While various taxa have been involved in the construction of reefs in the past, such as sponges, rudist bivalves, coralline algae and even tube-dwelling worms, the majority of reefs during the Phanerozoic have been produced by various orders of colonial anthozoan cnidarians, collectively referred to as "corals". The accumulation of skeletal material, broken and piled up by wave action and bioeroders, produces a massive calcareous formation that supports the living corals and a great variety of other animal and plant life. Although corals are found both in temperate and tropical waters, reefs are formed only in a zone extending at most from 30°N to 30°S of the equator; the reef-forming corals do not grow at depths of over 30 m (100 ft) or where the water temperature falls below 16 °C (72 °F).
The building blocks of coral reefs are the generations of reef-building corals, and other organisms that are composed of calcium carbonate. For example, as a coral head grows, it lays down a skeletal structure encasing each new polyp. Waves, grazing fish (such as parrotfish), sea urchins, sponges, and other forces and organisms break down the coral skeletons into fragments that settle into spaces in the reef structure. Many other organisms living in the reef community contribute their skeletal calcium carbonate in the same manner. Coralline algae are important contributors to the structure of the reef in those parts of the reef subjected to the greatest forces by waves (such as the reef front facing the open ocean). These algae contribute to reef-building by depositing limestone in sheets over the surface of the reef and thereby contributing also to the structural integrity of the reef.
Reef-building or hermatypic corals are only found in the photic zone (above 50 m depth), the depth to which sufficient sunlight penetrates the water for photosynthesis to occur. The coral polyps do not photosynthesize, but have a symbiotic relationship with single-celled algae called zooxanthellae; these algal cells within the tissues of the coral polyps carry out photosynthesis and produce excess organic nutrients that are then used by the coral polyps. Because of this relationship, coral reefs grow much faster in clear water, which admits more sunlight. Indeed, the relationship is responsible for coral reefs in the sense that without their symbionts, coral growth would be too slow for the corals to form impressive reef structures. Corals can get up to 90% of their nutrients from their zooxanthellae symbionts (Marshall & Schuttenberg, 2006). Further Natural selection favours organisms that can grow faster than competitors and form structures that are taller than competitors. Corals with tall structures can get more light for the algae that photosynthesize.
Although corals are found growing in most areas of a healthy coral reef, the elevation of the reef flat relative to sea level (and considering tidal range) imposes significant constraints on coral growth. In general, only a small number of hardy coral species can thrive on the reef flat, and these cannot grow above a certain height because the polyps can withstand only limited exposure to the air at low tide. Of course some reef flats carry a meter or so of water over the surface, and then coral growth can be prolific. It is the upward growth of coralline algae on the outer part of the reef flat that ultimately results in an overall rise in the surface elevation of a reef, which typically slopes gently downward in towards the shore or lagoon and very steeply downward in the seaward direction. Prolific growth of these algae is a response to water motion bringing in inorganic nutrients and removing waste products. The damaging effects of exposure at low tide on the algae is ameliorated somewhat by constantly breaking waves on the reef edge. Nonetheless, it is the case that mature reefs are in equilibrium with both sea level and wave regime with respect to their elevation, and excess production of limestone moves away from the margin to expand the reef laterally and fill in low areas.
The more prolific growths of corals are to be found in water deeper than where the bottom is exposed at low tides: on the frontal reef slope (forereef), in lagoons, and along reef channels that bisect the flat. Under conditions of clear, moving seawater, corals provide the bulk of the skeletal material comprising the reef and the structural complexity that results in a high diversity of reef associated fishes and invertebrates.
Coral reefs can take a variety of forms, defined in following;
- Apron reef – short reef resembling a fringing reef, but more sloped; extending out and downward from a point or peninsular shore.
- Fringing reef – reef that is directly attached to a shore or borders it with an intervening shallow channel or lagoon.
- Barrier reef – reef separated from a mainland or island shore by a deep lagoon.
- Patch reef – an isolated, often circular reef, usually within a lagoon or embayment.
- Ribbon reef – long, narrow, somewhat winding reef, usually associated with an atoll lagoon.
- Table reef – isolated reef, approaching an atoll type, but without a lagoon.
- Atoll reef – a more or less circular or continuous barrier reef extending all the way around a lagoon without a central island.
- Bank Reef – Bank reefs are larger than patch reefs and are linear or semi-circular in outline.
Ecology and biodiversity
Coral reefs support an extraordinary biodiversity although they are located in nutrient-poor tropical waters. The process of nutrient cycling between corals, zooxanthellae, and other reef organisms provides an explanation for why coral reefs flourish in these waters: recycling ensures that fewer nutrients are needed overall to support the community.
Cyanobacteria also provide soluble nitrates for the coral reef through the process of nitrogen fixation. Corals absorb nutrients, including inorganic nitrogen and phosphorus, directly from the water, and they feed upon zooplankton that are carried past the polyps by water motion (Castro and Huber, 2000). Thus, primary productivity on a coral reef is very high. Producers in coral reef communities include the symbiotic zooxanthellae, coralline algae, and various seaweeds, especially small types called turf algae, although scientists disagree about the importance of these particular organisms (Castro and Huber, 2000).
Reefs are home to a large variety of organisms, including sponges, Cnidarians, worms, crustaceans, molluscs, echinoderms, sea squirts and marine vertebrates. A few of these varied species feed directly on corals, while others graze on algae on the reef and participate in complex food webs (Castro and Huber, 2000; Spalding et al., 2001).
A number of invertebrates, collectively called cryptofauna, inhabit the coral rock substrate itself, either boring into the limestone surface or living in pre-existing voids and crevices. Those animals boring into the rock include sponges, bivalve molluscs and Sipunculans. Those settling on the reef include many other species, particularly crustaceans and polychaete worms (Nybakken, 1997).
Prior to the Late Ordovician, reef-type structures were formed by associations of non-anthozoan organisms, such as stromatolites and archaeocyaths. These structures did not attain the size and complexity of coral reefs. The first coral reefs in the Ordovician were constructed by tabulate and rugose corals, in association with stromatoporoids and bryozoans. Both tabulate and rugose corals declined sharply at the end of the Devonian. They recovered in numbers during the Carboniferous, but eventually became extinct at the end of the Permian.
During the Triassic, a new order of corals arose in the Scleractinia that became the dominant reef-forming organisms. Scleractinians declined in numbers during the late Jurassic and early Cretaceous, being supplanted for a while by the rudists, but they regained their superiority by the end of the Cretaceous and remain the dominant reef organisms in the present day.
Castro, P., & M. Huber. 2000. Marine Biology, 3rd ed. McGraw-Hill: Boston.
Marshall, P., & H. Schuttenberg. 2006. A Reef Manager’s Guide to Coral Bleaching. Great Barrier Reef Marine Park Authority: Townsville (Australia).
Nybakken, J. 1997. Marine Biology: An Ecological Approach, 4th ed. Addison Wesley: Menlo Park (CA).
Spalding, M., C. Ravilious & E. Green. 2001. World Atlas of Coral Reefs. University of California Press and UNEP/WCMC: Berkeley (CA).
Creditshttp://en.wikipedia.org/wiki/Coral_reefs; edited CKT070925. History CKT070925.