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Jitendra Kumar, A.S. Kumar Naik, Mahesh, V. Nayana, P.

Department of Fisheries Resources and Management,

College of Fisheries, Karnataka Veterinary, Animal and Fisheries Sciences University, Mangalore - 575002, India


Corals are marine animals in class Anthozoa of phylum Cnidaria typically living in compact colonies of many identical individual "polyps". The group includes the important reef builders that inhabit tropical oceans and secrete calcium carbonate to form a hard skeleton. Coral reefs are sometimes referred to as a "rain forest in the ocean" because, like tropical forests which have a large variety of life, coral reefs also have a large variety of life. In order to support all of this life, coral reefs are a complex system. They have living part called polyp that secrete a calcareous skeleton. Coral reefs are underwater structures made from calcium carbonate secreted by corals. Most coral reefs are built from stony corals, and are formed by polyps that live together in groups.To get energy to build the coral reef the corals need to eat. The polyps use their tentacles to catch tiny animals that float in the water called zooplankton. But corals get most of their food from marine plants that actually live inside the coral. A microscopic type of algae called zooxanthellae actually live inside corals, just under their skin. Corals are solar-powered just like trees on land. The algae that live inside the coral soak up the sun all day - just like sunbathers on the beach. Most ocean plants are called marine algae. A microscopic type of algae actually lives inside coral. These are called zooxanthellae and they give the corals some of their shades of brown and green. The most important reef-building plant is probably the microorganism called zooxanthellae (zo-zan-THEL-y). These plants live in the tissues of reef-building coral polyps. They are too tiny for the naked eye to see. Coral reefs wouldn't be able to develop without these algae because the corals could not be able to grow fast enough or make enough foundation material to build reefs. These plants use the light from the sun to develop oxygen and food that the polyps use. Coral polyps produce waste that the zooxanthallae need to survive. This means that these plants and animals help each other survive, and together they let a coral reef grow and work as a system. ** PLEASE DESCRIBE THIS IMAGE **

A coral reef is made of millions of tiny animals called coral polyps. These coral polyps are nocturnal animals. Corals produce children that are called planulae. These children then drift across the ocean to new shorelines where they settle and begin to grow. A coral "head" is a colony of myriad genetically identical polyps. Each polyp is a spineless animal typically only a few millimeters in diameter and a few centimeters in length. A set of tentacles surround a central mouth opening. An exoskeleton is excreted near the base. Over many generations, the colony thus creates a large skeleton that is characteristic of the species. Individual heads grow by asexual reproduction of polyps. Corals also breed sexually by spawning: polyps of the same species release gametes simultaneously over a period of one to several nights around a full moon.

The International Union for the Conservation of Nature (IUCN) has named coral reefs as one of the life-support systems essential for our own survival. Found around coastlines in the tropics, coral reefs provide homes for about a third of all fish species on Earth and numerous other marine organisms.

Reefs are physically as well as biologically important; coral reefs play a fundamental role in protecting coastlines from erosion and contribute to the formation of white sandy beaches.

Endangered (EN)

A taxon is considered endangered when it is not critically endangered but is facing a very high risk of extinction in the wild in the near future. When postulations exist in great numbers their futures are usually relatively secure; when they exist in small number they are regarded as 'rare' their futures are commonly insecure. However, there are exceptions, and abundance is only one of several indicators of the future status of a population.

An endangered species is a population of an organism (usually a taxonomic species), which because it is either few in number or threatened by changing environmental or predation parameters, leaving it at risk of becoming extinct.

Examples: Blue Whale, Asian Elephant, African Wild Dog, Green Sea Turtle, Asiatic Lion, Tiger, Steller's Sea Lion, Grevy's Zebra, Wild Water Buffalo, Giant Otter, Pygmy Hippopotamus, Hyacinth Macaw, Volcano Rabbit, Proboscis Monkey, Persian Leopard etc.


