Wednesday 19 February 2014

Coral Reefs

Coral reefs are colonies of tiny animals found in sea waters. They are underwater structures made from calcium carbonate secreted by corals. They have spectacular colours and multitude of living things that captivates almost everyone who sees a reef. Coral reefs are such massive structures, in fact, that they must be considered not only biological communities but geological structures.

Corals are cnidarians. Nearly all are in the class Anthozoa, making them closely related to sea anemones. Unlike many other cnidarians, they lack a medusa stage and live only as polyps. In reef-building corals, which are sometimes called hermatypic corals, the polyps produce calcium carbonate skeletons. Billions of these tiny skeletons build a massive reef.


In one way, growth and reproduction are the same thing in corals. The coral colony grows as its individual polyps divide to form new polyps. Thus, the colony grows as the polyps reproduce. The process crosses the fine line between growth and reproduction when a piece of coral breaks off and continues to grow. It is now a separate colony, though it is a genetically identical clone of its “parent”. Certain species of coral may depend heavily on this form of reproduction and may even be adapted to break easily. After a reef is damaged by a severe storm, an important part of its recovery is the growth of pieces of shattered coral colonies.

Corals can also reproduce sexually. Like other animals, they produce eggs and sperm, which fuse and eventually develop into planula larvae, the characteristic larvae of cnidarians. Some corals are hermaphrodites, and make both eggs and sperm, whereas other species have separate sexes. The method of fertilization also varies among corals. In some, whether or not they are hermaphrodites, the egg is fertilized and develops inside the polyp. Most corals, however, are broadcast spawners and release the eggs and sperm into the water.

The eggs and sperm may be released directly into the water or enclosed in little bundles that are released through the mouth. Depending on the coral species, the bundles may contain both eggs and sperm, or only one or the other. The bundles float to the surface and break up, allowing the eggs and sperm to mix.

Nobody knows why the corals all spawn together. Maybe the egg predators get so full that most of the eggs go uneaten. Maybe it has something to do with the tides. There may be an explanation that no one has thought of. Another interesting thing is that although the mass spawning happens on some reefs it does not occur on others.


Coral reefs are usually divided into three main categories: Fringing reefs, Barrier reefs, and Atolls. Some reefs do not fit neatly into any particular category, and some fall between two categories. Still, the division of reefs into the three major types works well for the most part.


Fringing reefs are the simplest and most common kind of reef. They develop near shore throughout the tropics, wherever there is some kind of hard surface for the settlement of coral larvae. Rocky shorelines provide the best conditions for fringing reefs. Fringing reefs also grow on soft bottoms if there is even a small hard patch that lets the corals get a foothold. Once they get started, the corals create their own hard bottom and the reef slowly expands.

As their name implies, fringing reefs grow in a narrow band or fringe along the shore. Occurring close to land, they are especially vulnerable to sediment, freshwater runoff, and human disturbance. Under the right conditions, however, fringing reefs can be impressive. In fact, the longest reef in the world is not the famous Great Barrier Reef in Australia but a fringing reef that runs some 4000 km along the coast of the Red Sea. Part of the reason that this reef is so well developed is that the climate is dry and there are no streams to bring in sediment and fresh water.

The typical structure of a fringing reef is shown in the above figure.


The distinction between barrier reefs and fringing reefs is sometimes unclear because the two types grade into one another. Like fringing reefs, barrier reefs lie along the coast, but barrier reefs occur considerably farther from shore, occasionally as far as 100 km or more. Barrier reefs are separated from the shore which may also have a fringing reef by a relatively deep lagoon. Largely protected
from waves and currents, the lagoon usually
has a soft sediment bottom. Inside the lagoon, coral formations variously known as patch reefs, coral knolls, or pinnacles depending upon their size and shape may grow up nearly to the surface.

The largest barrier reef in the Caribbean lies off the coast of Belize, Central America. Other major barrier reefs include the Florida Reef Tract and barrier reefs associated with the islands of New Caledonia, New Guinea, and Fiji in the Pacific. There are many other smaller barrier reefs, especially in the Pacific. Like the Great Barrier Reef, these usually are not single reefs but complex systems of smaller reefs.

Typical structure of Barrier reef is shown in the above figure.


An atoll is a ring of reef, and often islands or sand cays, surrounding a central lagoon. The vast majority of atolls occur in the Indo-West Pacific region, that is, the tropical Indian and western Pacific oceans. Atolls are rare in the Caribbean and the rest of the tropical Atlantic Ocean. Unlike fringing and barrier reefs, atolls can be found far from land, rising up from depths of thousands of meters or more. With practically no land around, there is no river-borne silt and very little freshwater runoff. Bathed in pure blue ocean water, atolls display spectacular coral growth and breathtaking water clarity. They are a diver’s dream.

When atolls were discovered, scientists were at a loss to explain them. It was known that corals can grow only in shallow water, yet atolls grow right in the middle of the ocean, out of very deep water. Therefore, the atoll could not have grown up from the ocean floor.

The puzzle of atoll formation was solved by Charles Darwin in the mid nineteenth century. Darwin is most famous, of course, for proposing the theory of evolution by natural selection, but his theory of atoll formation is also an important contribution to science. Darwin reasoned that atolls could be explained by reef growth on a subsiding island. The atoll gets its start when a deep-sea volcano erupts to build a volcanic island. Corals soon colonize the shores of the new island, and a fringing reef develops.

Typical structure of an Atoll is shown in the above figure.

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