Corals and coral Reefs – Definition, Structure, Types, Formation

Sourav Bio

What is Corals and coral Reefs?

  • Corals are marine invertebrates belonging to the Cnidaria phylum. There is a close relationship between them and jellyfish and sea anemones. Corals are commonly found in warm, shallow seas, where they form colonies composed of several coral polyps. These polyps exude calcium carbonate exoskeletons that constitute the framework of coral reefs.
  • Coral reefs are among the world’s most diversified ecosystems and are home to a broad diversity of marine creatures, including fish, crabs, and mollusks. Coral reefs provide crucial habitats for many of these species, as well as important ecological services, such as preventing erosion and storm damage to shorelines.
  • However, coral reefs are threatened by a variety of factors, such as rising ocean temperatures, ocean acidification, overfishing, and pollution. These dangers are causing widespread coral bleaching, which happens when coral polyps expel algae from their tissues, leading coral to turn white and become susceptible to illness and mortality.
  • To protect and rebuild coral reefs, conservation efforts are currently ongoing, including attempts to reduce greenhouse gas emissions and improve water quality. In addition, some researchers are investigating approaches such as assisted evolution and coral gardening that could help corals adapt to changing environmental conditions.
  • Corals are marine invertebrates belonging to the Cnidaria phylum.
  • Corals produce colonies composed of several coral polyps.
  • These calcium carbonate exoskeletons are secreted by coral polyps and create the structure of coral reefs.
  • Coral reefs are among the planet’s most diversified ecosystems.
  • Many marine animals, including fish, crabs, and mollusks, inhabit coral reefs.
  • Important ecological benefits provided by coral reefs include protection of shorelines from erosion and storm damage.
  • There are various dangers to coral reefs, including rising ocean temperatures and ocean acidification.
  • Coral bleaching occurs when coral polyps expel algae from their tissues, causing coral to turn white and become susceptible to illness and mortality.
  • Moreover, overfishing and pollution are significant dangers to coral reefs.
  • Many coastal communities rely on coral reefs as a source of income and as a tourism attraction.
  • It can take thousands of years for coral reefs to emerge, yet they can be destroyed within a few years or even months.
  • Certain corals can create compounds with potential medical applications.
  • Scientists examine coral reefs to gain a better understanding of coral biology and to discover ways to safeguard them.
  • Protection and restoration activities are now ongoing for coral reefs.
  • These efforts include the reduction of greenhouse gas emissions, the improvement of water quality, and the exploration of techniques such as aided evolution and coral gardening.
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Structure of coral Polyp

  • A typical coral polyp creates a colony that is around 10 mm in length and 1 to 3 mm in width. Each coral polyps can reach a maximum diameter of 25 cm.
  • All coral polyps lack a basal disc because the base of each polyp is encased by a calcareous exoskeleton.
  • Oral disc has multiple rows of tentacles surrounding an extended oral or circular mouth. Pharynx is limited in length without siphonoglyphs.
  • Mesenteries are confined to the coelenteron’s top region. Polyps are exclusively present on the surface layers of coral colonies.
  • They eat by both raptorial and suspension methods at night. The majority of corals are symbiotic with zooxanthellae.

Structure of coral skeleton

Coral created by a single Polyp is known as corallite. A collection of colonial Polyps that create the skeletal mass known as corallum. The base of each corallite is known as the basal plate. Theca encompasses the aboral region of Polyp. The cavity of the cup contains sclerosepta, which are vertical radiating ridges. The inner ends of the sclerosepta merged in the middle to produce the columella.

Formation of coral skeleton

  • All coral polyps engage in sexual reproduction through the fusing of gametes.
  • The zygote transforms into a free-swimming, ciliated planula larva, which attaches to a substratum after a brief period of swimming.
  • This planula larva transforms into a juvenile coral polyp. In corals, there is no medusa stage. Through the process of asexual budding, a single parent becomes the parent of the entire colony.
  • This coral polyp’s ectoderm begins to secrete prototheca, a skeletal rudiment. After this secretion, radial folds that secrete sclerosepta form.
  • Concurrently, a rim is constructed on the thecal wall surrounding the polyp. In the interim, more skeletal material is inserted into the spaces between the sclerosepta of the skeleton.
  • By the proliferation of new polyps, the coral colony expands continuously along the edges and at the surface. The patterns of budding are the cause of the structural differences and variances within the coral colony.

Types of corals in different groups

  • Hydrozoancorals: Certain hydrozoan organisms, known as hydroid corals, secrete corals known as hydrozoan corals. Calicoblasts are modified epidermal cells that secrete skeleton. Each and every hydroid coral is colonial and branching. Examples:Millepora, stylaster.
  • Octocoralliacorals: Octocorallia corals are colonial, soft corals belonging to the order Alcyonacea. Protein (collagen) skeleton containing calcareous spicules are secreted internally. Alcyonium (Dead man’s finger), Tubipora (organ pipe coral), Heliopora (Blue cord), Gorgonia (Sec fan), Corallium (Red coral), Eunicea (Sea Rods), and Pennatula (Sea rods) are all examples of corals (Sea pens).
  • Hexacorallia Corals: Order madrepora contains stone corals or genuine corals. Polyps secrete a calcareous, hefty exoskeleton. They are either solitary or colony in nature. Madrepora (stone coral or stag horn coral), Favia, Oculina, Acropora, and Meandrina are examples of Colonial corals (Brain coral). Fungia (mushroom coral), Flabellum, and Caryophylla are solitary corals.

