Coral Reefs – UPSC World Geography Notes

Coral reefs, comprised of the skeletons of colonial marine invertebrates called coral, form intricate underwater ecosystems. Recognized for their exceptional biological diversity, coral reefs stand as one of the most varied marine ecosystems globally. These colonies of small living animals, known as coral polyps, thrive in oceanic environments, constituting formations interconnected by calcium carbonate. Playing a crucial role in marine ecosystems, coral reefs offer essential habitats for marine plants and wildlife. This article aims to provide insights into Coral Reefs, contributing valuable information for those preparing for the UPSC Civil Service Exam in Environmental studies.


  • Coral: Living creature with a symbiotic relationship with ‘zooxanthellae’
  • Zooxanthellae: Microscopic algae living on coral, aiding in nutrient production through photosynthesis
  • Coral Tissues: Clear (white), with vibrant colors coming from zooxanthellae within
  • Host Coral Polyp: Provides a safe environment and carbon dioxide for zooxanthellae in exchange for benefits
  • Types of Corals: Hard corals and soft corals (including sea fans and gorgonians); only hard corals can form reefs
  • Coral Reefs Formation: Built by tiny animals known as polyps, leaving behind limestone skeletons
  • Reef Structure: Composed of layers of skeletons eventually covered by living polyps
  • Reef-Building Corals (Hermatypic Corals): Exhibit various shapes, including branched, table-like, massive cups, boulders, or knobs.

Cold Water Corals

Cold-water corals, also known as deep-water corals, are marine invertebrates characterized by the presence of cnidocytes—harpoon-shaped cells used for protection and hunting.

  • Found in the Atlantic, Indian, and Pacific Oceans
  • Inhabit deep, cold waters (39-55 degrees Fahrenheit)
  • According to the United Nations Environment Programme, outnumber tropical reefs globally
  • Members of Phylum Cnidaria, including stony, black, and thorny corals, as well as soft corals
  • Similar to tropical corals, they provide habitat for other species but do not rely on zooxanthellae for survival
  • Approximately six coral species associated with reef construction
  • The Rost Reef off the coast of Norway holds the title of the world’s largest coldwater coral reef.

Features of Coral Reefs

They are located in shallow tropical areas with clean, clear, and warm sea water.

  • The coral reef cover in Indian waters spans an estimated 19,000 square kilometers.

Coral reefs stand as some of the most productive and complex coastal ecosystems, boasting a wide range of biological diversity.

  • High productivity results from a blend of primary production within the reef and external environmental support.

Reef-building corals symbolize a union between polyps (coral animals) and zooxanthellae (microscopic algae).

  • Corals, slow-growing animal colonies, partner with the fast-growing plants known as zooxanthellae.

Despite residing in nutrient-poor waters, corals exhibit significant nutrient recycling capabilities, involving the entire nutrient community.

  • The close association of many invertebrates, vertebrates, and plants with corals, featuring tight resource coupling and recycling, earns coral reefs the moniker “the Tropical Rainforests of the Oceans.”

Formation of Coral Reefs

Corals are remarkable plant and animal superorganisms positioned between population and community in the ecological hierarchy or organization.

  • The animal component belongs to the class Anthozoa of the phylum Cnidaria.
  • The plant component is an algae called zooxanthellae, possessing chlorophyll pigment and photosynthesis abilities.

This algae, through photosynthesis, provides carbon compounds to coral polyps, offering them energy. In return, the polyps protect the zooxanthellae.

Corals essentially form calcareous rocks originating from the skeletons of tiny sea creatures called polyps.

  • Polyps extract calcium salts from saltwater to construct hard skeletons, safeguarding their delicate bodies.

When free-moving coral larvae attach themselves to sedimentary rocks or hard surfaces near coasts, coral reefs take shape. The skeletons of deceased polyps give rise to new generations, perpetuating the cycle and forming coral layers.

The shallow rock resulting from these depositions is termed a reef, which eventually transforms into islands.

Corals are solid, calcareous stone masses that grow upward and outward as tubular skeletons.

  • Coral reefs, influenced by the nature of salts, may exhibit a variety of shapes and colors.

