CORAL

Corals are marine organisms from the class Anthozoa and exist as small sea anemone-like polyps, typically in colonies of many identical individuals. The group includes the important reef builders that are found in tropical oceans, which secrete calcium carbonate to form a hard skeleton.

A coral "head", commonly perceived to be a single organism, is formed from myriads of individual but genetically identical polyps, each polyp only a few millimeters in diameter. Over thousands of generations, the polyps lay down a skeleton that is characteristic of their species. An individual head of coral grows by asexual reproduction of the individual polyps. Corals also breed sexually by spawning, with corals of the same species releasing gametes simultaneously over a period of one to several nights around a full moon.

Although corals can catch small fish and animals such as plankton using stinging cells on their tentacles, these animals obtain most of their nutrients from photosynthetic unicellular algae called zooxanthellae. Consequently, most corals depend on sunlight and grow in clear and shallow water, typically at depths shallower than 60 m (200 ft). These corals can be major contributors to the physical structure of the coral reefs that develop in tropical and subtropical waters, such as the enormous Great Barrier Reef off the coast of Queensland, Australia. Other corals do not have associated algae and can live in much deeper water, with the cold-water genus Lophelia surviving as deep as 3000 m. Examples of these can be found living on the Darwin Mounds located north-west of Cape Wrath, Scotland. Corals have also been found off the coast of Washington State and the Aleutian Islands in Alaska. Corals coordinate behaviour by communicating with each other.

PHYLOGENY

Corals belong to the class Anthozoa and are divided into two subclasses, depending on the number of tentacles or lines of symmetry, and a series of orders corresponding to their exoskeleton, nematocyst type and mitochondrial genetic analysis. Those with eight tentacles are called octocorallia or Alcyonaria and comprise soft corals, sea fans and sea pens. Those with more than eight in a multiple of six are called hexacorallia or Zoantharia. This group includes reef-building corals (Scleractinians), sea anemones and zoanthids.

ANATOMY

While a coral head appears to be a single organism, it is actually a head of many individual, yet genetically identical, polyps. The polyps are multicellular organisms that feed on a variety of small organisms, from microscopic plankton to small fish. Polyps are usually a few millimeters in diameter, and are formed by a layer of outer epithelium and inner jellylike tissue known as the mesoglea. They are radially symmetrical with tentacles surrounding a central mouth, the only opening to the stomach or coelenteron, through which both food is ingested and waste expelled.

The stomach closes at the base of the polyp, where the epithelium produces an exoskeleton called the basal plate or calicle (L. small cup). This is formed by a thickened calciferous ring (annular thickening) with six supporting radial ridges. These structures grow vertically and project into the base of the polyp. When polyps are physically stressed, they contract into the calyx so that virtually no part is exposed above the skeletal platform. This protects the organism from predators and the elements (Barnes, R.D., 1987; Sumich, 1996).

The polyp grows by extension of vertical calices which are occasionally septated to form a new, higher, basal plate. Over many generations this extension forms the large calciferous (Calcium containing) structures of corals and ultimately coral reefs. Formation of the calciferous exoskeleton involves deposition of the mineral aragonite by the polyps from calcium ions they acquire from seawater. The rate of deposition, while varying greatly across species and environmental conditions, can be as much as 10 g / m² of polyp / day (0.3 ounce / sq yd / day). This is light dependent, with night-time production 90% lower than that during the middle of the day. Nematocyst discharge: A dormant nematocyst discharges response to nearby prey touching the cnidocil, the operculum flap opens and its stinging apparatus fires the barb into the prey leaving a hollow filament through which poisons are injected to immobilise the prey, then the tentacles manoeuvre the prey to the mouth.

The polyp's tentacles trap prey using stinging cells called nematocysts. These are cells modified to capture and immobilize prey, such as plankton, by injecting poisons, firing very rapidly in response to contact. These poisons are usually weak but in fire corals are potent enough to harm humans. Nematocysts can also be found in jellyfish and sea anemones. The toxins injected by nematocysts immobilize or kill prey, which can then be drawn into the polyp's stomach by the tentacles through a contractile band of epithelium called the pharynx.

The polyps interconnect by a complex and well developed system of gastrovascular canals allowing significant sharing of nutrients and symbiotes. In soft corals these range in size from 50-500 ?m in diameter and to allow transport of both metabolites and cellular components. Close-up of Montastrea cavernosa polyps. Tentacles are clearly visible.

