Darwin's three stages of atoll formation.
Coral reefs are formed by colonies of hermatypic coral polyps. Though the individual polyps are tiny, a coral colony can grow to be very large and weigh up to several tons. The polyps in a single colony are genetically identical; a single free-swimming coral larvae attaches to submerged rocks or other hard surfaces along the edges of islands or continents and asexually reproduces by budding. These new polyps are identical to the parent polyp and, over time, this community of polyps forms a coral colony. A colony is composed of a single species of coral, but a reef's structure can be comprised of colonies of multiple species of hard coral, with additional habitat created by soft corals, sponges, and algae that grow on or next to the hermatypic corals.
Hermatypic coral polyps secrete calcium carbonate from their base, forming the calyx. Periodically, the polyp will lift off its basal plate and secrete a new one, creating a tiny chamber in the coral skeleton. All the polyps in a colony deposit calcium carbonate and add chambers to the skeleton, growing upwards. When the polyp dies, if the colony is still living, a new polyp will grow on top of the skeleton of the dead polyp. The living polyps form a thin layer of tissue over the skeletons of dead polyps. Over time, this growth forms the primary structure of a coral reef. Other species of coral and organisms such as sponges and algae make up other parts of the physical reef structure.
Left: The reefs off the coast of Florida are visible in this image photographed from the International Space Station by an Expedition 18 crew member. One of the station's solar panels obstructs the view in the top left of the image. (NASA image ID: ISS018-E-02522) Right: NOAA scientists have developed a method to determine water depth from NASA images, like this image of Pearl and Hermes Atoll in the Hawaiian archipelago that was taken from the International Space Station during Expedition 5. (NASA image ID: ISS005-E-13929) Photos Courtesy of: Image Science & Analysis Laboratory, NASA Johnson Space Center
How old are today's reefs? The geological record indicates that ancestors of modern coral reef ecosystems were formed at least 240 million years ago. The coral reefs existing today began growing as early as 50 million years ago. Most established coral reefs are between 5,000 and 10,000 years old. Although size sometimes indicates the age of a coral reef, this is not always true. Different species of coral grow at different rates depending on the type of coral, water temperature, water turbidity, availability of light, oxygen level, amount of turbulence, and availability of food. In general, massive corals tend to grow slowly, increasing in size from 0.5 cm to 2 cm per year. However, under favorable conditions (high light exposure, consistent temperature, moderate wave action), some species can grow as much as 4.5 cm per year. In contrast to the massive species, branching colonies tend to grow much faster, and under favorable conditions, these colonies can grow vertically by as much as 10 cm per year. [a]
Coral reefs are the largest biological structure on earth; some of them can even be seen from space. [b] more...
There are three main types of reefs that form as hermatypic coral colonies grow: fringing reefs, barrier reefs, and atolls. Charles Darwin determined that these three reef types are also related stages in the process of atoll formation. Fringing reefs, which are the most common, project seaward directly from the shore, forming borders along the shoreline and surrounding islands. Sometimes they are separated from the shore by a narrow, shallow lagoon. Barrier reefs also border shorelines, but at a greater distance. They are separated from their adjacent land mass by a lagoon of open, often deep water. If a fringing reef forms around a volcanic island that subsides completely below sea level while the coral continues to grow upward, an atoll forms. Atolls are usually circular or oval, with a central lagoon. Parts of the reef platform may emerge as one or more islands, and breaks in the reef provide access to the central lagoon. [a]
Though much remains to be studied about deep-sea corals due to the depth at which they occur, scientists are learning more about these communities each year as advances in deep-sea technology progress.
Deep corals include both reef-building and non-reef-building species, though only a few form deep-water reef-like structures called bioherms, coral banks, and lithoherms. Deep-sea coral species can occur as individual small colonies that are less than a meter in diameter, or they may form aggregations and communities that are tens of kilometers across and tens of meters in height. Some species have complex branching shapes and can form dense groves or thickets, sometimes called forests. Other species of deep-sea corals, called sea pens, resemble feathers and grow on soft-sediment.
Like shallow coral reefs, deep-sea corals are often extremely long-lived—some species like Lophelia pertusa can live for centuries—and slow-growing.
Like their shallow counterparts, deep-sea coral communities are associated with a large number of other marine species and are ecologically important as biodiversity hotspots in the deeper oceant. [c]