Coral Reefs: Coral colonies continuously increase in size through the reproduction of new polyps, forming coral reefs. CaCO3 is the primary component of coral reefs, which is secreted by coral polyps. The majority of coral reefs consist of stone corals. Additional coral reef contributions include hydrocorallines and alcyonarians. The vast majority of coral reefs are located in shallow tropical and subtropical waters. Corals grow optimally at temperatures between 220C and 280C, and at depths ranging from 50 to 200 feet. Coral reefs are frequently referred to as the rainforests of the sea because they provide habitat for at least 25 percent of marine life. In the early Ordovician epoch, 485 million years ago, coral reefs first formed. Due to eutrophication, rising temperatures, ocean acidification, overfishing, and the El Nio effect, coral reefs are now threatened.

Factors affecting Coral Reefs

  • Extreme climate conditions: Severe climate circumstances have led to the decline of these corals, as they cannot survive in water with a high temperature. As predicted by scientists, the majority of the world’s coral reefs will soon decline as ocean temperatures continue to rise.
  • Overfishing: Another important worry is overfishing, which is causing an ecological imbalance in coral reefs.
  • Coastal development: Construction of coastal infrastructure and tourist resorts on or near these coral reefs causes substantial harm.
  • Pollution: Toxic chemicals that are discharged directly into the ocean can contribute to the poisoning of coral reefs by elevating the nitrogen concentration in the saltwater, thereby fostering an algal bloom.
  • Sedimentation: Building around the beaches and islands causes soil erosion, which increases sediments in the river. As a result, it can suffocate corals by preventing them from receiving the necessary light to survive.

Growth conditions for Coral Reefs

  • Temperature: Coral reefs require water temperatures between 68 and 82 degrees Fahrenheit (20 and 28 degrees Celsius) to survive. Outside of this range, temperatures can damage or destroy corals.
  • Sunlight: Sunlight is necessary for photosynthesis, which is essential for the growth and survival of coral and its symbiotic algae.
  • Clean and clear water: Coral reefs require clean and clear water to allow sufficient light to reach the coral polyps and to prevent sedimentation and nutrient excess, which are detrimental to coral development and survival.
  • Shallow water: Coral reefs grow in shallow waters when sufficient sunlight is available for photosynthesis. In addition, they require a firm substrate, like as rocks or the remains of other corals, to connect to.
  • Salinity: Coral reefs require a consistent and adequate level of salinity, or salt concentration, in the water. Salinity fluctuations can stress or destroy corals.
  • Nutrient availability: Coral reefs require a balance of nutrients in the water for growth, but excess nutrients can result in destructive algal blooms and other detrimental effects.
  • pH balance: Coral reefs require a stable and suitable pH balance in the water, which can be affected by ocean acidification resulting from higher atmospheric carbon dioxide levels.
  • Wave action: Coral reefs need wave motion to bring in fresh water and nutrients, but excessive wave action or storms can cause physical damage to the corals.

Coral reefs, which face various problems including climate change, overfishing, pollution, and habitat destruction, require certain growth conditions to survive. Coral reef conservation and protection are essential for the health of our seas and the multitude of species that depend on them.

Types of Coral Reefs

There are three primary varieties of coral reefs, each having its own environment and characteristics:

  • Fringing reefs: Fringing reefs are the most common type of coral reef and are situated near to the shore, either surrounding islands or along the coastline. These reefs grow immediately from the coast and, depending on the depth of the surrounding water, can be shallow or deep. Between the reef and the beach, there is typically a narrow, shallow lagoon.
Barrier reefs
Barrier reefs
  • Barrier reefs: Barrier reefs are similar to fringing reefs, with the exception that they are separated from the shore by a deeper lagoon or channel. These reefs are frequently located further offshore, separated from the mainland by a deep canal. Australia’s Great Barrier Reef is the world’s largest barrier reef.
Atoll reefs
Atoll reefs
  • Atoll reefs: Atoll reefs are circular or horseshoe-shaped reefs which encircle a central lagoon. They are constructed atop a submerged volcano or seamount that has subsided through time and are typically found in the open ocean. In the Indian and Pacific Oceans, including the Maldives and the Marshall Islands, there are atoll reefs.
Atoll reefs
Atoll reefs

Each type of coral reef provides a distinct habitat for a wide range of marine organisms, and they play a crucial role in the health and functionality of our oceans. Climate change, overfishing, and pollution are however severe dangers to all types of coral reefs. Important ecosystems must be protected and preserved through conservation efforts.

