A coral reef is an underwater ecosystem characterized by reef-building corals. Reefs are formed of colonies of coral polyps held together by calcium carbonate. Contrary to the general belief that corals are plants or even rocks, corals are actually tiny animals called polyps that live in colonies underwater, either in patches or in extensive reefs.
Each colony is composed of thousands of tiny polyps. Polyps secrete salivary calcium carbonate material that hardens to form the rigid structural mass or skeletons of the reefs. Inside the polyps are many single-celled algae called zooxanthellae, which are capable of photosynthesis, thus providing an energy source for both the algae and the coral. It is the algae that give corals their varied colors.
The coral colony grows as one polyp buds off another polyp. The polyps build a united skeleton which gives the colony the strength to withstand waves and storms. Coral reefs occur along shallow, tropical coastlines where the marine waters are oxygenated, clear, warm, and free from sediments and pollutants.
The actual reef consists of large and rigid structural mass of calcium carbonate formed by the cemented skeletal remains resulting from the successive growth and development of reef-building corals. As the colony grows, it provides structure and niches that serve as homes for many different organisms including fish, sea snakes, mollusks, marine worms, crustaceans, algae and sponges. It is this ability to provide diverse structure that supports the vast biodiversity associated with reefs.
Considered as the “rainforests of the sea”, coral reefs are among nature’s most spectacular and beautiful creations and rank as one of the most complex and diverse ecosystems in the world. They are home to a dazzling array of marine life consisting of nearly a million species. Up to 3,000 species may co-exist on a single reef, where the density of fishes can be 100 times the ocean average.
Coral reefs are extremely efficient in capturing nutrients and sunlight and then cycling them for use by many different organisms. They are also an essential protein source for fish and other marine food products
In soft corals, there is no stony skeleton but the tissues are often toughened by the presence of tiny skeletal elements known as sclerites, which are made from calcium carbonate. Soft corals are very variable in form and most are colonial. A few soft corals are stolonate, but the polyps of most are connected by sheets of coenosarc. In some species this is thick and the polyps are deeply embedded. Some soft corals are encrusting or form lobes. Others are tree-like or whip-like and have a central axial skeleton embedded in the tissue matrix. This is composed either of a fibrous protein called gorgonin or of a calcified material. In both stony and soft corals, the polyps can be retracted, with stony corals relying on their hard skeleton and cnidocytes for defence against predators, and soft corals generally relying on chemical defences in the form of toxic substances present in the tissues known as terpenoids
Hard corals are made of a rigid calcium carbonate (limestone) and appear very much like rocks. Each polyp secretes a hard exoskeleton made up of calcium carbonate and a chalky internal skeleton that stays in place even after they die. As each generation of polyps dies and their exoskeleton remains, the coral grows a bit larger and because each polyp is so small, hard corals grow at a very very slow rate. Hard corals are scientifically known as “scleractinians”.
Types of Hard Coral
Staghorn coral (Acropora cervicornis): Staghorn coral is a branching coral with cylindrical branches ranging from a few centimeters to over 6.5 feet (2 m) in length.This coral exhibits the fastest growth of all known western Atlantic corals, with branches increasing in length by 4-8 inches (10-20 cm) per year and is one of the three most important Caribbean corals in terms of its contribution to reef growth and fish habitat.
Pillar corals (Dendrogyra cylindricus): This type of coral grows up from the sea floor, but without any secondary branching. They can grow to be up to 2.5 m (8 ft) tall. They can grow on both flat and sloping sea floors at a depth of between 1 and 20 m (65 ft). They are one of the few types of hard coral whose polyps can commonly be seen feeding during the day.
Table Coral (Acropora):Table Coral is the same branching type of coral as Staghorn coral, however it grow as flat plates. The shape of table coral is ideal to expose as much of their surface as possible to sunlight. The usual color of table coral is a dull brown or green, but it is brightened up by the numerous reef fish that shelter under and around its plates.
Brain coral (family- Faviidae): Named because of this corals spheroid shape and grooved surface which resembles an animal brain. The life span of the largest brain corals is 900 years. Colonies can grow as large as 6 or more feet (1.8 m) high.
Blue coral (Heliopora coerulea): Blue corals is named for their distinctive, permanently blue skeleton, which is generally hidden by greenish-grey or blue polyps. Blue corals occur in tropical waters, on intertidal reef flats and upper reef slopes.
Great star coral (Montastraea cavernosa): This type of coral is a colonial stony coral found in the Caribbean seas. It forms into massive boulders and sometimes develops into plates. It’s Polyps are the size of a person’s thumb and can be seen fully extend at night.
Tube Coral (Tubastraea): Tube Coral is a large polyp stony coral, and is found in a variety of colors and forms depending upon species. The tubastraea faulkneri is known as the Orange Cup or Sun Coral, pictured above.
Elkhorn coral (Acropora palmata): This coral is considered to be one of the most important reef-building corals in the Caribbean. This species of coral is structurally complex with many large branches. The coral structure closely resembles that of elk antlers. A popular choice as a home for lobsters, parrot-fish, snappers and other reef fish. Elkhorn coral colonies are incredibly fast growing with an average growth rate of 5 to 10 centimetres (2.0 to 3.9 in) per year and can eventually grow up to 3.7 metres (12 ft) in diameter.
Coral growth forms
Hard coral growth forms
As hard coral colonies expand and grow, their structures can develop into different morphologies or growth forms. These growth forms are generally referred to as branching, columnar, encrusting, massive, laminar, foliaceous and free living. Scientists today, use these growth forms to describe the appearance of different species of coral.
Branching corals, such as the thin birds-nest coral (Seriatopra hystrix), often found in areas of high wave action, are antler or staghorn-like in their appearance. Columnar corals, such as the catch bowl coral (Isopora palifera), are pillar or finger-like corals that form. Columnar corals do not have the secondary branches seen in the branching coral growth type. Encrusting corals, such as Leptoseris incrustans, are lichen-like in their form and are low spreading. Massive corals, such as the grooved brain coral (Diploria labyrinthiformis), grow slowly to form large, dome-shaped boulders. This shape gives them stability. Laminar corals, such as Acropora cytherea, have a flat upper surface which gives them a table-like structure. Due to their table-like shape, laminar coral are often referred to as table corals. Foliaceous corals, such as the pagoda coral Tubinaria mesenterina, are scroll-like in their appearance. Finally, solitary coral polyps that do not form colonies may be referred to as free-living. Solitary species include the mushroom coral Fungia scutaria species.
Reefs at Risk.
Around the world, coral reefs are under assault from a multitude of sources
— coral mining, fishing with dynamite and cyanide, coastal development, pollution or overfishing in general. It is reported that globally, 58 percent of the world’s reefs are at risk from human activities, with percent of its reefs are high or very high risk.
Reef degradation is caused by a variety of factors. Resource use on coral reefs is intensive and mostly extractive such as fishing, shell gleaning, collection of ornamental fish or other reef products and coral mining. Highly destructive fishing methods such as trawling, blast/dynamite fishing, cyanide or poison fishing, destroy the structure and marine life in the reefs. When uncontrolled or unregulated, drive-in nets, push-nets, beach seines and other similar methods that tend to scour the sea bottom can physically destroy the nooks and crannies that serve as “homes” to marine life.