Aside from feeding on plankton, many corals as well as other cnidarian groups such as sea anemones (e.g. Aiptasia), form a symbiotic relationship with a class of algae, zooxanthellae, of the genus Symbiodinium. The sea anemone Aiptasia, while considered a pest among coral reef aquarium hobbyists, has served as a valuable model organism in the scientific study of cnidarian-algal symbiosis. Typically a polyp harbors one particular species of algae. Via photosynthesis, these provide energy for the coral, and aid in calcification. The algae benefit from a safe environment, and use the carbon dioxide and nitrogenous waste produced by the polyp. Due to the strain the algae can put on the polyp, stress on the coral often triggers ejection of the algae, known on a large scale as coral bleaching, as it is the algae that contribute to the brown coloration of corals; other colors, however, are due to host coral pigments, such as GFPs (green fluorescent protein). Ejecting the algae increases the polyps' chances of surviving stressful periods - they can regain the algae at a later time. If the stressful conditions persist, the corals eventually die.

REEFS

The hermatypic, stony corals are often found in coral reefs, large calcium carbonate structures generally found in shallow, tropical water. Reefs are built up from coral skeletons and held together by layers of calcium carbonate produced by coralline algae. Reefs are extremely diverse marine ecosystems being host to over 4,000 species of fish, massive numbers of cnidarians, mollusks, crustaceans, and many other animals.

ENVIRONMENTAL INFLUENCES

A healthy coral reef has a striking level of biodiversity in many forms of marine life. Corals are highly sensitive to environmental changes. Scientists have predicted that over 50% of the world's coral reefs may be destroyed by the year 2030; as a result most nations protect them through environmental laws. Algae can overwhelm a coral reef if too many nutrients are present. Coral will also die if the water temperature changes by more than a degree or two beyond its normal range or if the salinity of the water drops. In an early symptom of environmental stress, corals expel their zooxanthellae; without their symbiotic unicellular algae, coral tissues become colorless as they reveal the white of their calcium carbonate skeletons, an event known as coral bleaching.

Many governments now prohibit removal of coral from reefs and use education to inform their populations about reef protection and ecology. However, many other human activities damage reefs, including mooring, fishing, diving, mining and construction. The narrow niche that coral occupies, and the stony corals' reliance on calcium carbonate deposition, means they are susceptible to changes in water pH. The increase in atmospheric carbon dioxide has caused enough dissolution of carbon dioxide to lower the ocean's pH, in a process known as ocean acidification. Lowered pH reduces the ability of corals to produce calcium carbonate, and at the extreme, can entirely dissolve those skeletons. Without deep and immediate cuts in anthropogenic CO2 emissions, many scientists fear that ocean acidification will result in the severe degradation or destruction of coral species and ecosystems.

Climatic variations can cause temperature changes that destroy corals. For example, during the 1997-98 warming event all the hydrozoan Millepora boschmai colonies near Panamá were bleached and died within six years - this species is now thought to be extinct.

LIVE CORALS

Local economies near major coral reefs benefit from an abundance of fish and other marine creatures as a food source. Reefs also provide recreational scuba diving and snorkeling tourism. Unfortunately these activities can also have deleterious effects, such as accidental destruction of coral. Coral is also useful as a protection against hurricanes and other extreme weather.

Live coral is highly sought after in the aquarium trade. Provided the proper ecosystem, live coral makes a stunning addition to any salt water aquarium. Soft corals are considered easier to maintain in captivity than hard corals.

Deep sea bamboo corals (Isididae) may be among the first organisms to display the effects of ocean acidification. They produce growth rings similar to those of tree and can provide a view of changes in the condition in the deep sea over time. Other coral biology research presents the possibility that Isididae corals, because of their potential to mimic biological properties, may be usable as living bone implants and in aquatic cultivation.

CORAL AS A GEMSTONE

Intensely red coral is sometimes called fire coral (but this is not at all the same thing as fire coral). Red coral is very rare now because of overharvesting due to the great demand for perfect red coral in jewelry-making.

ANCIENT CORAL

Ancient coral reefs on land are often mined for lime or use as building blocks ("coral rag"). Coral rag is an important local building material in places such as the East African coast. The annual growth bands in bamboo corals and others allow geologists to construct year-by-year chronologies, a form of incremental dating, which can provide high-resolution records of past climatic and environmental changes usinggeochemical techniques. Certain species of corals form communities called microatolls. The vertical growth of microatolls is limited by average tidal height. By analyzing the various growth morphologies, microatolls can be used as a low resolution record of patterns of sea level change. Fossilized microatolls can also be dated using radioactive carbon dating. Such methods have been used to used to reconstruct Holocene sea levels.