• Corals are calcareous rocks formed from the skeletons of sea animals called polyps.
• Polyps extract calcium salts from seawater to form hard skeletons that protect their soft bodies.
• Corals live in colonies on the seafloor, and new generations develop on the skeletons of dead polyps.
• The tubular skeletons grow to create a cemented calcareous mass known as corals.
• These corals form shallow rocky deposits called reefs, which eventually evolve into islands.
• Corals come in different forms and colors, depending on their constituents.
• Small marine plants (algae) also contribute to coral growth by depositing calcium carbonate.

Ideal conditions for coral growth

• Corals thrive in tropical waters-between 30°N and 30°S latitudes.
• The ideal depths for coral growth are 45 m to 55 m below sea surface, where there is abundant sunlight available.
• The temperature of water should be around 20°C.
• Clear salt water is suitable for coral growth, while both fresh water and highly saline water are harmful for polyp growth.
• Adequate supply of oxygen and microscopic marine food, called plankton, is essential for growth and existence. As the food supply is more abundant on the seaward side, corals grow more rapidly on the seaward side.

Types of Coral Reefs

• Coral reefs can be classified based on their large-scale morphology and their relationship to nearby land.
• This classification is usually sufficient to distinguish one reef type from another, though there may be overlap in dominant animals and plants.
• There are three major types of coral reefs: fringing reefs, barrier reefs, and atolls.

Fringing Reef

• Fringing reefs are the most common of the three major types of coral reefs.
• They are attached to a continental coast or an island and may be separated by a narrow, shallow lagoon or channel.
• Fringing reefs form a narrow belt that is typically 0.5 km to 2.5 km wide.
• This type of reef grows from the deep sea bottom with a steep slope on the seaward side.
• Coral polyps do not extend outward due to sudden and large increases in depth.
• The surface of a fringing reef is rough and covered with coral remains, forming a boulder zone or reef flat.
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Barrier Reef

• Barrier reefs are the largest of the three reef types, running for hundreds of kilometers and several kilometers wide.
• They extend as a broken, irregular ring around the coast or island, running almost parallel to it.
• Barrier reefs are characterized by their distant location from the coast and a broader and deeper lagoon, sometimes connected to the seawater through one or more channels cutting across the barrier reef.
• This type of reef is very thick, going even below 180 meters from the surface with a steep slope on the seaward side.
• The surface of a barrier reef is covered with coral debris, boulders, and sand.
• The Great Barrier Reef off the coast of northeastern Australia is the most famous example of this type of reef, measuring 1900 km long and 160 km wide.
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Atolls

• Atolls are a ring-like reef that encloses a lagoon, which may have a level surface, but the seaward side of the reef slopes steeply into the deep sea.
• The lagoon of an atoll is 80-150 meters deep and may be joined with seawater through channels cutting across the reef.
• Atolls are located far from deep-sea platforms, and the formation may be aided by submerged islands or volcanic cones suitable for coral growth.
• Atolls are more common in the Pacific Ocean than any other ocean, and examples include the Fiji atoll, Funafuti atoll in the Ellice Islands, and numerous atolls in the Lakshadweep islands.
• Most atolls in the South Pacific occur in mid-ocean, with examples found in French Polynesia, the Caroline and Marshall Islands, Micronesia, and the Cook Islands.
• The Indian Ocean also has numerous atoll formations, including examples found in the Maldives and Chagos island groups, the Seychelles, and the Cocos Island group.
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Formation of Lakshadweep Islands (Atoll Formation)

• Charles Darwin proposed the basic coral reef classification scheme still used today.
• Fringing reefs grew near shorelines of new islands with ideal ecological conditions for hard coral growth.
• As the island slowly sank, coral growth kept pace and the reef moved farther from shore, becoming a barrier reef.
• When the island disappeared below the sea surface, leaving only the ring of coral encircling the central lagoon, an atoll was formed.
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Distribution of Coral Reefs

• The majority of reef-building corals are found in tropical and subtropical waters between 30° north and 30° south latitudes.
• The Indonesian/Philippines archipelago has the world’s greatest concentration of reefs and the greatest coral diversity.
• Other areas with high concentrations of reefs are the Great Barrier Reef of Australia, the Red Sea, and the Caribbean, with the latter having lower coral diversity compared to major Indo-Pacific regions.
• The world’s major coral reef regions are the Caribbean/western Atlantic, Eastern Pacific, Central and Western Pacific, Indian Ocean, Arabian Gulf, and the Red Sea.
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Coral Reefs Bleaching

• Coral reef ecosystems have been degraded in recent decades due to both anthropogenic and natural disturbances.
• Accelerated coral reef decline is primarily due to anthropogenic impacts such as overexploitation, overfishing, increased sedimentation, and nutrient overloading.
• Natural disturbances that damage coral reefs include violent storms, flooding, high and low-temperature extremes, and El Nino Southern Oscillation (ENSO) events.
• Coral bleaching occurs when the coral’s relationship with marine algae breaks down, resulting in the coral tissue becoming transparent and the skeleton becoming visible.
• Coral bleaching is a common response to many disturbances mentioned above.
• Bleached corals begin to starve and most struggle to survive without their algae, resulting in decreased coral growth and reproduction and increased susceptibility to disease.
• Corals can recover if conditions return to normal, but if the stress persists, bleached corals often die.
• Coral reefs with high rates of coral death following bleaching can take many years or decades to recover.
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Causes of Coral Bleaching

• Coral reef bleaching can be caused by a variety of factors.
• Some of the factors implicated in coral reef bleaching include temperature, solar irradiance, subaerial exposure, and fresh water dilution.
• Temperature changes, either low or high, can induce coral bleaching.
• Bleaching can occur disproportionately in shallow-living corals and on the exposed summits of colonies during seasonal temperature and irradiance maxima.
• Sudden exposure of reef flat corals to the atmosphere can potentially induce bleaching.
• Rapid dilution of reef waters from storm-generated precipitation and runoff can cause coral reef bleaching.
• Other factors that can cause bleaching include the increase in the concentration of inorganic nutrients, sedimentation, oxygen starvation caused by an increase in zooplankton levels due to overfishing, ocean acidification, changes in salinity, sea level change due to global warming, cyanide fishing, and more.

Spatialand Temporal range of Coral Reef bleaching

• Mass coral mortalities have been reported in all major reef provinces since the 1870s.
• The frequency and scale of bleaching disturbances has increased dramatically since the late 70s.
• More than 60 coral reef bleaching events out of 105 mass coral mortalities were reported during 1979-1990.
• Only three bleaching events were reported among 63 mass coral mortalities recorded during the preceding 103 years.