Kingdom -Animalia

Phylum - Cnidaria

Class -Anthozoa (Ehrenberg, 1831)

Subclass - 1-Octacorilla (soft corals) 2-Hexacorilla (Hard corals)

Class - Anthozoa

  • This is the largest class of the phylum cnidaria and the members are dominantly marine.

  • The ectoderm may secrete an exoskeleton of CaCO3 or gorgonin or both.

Subclass - Octocorallia (Alcyonaria)

  • Includes a group of brightly coloured animals.

  • Tentacles and mesenteries 8 or in multiples of 8.

  • Colony may be simple or plant like.

  • This subclass includes soft coral (alcyonarians), horny coral (gorgonids) and sea pen (Pennatulacean).

Subclass - Hexacorallia (Zoantharia)

  • This subclass embraces solitary and colonial forms.

  • Tentacles and mesenteries 6 or in multiples of 6; dimorphism is unknown.

  • Exoskeleton solid, massive; made of CaCO3.

  • This sub class includes sea anemones, true corals or stony corals, black coral (antipatharia) etc.

Evolutionary History of corals

Although corals first appeared in the Cambrian period,some 542 million years ago. When rugose and tabulate corals became widespread. Tabulate corals occur in the lime stones. Their numbers began to decline during the middle of the Silurian period. The skeletons of tabulate corals are composed of a form of calcium carbonate known as calcite.

Types and Identification of corals

Based on their appearance, the reed building corals are classified.

Hard corals

Hard corals are most often referred to as corals that contain a 'hard' calcium skeleton and they are also referred to as stony corals and are member of the order Scleractinia. In most cases these skeleton grow very slowly, i.e. 1 cm a year.

Massive corals

Massive corals are characteristically ball or rock shaped and relatively slow growing. They have very stable profiles; massive corals are relatively undamaged by strong wave action unless they are dislodged from their holdfasts.

Branching corals

Branching corals are characterized by having numerous branches, usually with secondary branches. They are attractive, colorful and fragile. Its highly used for ornamental purpuses.

Digitate corals

Digitate corals are characterized by finger like branches and they are distinguished from branching corals in that they have no secondary branches. It does not grow very long and is dull in colour.

Table corals

Corals that form broad horizontal surfaces are commonly called Table corals, they resemble that of a table. The size of the Table corals may vary from small round shaped plates to large round tables.

Foliose corals

Corals with foliose or coil-like growth patterns from beautiful structures that have been compared to the open petals of a flower. The corals folds and densities greatly increase its surface area, and the space in between the coils may provide shelter for fish and invertebrates.

Cup corals

Cup corals resemble exactly like that of cup. The size may vary from small cups to large cups and they are locally called as 'Vattai'.

Solitary corals

Solitary corals are often flat or dome-shaped and circular or slightly oval in shape, resembling the cap of a mushroom. Most mushroom shaped corals are solitary forms living unattached to any underlying substrate.

Soft corals

Soft corals are dominant elements of the reef environment, providing all sorts of shapes and colors ranging from red and yellow to orange and purple, they resemble like that of a sponge. They contain minute, spiney skeleton elements called sclerites. Thus they are not reef building corals and do not lay new foundations for future corals. Soft corals are found worldwide in the reef environment. This near-surface-depth allows for currents which provide the soft corals with food and oxygen.

Sea fans

Sea fans look a lot like plants with colorful, forked branches but they are actually animals, they look very much like manual fans. They are made up of many tiny individual animals that work together as one. The individual animal lives along the sea fan's branches and look like little anemones. The main structural skeleton of a sea fan colony is formed from a flexible, horny substance called gorgonin with living polyps covering the surface. Sea fans do not attach themselves to a hard substrate; instead, they anchor themselves in mud or sand. Each sea fan polyps has eight tentacles which catch plankton that is consumed.

