Coral reefs, the largest known structures of biological origin on Earth, are also amongst the most beautiful natural structures that exist on the planet as well as being, possibly, the most diverse and valuable ecosystems that exist anywhere on Earth. Commonly compared with the rain forests that are found on land, Earth's coral reefs are known to support more species per unit area than any other marine environment including several thousands of fish species, hundreds of hard coral species, and hundreds of other species who make their home amongst these reefs. But this great diversity of life may be only the tip of the iceberg; according to estimates made by marine researchers, the reefs may be home to between a million and 8 million additional as yet undiscovered species. In addition to the immense biological wealth that is contained in and around the reefs, the reefs also contribute enormously to the economic well-being of millions of people providing people with reef related tourism alone generating an estimated over $300 billion per annum. For an environment that covers less than 1% of the Earth's surface, the biologic and economic wealth that coral reefs provide is truly phenomenal.
Three types of coral reefs exist. The most common reefs are fringing reefs which project directly from the shore with borders that run along the shorelines and surrounding islands. Barrier reefs are similar in that they also border the shorelines but they lie at greater distances from land than do fringing reefs. Barrier reefs are separated from adjacent land masses by greater distances than are fringing reefs by open, often deep-water, lagoons. Thirdly, there are the formations called atolls. These are fringing reefs that have formed around a volcanic island that has subsided entirely into the sea while the coral reef grows upward around it. Consequently, atolls are often circular or oval in shape around a central lagoon that has formed in the space formerly occupied by the now submerged volcanic island with gaps in the reef providing access to the central lagoon; sometimes, parts of the reef platform itself may emerge above water to form islands.
Corals are members of a large class of organisms (over 6000 members) known as anthozoans, which class also includes the anemones and sea pansies. Going by the fossil record, corals are very ancient animals. Solitary forms of corals have been found that date back more than 400 million years, but the reef builders that we know today seem to have evolved over the last 25 million years. Stony corals are the ones who are responsible for the building of reefs and they are, in the main, colonial organisms that cam comprise of hundreds or even hundreds of thousands of individual members mostly ranging in size between 1 to 3 centimeters. Notwithstanding the small size of individual corals, the colonies that they form, known as polyps, can grow extremely large and weigh several tons.
The formation of a reef starts when free-swimming coral larvae attach themselves to the rocky or other hard surfaces along the edges of island or continental masses and, as they grow and spread out, they develop one of the three characteristic shapes of reef formations. The growth of the reef structure starts as each coral polyp excretes a calcium carbonate (CaCO3) skeleton from its lower portion. This process results in a cup, known as a calice, in which the polyp sits on the floor, known as the basal plate, surrounded by the walls, known as the theca. From time to time, the polyp lifts itself from the basal plate and secretes a new skeleton thus providing a new floor above the old one. As long as the polyp lives, additional amounts of CaCO3 are deposited thereby creating new partitions and raising the coral. The partitions between the succeeding floor levels provides chambers which living polyps can retreat into when they are under threat from predators or the elements so that virtually no parts of the living organism is exposed above the skeletal formation. When necessary, as when they need to feed on plankton, for instance, the polyps can extend themselves from out of these protective chambers and get on with the business at hand.
Reef-building is extremely slow work, at least from a human viewpoint. With growth rates between 0.3 centimeters and 10 centimeters per annum, it can take up to 10,000 years for a coral reef to form from the time a group of larvae first make their home on some stony outcrop of an island or continental shelf and, depending on the size of the reef, it may last anything between 100,000 to 30,000,000 years before such a reef is fully formed. But the reef-building corals have the time to go about their work. Individual polyps can live for hundreds of years, while a colony has a life span that can last for several centuries although climatic change and several human activities have put these creatures and the wonderful habitats that they create in danger of vanishing forever.
Reefs will not form in just any location; fairly stringent conditions must be met in order that the stony corals who do the building can go about their work. Optimal water temperatures are between 23° and 29° Celsius, although some corals can tolerate tolerate temperatures as high as 40° Celsius for short periods of time. Reef-building corals are unable to operate in waters whose temperatures are less than 18° Celsius. Additionally, reef-building corals thrive in very salty water (32 to 42 parts of salt per one thousand parts of water) as well as a very good exposure to light. Thus reef-builders are found in those areas of the seas where sunlight is able to penetrate to a depth of approximately 70 meters. Consequently, coral reefs are generally to be found in clear tropical/semi-tropical waters. As we move to the higher latitudes north or south of the equator the numbers of species of reef-building corals decrease up to about 30° north or south. Beyond latitude 30°, reefs are commonly not found although there are exceptions such as Bermuda (32° north) whose reefs are facilitated by the fact that the island lies directly in the path of the Gulf Stream's warming waters.
Adult reef-building corals are almost all confined to the same spot on the floor of the sea, so the question arises as to how new favorable positions can be colonized and new reef-building put into process. Well, these corals have developed breeding habits that meet this need. Corals engage in asexual and sexual reproduction. When polyps grow up to a certain size, asexual reproduction takes place as new polyps, clones of the parent polyp, break off to expand or to form new colonies. Asexual reproduction continues for the coral's entire life.
Side by side with asexual reproduction, reef-building corals also practice sexual reproduction and an estimated 75% of all reef-building corals produce male and/or female gametes. The usual process is for massive amounts of eggs and sperm to be released in the water so that offspring can be distributed over the widest possible geographical area. The male and female gametes join together to form free-floating larvae called planulae. Planulae face enormous hazards from predators so vast numbers are produced to enhance their chances of survival. The release of eggs and sperm is often synchronized among many reefs and, because male and female gametes cannot move to meet each other but must depend on the vagaries of the movement of the sea, the timing of the release of these gametes is of the highest importance. It is thought that these mass release of gametes is determined by a combination of various environmental cues. Planulae follow the light and swim to the surface where they are carried along by the current. After floating along the surface for a time, they swim back to the bottom of the sea where, if the conditions are favorable, they will settle. Once settled, they grow into polyps, form colonies which increase in size as the business of reef-building gets underway. For most coral species, this process takes just about a couple of days, though some will be on their way for much longer.