Conditions Required for Growth of Coral

  • Shallow water: Coral reefs thrive in shallow waters, with the reef surface ideally not exceeding 80 meters above the water’s surface. Adequate sunlight is crucial for zooxanthellae to perform photosynthesis.
  • Semi-hard or hard surface: A semi-hard or hard surface is essential for the compaction, cementation, and solidification of coral polyp skeletons.
  • Low range of temperature: Corals, located in tropical and subtropical regions, require a mean annual temperature of 20-22°C. They are stenothermal and cannot tolerate a wide temperature range.
  • Clear water: The presence of sediments and opaque water in shallow waters can destroy the beautiful coral polyps. Clear water is necessary to allow sufficient sunlight to reach the algae (zooxanthellae), ensuring their survival.
  • Saline waters: Moderate ocean water salinity, ranging from 27 to 30 parts per thousand (PPT), is crucial for the development of coral polyps. Polyps extract calcium from the water to protect their skeletons.
  • Rich supply of nutrients: Coral reefs flourish in seawater due to the continuous source of rich nutrients provided by ocean waves. The multiplication of coral polyps is accelerated when ample nutrients are available.

Classification of Coral Reefs

Fringing Reefs (Shore Reefs)

Fringing reefs develop near the continent, remaining close to the coastline. Small, shallow lagoons separate these reefs from the coastline. They are the most common among all corals.

Examples: Andaman and Nicobar Island, Gulf of Kutch, Mannar, and Palk Bay.

Barrier Reefs

Barrier reefs are the largest, highest, and widest among coral reefs. They are linear offshore structures parallel to the continental margin, separated by wide, deep non-navigable barriers. The water body between the reef and the shore is termed a lagoon. Barrier reefs extend as a broken, irregular ring around the coast or an island, running almost parallel to it. The Great Barrier Reef, on the eastern coast of Queensland, Australia, is the largest coral reef globally.

Patch Reef

Patch reefs typically form on the island platform or continental shelf between fringing and barrier reefs. They are commonly found in shallow lagoons within larger collective reefs or atolls. Patch reefs are in waters ranging in depth from 10 to 20 feet and are usually located close to land.

Examples: Caribbean and Bahamas, Bermuda, and the Pacific Islands.

Atoll Reefs

Atoll is a ring of narrow-growing coral generally in the form of a horseshoe pattern or elliptical manner. Corals are found on a submarine bench or platform, and in the middle of the coral lagoon, there is no land within the Atoll. A chain of various Atolls collectively is called Faros.

Example: Lakshadweep.

Functions of Coral Reefs

Coral reefs, thriving in shallow waters of tropical or subtropical regions, stand out for their high biomass production and rich floral and faunal activity.

  • Natural protective barrier against erosion.
  • Some corals serve as a source of high-quality proteins.
  • Coral acts as a significant hot spot for marine biodiversity and fishery resources.
  • The largest biogenic calcium carbonate producer in the marine environment.
  • Provides substrate for mangroves.
  • Habitat for a diverse array of animals and plants, including avifauna.
  • Remarkable role in in-situ conservation of marine biodiversity.
  • Used as a bone substitute in reconstructive bone surgery, with pores and channels in some corals resembling those of human bone.

Threats to Coral Reef

Natural causes could involve the emergence of reef-destroying mechanisms like “bleaching” and the depletion of essential symbiotics.

Anthropogenic causes encompass a range of activities such as chemical pollution (pesticides, cosmetics, etc.), industrial pollution, mechanical damage, nutrient loading or sediment loading, dredging, shipping, tourism, mining or collection, thermal pollution, and intensive fishing.

Coral reef ecosystems globally have witnessed unprecedented degradation in recent decades, with disturbances stemming from both anthropogenic and natural events.

Natural disturbances causing coral reef damage include violent storms, flooding, extreme temperature swings, El Nino Southern Oscillation (ENSO) events, subaerial exposures, predatory outbreaks, and epizootics.

Coral reef bleaching is a common stress response of corals to many of the aforementioned disturbances.

Other threats encompass:

  • Deforestation
  • Oil and chemical spill (toxicants)
  • Road construction (sedimentation)
  • Agriculture (nutrient & sedimentation)
  • Stormwater runoff (sediment, toxins, nutrients & pathogen)
  • Chemical from sunscreen (toxicants)
  • Coastal development & Impervious surface (sedimentation & toxicant)
  • Sailed septic supeny (nutrients & pathogens)

Causes of Decline in Coral Reefs


Coral species thrive within a relatively narrow temperature range, and anomalously low & high sea temperatures can induce coral bleaching. Bleaching is more frequently reported during elevated seawater temperatures. Bleaching events also occur during sudden temperature drops accompanying intense upwelling episodes and seasonal cold air outbreaks.

