The modern understanding of coral reefs begins in Charles Darwin's book, On the Structure and Distribution of Coral Reefs. In this classic book written in 1842, he distinguished three main types of reef: the fringing reef, the barrier reef, and the atoll. The fringing reef occurs near the shoreline and basically follows the profile of the shore. Its stony corals need a firm base on which to establish themselves, and they must compete with many other sedimentary organisms looking for a firm substrate on which to settle. In off-shore waters, that substrate is usually provided by the limestone secreted by earlier stony corals on rock (often volcanic). However, if there is a firm, rocky base present, as in fringing reefs, sedentary rivals can settle in great numbers without waiting for reef-building corals to lay the foundations (Stafford-Deitsch 20).
Thus, the stony corals do not have to be the major constituents of the reef. Sponges, soft corals, and corr aline algae are abundant throughout the fringing reef. Some of the finest fringing reefs in the world are along the edges of the Red Sea, where the conditions are premier for the growth of the reef. The water is enclosed by desert. Therefore, there is barely any rainfall to wash either the sand or fresh water into the sea.
Also, there are very little clouds in the area allowing sunlight to reach the surface, resulting in much warmer water than one might think to find at this latitude. The reefs in the Red Sea are some of the richest and most diverse. If one were to swim over the crest (the open-water edge), one might panic being that the reef drops drastically into unknown depths. Thus, not allowing wave action to stir up much sediment which would damage the reef being that the sediment settles below the reach of the waves. Little sediment at the top of the reef causes water to be clear, maximizing the amount of sunlight that reaches the reef (Stafford-Deitsch 21). The second type of reef according to Darwin is the barrier reef.
It is different than the fringing reef in one main way: the area between the living coral of a fringing reef and the shore is the reef flat, but the area between the barrier reef and the shore is a lagoon (Stafford-Deitsch 21). The lagoon is a relatively shallow area of water, only a few meters in depth. However, its width can be enormous as in Australia's Great Barrier Reef, some forty miles wide. The final type of reef distinguished by Darwin was the Atoll. It is a more-or-less circular crest of coral with a lagoon located at its center, only about a few kilometers in diameter. They are usually far from human influence and very deep, making them ideal for diving.
There might be areas on an atoll reef where sand is exposed to the air. The reef acts like a trap for sand and with the occasional storm, and persistent currents, banks of exposed sand and small sand islands can be formed. Coral is formed in an astonishing way. The animals that form coral belong to the same animal group as the hydras, jellyfish, and sea anemones. Most individual coral animals, polyps, are less than one inch in diameter, but a small percentage of them measure one foot.
A coral polyp has a cylinder-shaped body with a mouth surrounded by tentacles at one end, and the other end is used to attach itself to the hard surfaces at the sea bottom. Most coral polyps live in colonies. The stony corals attach themselves to each other with a flat sheet of tissue that connects to the middle of each body. Half of the coral polyp extends above the sheet and half below. The polyps build their limestone skeletons by taking calcium out of the seawater. They than deposit calcium carbonate (limestone) around the lower half of the body.
As new polyps grow, the limestone formation becomes larger and larger. The polyps feed primarily on tiny swimming organisms like the larvae of many kinds of shellfish. Reef corals cannot live without algae. They use some food manufactured by algae that live in the polyps' own tissue. These algae produce chemicals that help the coral animals secrete their limestone skeletons. Coral reefs grow only in water with enough light for photosynthesis to occur in the algae.
Coral polyps reproduce either form eggs or by budding. Small, knoblike growths called buds appear on the body of an adult polyp, or on the connecting sheet from tome to time. These buds grow larger, separate form the parent, and begin to deposit their own limestone in the colony making it grow. New colonies of coral polyps form when the adult polyps of an old colony produce eggs.
These eggs grow into tiny organisms that swim away. The animal than settles to the sea bottom and begins to form new colonies by budding.