Types of reef

About 150 years ago, Charles Darwin, the father of evolution, divided the reefs into three types. The three different types of reefs are fringing reefs, barrier reefs, and atolls. Coral reef organisms can be found in different parts of the reef called zones. The zones are called the reef flat (0-2 meters in depth), reef bench (2-10 meters in depth), reef slope (10-30 meters in depth), and the rubble (30-40 meters in depth) zones. The coral community varies according to distance and depth from shore. Different environmental conditions can be found in the zones such as wave action, salinity, and temperature. Many reef organisms can survive only in a certain zone. The three principal reef types are:

  1. Fringing reef - A reef that is directly attached to a shore or borders it with an intervening shallow channel or lagoon. Fringing reefs are closest to shore, sometimes touching the shore. Fringing reefs are found around new and developing islands, compared to the age of the world. These reefs are formed from decaying sea life and polyps. Young fringing reefs that are still growing are called apron reefs. Fringing reefs are the most prevalent type of reef found in the Hawaiian Islands. They are found 150-1,500 feet (45-450 meters) from shore. They need warm water (73-80 degrees Fahrenheit) and can survive in rough, wave activated waters. This type of coral reef is the most common type of reef found in the Caribbean and Red Sea.

  2. Barrier reef - The second type of reef is the barrier reef. A reef separated from a mainland or island shore by a deep channel or lagoon. Barrier reefs stretch out farther into the ocean than fringing reefs, cutting off all water circulation from the shore. Only a lagoon separates two different barrier reefs. They are usually found 1/2 to 3 miles from shore, but can also be found farther out than that.

Barrier reefs are the biggest type of reef. The coral growth of a barrier reef continues outward. Barrier reefs need warm water (73-80 degrees Fahrenheit) to survive. They can also survive rough waters.

  1. Atoll reef - Atolls are reefs that are found far from any major land masses. If a land mass is a small island, it may disappear under the surface of the ocean. It then becomes an atoll. This is how an atoll is formed. Atolls can also develop when patch reefs sink below the surface of the water. Atolls are roughly circular or horseshoe shaped, and they surround a lagoon. The result is that there are several low coral islands surrounding a lagoon. Atolls can survive in rough water.

Why are coral reefs important?

Habitat: They are home to 33% of all known fish species.

Nursery: And a nursery ground for over 25% of all marine species.

Income: They provide millions of dollars of income annually for people living by coral reefs.

Medical Research: Coral reefs have the potential to be used as medical cures to treat cancer, heart disease, HIV and arthritis among others.

Protection: They protect 20% of the world's coast from wave erosion.

Food: They are a food source for millions of people.

Tourism: Coral reefs attract tourists from all over the world.

What is a healthy environment for coral reefs?

The conditions for healthy coral to grow.

Light: Reef-building corals contain single-celled algae in their tissues called zooxanthellae. The algae live in a symbiotic relationship with the coral (i.e. they are reliant on each other for their mutual survival). The algae provide the coral with food manufactured from photosynthesis and in return the algae receive a safe home as well as nutrients from the waste created by the coral polyp. The zooxanthellae relies on light, therefore these reef-building corals are restricted to the shallow depths of coral reefs, about 60 m or 200 ft.

Low nutrient and clear water: The numerous organisms that inhabit coral reef waters are incredibly efficient at taking up nutrients as soon as they become available from decomposing organisms or dissolved in the water column. Nearly all nutrients are taken up by living organisms leaving the water nutrient poor and clear. Clear water is important to allow light to penetrate to the corals so that the zooxanthellae can photosynthesize.

Salinity: Salinity of between 32 - 42 ppm (parts per million) is optimal for most coral reef organisms. Coral growth is reduced in areas with large freshwater inputs.

Temperature: Optimal temperatures for tropical coral reefs range between 23° and 29°C. Some corals are adapted to survive outside this range. Some reefs have recently been discovered in deep, cold water areas, but these are different to the coral types found in shallow, tropical waters. Excessive temperatures from 33°C upwards (although this varies according to the locality and tolerance of individual corals) can cause the loss of zooxanthellae, bleaching and potentially death.

Water circulation: Waves and currents bring in oxygenated water, prevent sediment deposition and renew the planktonic food supply. Reef development is usually greatest in areas subject to moderate wave action.

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