Solar Irradiance

Bleaching during the summer months, coinciding with seasonal temperature and irradiance maxima, disproportionately occurs in shallow living corals and on the exposed summits of colonies. Solar radiation is suspected to play a role in coral bleaching, with both photosynthetically active radiation (PAR, 400-700mm) and ultraviolet radiation (UVR, 280-400-700mm) implicated in the process.

Subaerial Exposure

Sudden exposure of reef flat corals to the atmosphere during events such as extreme low tides. ENSO-related sea level drops or tectonic uplift can potentially induce bleaching.


While relatively few instances of coral bleaching are linked solely to sediment, it is possible (though not demonstrated) that sediment loading could increase the likelihood of Zooxanthellae species bleaching.


Pathogens, such as those inducing bleaching-like white band disease and black band disease, are considered as contributors to coral bleaching.

Conservation (Note: The last sentence seems incomplete, please provide the missing part if necessary)

Conservation of Corals

The Environmental Protection Act, 1986 prohibits the use of corals and sands from the beaches and coastal waters for construction and other purposes.

India is actively participating in the Green Coast project.

Coastal Regulation Zone Rules (CRZ) prohibit the collection and destruction of corals, along with dredging and underwater blasting in and around coral formations.

Concepts of marine protected areas and marine national parks play a crucial role in coral conservation.

For example, the Gulf of Mannar (Gujrat), Great Nicobar Biosphere Reserve, and Mahatma Gandhi Marine National Park (Andaman) are vital in the conservation of corals.

India has ratified the Paris Agreement, becoming the 62nd country in the world to do so, signaling a significant step toward climate change mitigation.

India has also ratified the second commitment period of the Kyoto Protocol (2013-20).

Coral Bleaching

Bleaching, or coral color paling, occurs when the density of zooxanthellae decreases and/or the concentration of photosynthetic pigments within the zooxanthellae decreases.

Corals typically lose 60-90 percent of their zooxanthellae during bleaching, with each zooxanthella losing 50-80 percent of its photosynthetic pigments.

If the stress-induced bleaching is not too severe and fades over time, affected corals usually regain their symbiotic algae within a few weeks or months.

Continued loss of zooxanthellae, indicating persistent stress and the failure of depleted zooxanthellae populations to recover, can lead to the eventual death of the coral host.

High temperature and irradiance stressors have been associated with the disruption of enzyme systems in zooxanthellae, providing protection against oxygen toxicity.

At temperatures above 30 degrees Celsius, photosynthesis pathways in zooxanthellae are impaired, potentially leading to the disassociation of the coral/algal symbiosis.

Low or high temperature shocks can cause zooxanthellae to die due to cell adhesion dysfunction, involving the separation of cnidarian endodermal cells from their zooxanthellae, followed by the expulsion of both cell types.


For over a million years, coral reefs have been vital in supporting life. The rich environment they provide ensures the flourishing of marine life. Interfering with coral reefs is not in our best interests. To safeguard these beautiful polyps, conservation measures should be implemented. Laws governing coral reef protection should be strictly enforced, and if they are not followed, penalties should be levied as early as possible.

FAQs Coral Reefs

Question: What are coral reefs?

Answer: Coral reefs are diverse ecosystems formed by the accumulation of the skeletons of tiny marine organisms called polyps. These polyps secrete calcium carbonate, creating complex structures that provide habitats for a wide variety of marine life.

Question: What causes a coral reef to form?

Answer: Coral reefs form through the gradual accumulation of coral skeletons over time. This process primarily occurs as coral polyps extract calcium salts from the water and build hard skeletons. The continuous growth and deposition of these skeletons contribute to the development of coral reefs.

Question: What are the types of coral reefs?

Answer: There are four main types of coral reefs:

  1. Fringing Reefs (Shore Reefs): Found close to the coastline and separated by small, shallow lagoons.
  2. Barrier Reefs: Linear offshore structures running parallel to continental margins, often separated by wide and deep non-navigable barriers.
  3. Patch Reefs: Typically forming between fringing and barrier reefs on island platforms or continental shelves.
  4. Atoll Reefs: Circular or horseshoe-shaped coral formations, usually with a central lagoon, often found in open ocean areas.

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