbiotic association with Zooxanthellae, single celled algae, which live in the tissue of the corals. The zooxanthellae produce the oxygen, that the corals need to survive, by photosynthesis; in return the algae are protected from grazing species and can access the nutrients that the coral excretes - a mutually beneficial association. Corals feed on zooplankton with the use of their tentacles. During daylight they mostly remain within their protective skeleton to avoid predation, but at night the tentacles are extended to allow them to feed. Coral colonies grow by having the polyps bud off new polyps asexually. New colonies are established by the fragmentation of skeletal pieces or through the settling of planktonic coral lava on a hard substrate. The lava are the result of sexual reproduction.
What do corals need to grow?
There are six major factors that limit coral reef development; water temperature and salinity, depth, light, sedimentation and emergence into air.
Coral reefs are only found between about 30° north and south of the equator, where the water temperature is at least 20°C, and optimal reef development occurs in waters where the mean annual temperatures are about 23-25°C.
Corals are intolerant of salinities that deviate significantly from that of seawater and gaps will occur in reefs where, for example, freshwater from a river enters the sea.
Extensive ‘fields’ of plate corals growing in clear water,
at depths of 15-25m, around the island of Niue.
Depth is also critical, coral reefs will not develop in water that is deeper than about 50-70m, and they grow most energetically at depths of 25m or less. Light, which is related to depth of water, is necessary for the zooxanthellae to photosynthesize. Without light the photosynthetic rate is reduced and with it the corals ability to secrete calcium carbonate. Corals also require clear water - sediment clogs their feeding structures and smothers them. For this reason corals usually grow most actively in areas of strong wave action, such as the windward side of a reef, where sediment is prevent from settling on the colonies.
Finally corals reefs are limited in an upward direction by emergence into air. Most corals are killed by long exposure to air and so their upward growth is limited to the level of the lowest tides.
The formation of a coral reef
Coral reefs are grouped into one of three categories: atolls, barrier reefs and fringing reefs.
Atolls, such as the Tuamotu Islands, are modified ring-shaped reefs that rise out of very deep water, far from land, and enclose a lagoon. The lagoon itself may contain lagoon reefs or patch reefs.
Barrier reefs and fringing reefs occur adjacent to a landmass, whether it is a volcanic island such as Bora Bora or Tahiti, or a continental landmass such as Australia. The main difference between the two is that a barrier reef is separated from the landmass by a greater distance and a deeper water channel than the fringing reef.
Most of the islands in the Indo-Pacific that have fringing or barrier reefs are volcanic in origin - as indeed are atolls. As mentioned above, corals require an environment with a hard substrate, and water depths of less 70m, so they are unable to grow up from the ocean floor where water depths may be as much as 3000m.
Plate coral in the Tuamotus
The distribution of volcanic islands is related to the configuration of the earth’s tectonic plates. Volcanic islands are typically located at mid-oceanic spreading ridges for example, Iceland; at ocean trench subduction zones e.g. Tonga and the Marianas Island Arc; or within tectonic plates, over “hot spots” for example the Hawaiian Islands. Hot spots are regions where there is a pronounced upwelling of hot magma from the earth’s mantle.
In most cases the volcano begins its life in the ocean depths with the bulging of the ocean floor above the source of upwelling magma. The dome grows until the ocean floor fractures and magma pours out on the ocean floor cooling and fragmenting. Continued outpouring of magma results in a growing pile of basaltic rubble and submarine lavas, which may eventually reach sea level and emerge as an aerial volcano. At this stage (provided the submarine slopes of the volcano are stable, and there is not too much ash or sediment being produced) the coral polyps move in and begin building a fringing reef. With time, the volcano becomes extinct, and begins to subside. If the subsidence is not too fast, reef growth will keep pace with the subsidence, forming next a barrier reef. The prolific coral polyps manage to keep their upper domain alive near the water’s surface even though the host landmass is slowly sinking into the depths. When the volcanic island finally disappears below the surface corals continue to grow on the outside keeping the reef at the surface and forming an atoll. On the inside, where the island used to be, quiet water conditions and high sedimentation prevail. These conditions prevent vigorous coral growth and a lagoon develops.
Atoll in the Tuamotus
References / Acknowledgements
Nybakken JW (1997) Marine Biology - An Ecological Approach, Fourth Edition. Addison-Wesley Educational Publishers Ltd.
Thanks to Megan Robertson (South Cheshire College, Cheshire, UK) for searching the Web for reference material.
This page will be updated if I receive further information.
Dr. Janet Sumner-Fromeyer