Importance of Coral Reefs

Coral reefs are vital ecosystems that give countless advantages to the earth and its inhabitants. Here are some of the most essential reasons why coral reefs are crucial:

  • Biodiversity: Coral reefs are among the most biodiverse ecosystems on the globe, as they provide home and protection for a huge array of marine organisms. They are commonly known as the “rainforests of the sea.”
  • Fisheries: Coral reefs are significant fisheries, supplying food and a means of subsistence for millions of people throughout the world. They contribute to the fishing industry by serving as a habitat and breeding ground for numerous economically valuable fish species.
  • Coastal protection: Coral reefs create a natural barrier that aids in preventing erosion and storm damage to coastlines. In addition, they assist absorb the energy of waves and storms, mitigating their negative effects on coastal areas.
  • Tourism: Coral reefs are a popular tourist attraction and a substantial contributor to the worldwide tourism sector. They enable snorkelling, scuba diving, and other recreational activities.
  • Scientific research: Scientists do research on coral reefs to better comprehend the biology of coral and the marine creatures that inhabit there. This research can benefit in the creation of novel medicinal therapies and conservation activities.
  • Climate regulation: Coral reefs have a significant part in regulating the climate of the Earth. They are able to absorb and store enormous quantities of carbon dioxide, so contributing to the reduction of greenhouse gases in the environment.

Despite their significance, coral reefs are threatened by climate change, overfishing, pollution, and habitat loss. Preserving and conserving coral reefs is essential for the health of our seas and the multitude of species that depend on them.

Coral Reefs in India

India’s coastline stretches about 7500 kilometres. India has extremely few coral reefs because of its subtropical climate. Palk Bay, the Gulf of Mannar, the Gulf of Kutch, the Andaman and Nicobar Islands, and the Lakshadweep Islands are the principal coral reefs in India. The Lakshadweep reef is an example of an atoll reef, whereas the others are all bordering reefs.

  • Palk Bay: Palk Bay, which is located on the south-eastern coast of India, is divided from the Gulf of Mannar by the Mandapam Peninsula and the Rameshwaram Island and is centred at 9 °17’N and 79 °15’E. The one bordering reef in Palk Bay is 25 to 30 kilometres long and fewer than 200 metres broad; it sits in the Pamban channel’s east-west direction. This reef’s maximum depth is approximately 3 metres.
  • The Straits of Mannar: Located between Tuticorin and Rameswaram for a distance of 140 kilometres, the Gulf of Mannar encompasses a chain of 21 islands. These 21 islands are between latitudes 8°47′ N and 9° 15′ N and longitudes 78° 12′ E and 79° 14’E; they are a part of the 140-kilometer-long and 25-kilometer-wide Mannar Barrier Reef.
  • Islands of the Andaman and Nicobar Group: The Andaman and Nicobar Islands are located between latitudes 6°-14° N and longitudes 91°-94° E. They are located in the south-eastern portion of the Gulf of Bengal and consist of 350 uninhabited islands. These islands reach southward from Burma’s Irrawaddy Delta to the Arakan Yoma Mountains. Almost every island in the Andaman and Nicobar island groupings is surrounded by reefs.
  • The Strait of Kutch: The Gulf of Kutch is located in the northern portion of the Saurashtra Peninsula, between 22°15′-23°40′ N latitude and 68°20′-70′40′ E longitude, with an approximate area of 7350 square kilometres. These reefs are approximately 170 kilometres long and 75 kilometres wide at the mouth, which narrows at a longitude of 72 degrees 20 minutes. Due to the mud deposits on numerous coral reefs, the state of these reefs is severely deteriorated.
  • Lakshadweep Islands: The Lakshadweep Islands, which are scattered in the Arabian Sea and located between 8°N and 12°3’N in latitude and 71°E and 74°E in longitude, are 225 to 450 kilometres from the Kerala coast. 36 little islands, 12 atolls, three reefs, and five submerged banks make up the 32 km2 in size of the islands, which are surrounded by 4200 km2 of lagoons. Due to the warm, humid climate of these islands, the water temperature ranges from 28 to 31 degrees Celsius, and the salinity ranges from 34% to – 37%.

Formation of coral reef

Many hypotheses have been proposed to explain the origin of coral reefs. Yet, just a few are suitable. These are the hypotheses that adequately explain the creation of coral reefs.

  • Subsidence theory : Darwin presented this hypothesis regarding subsidence. According to this notion, all known coral reefs are located in places that have experienced land subsidence. He argues that corals initially form as bordering reefs around the coasts of islands in tropical shallow waters. Subsidence of the islands began, resulting in the transformation of the bordering reef into a barrier reef separated by a lagoon. Although additional sinking of the island results in the island’s disappearance, an atoll is formed.
  • Glaciation control theory : The hypothesis of glaciation control was proposed by Albrecht Penck and Reginald Daly. They argued that the creation of ice caps during the glacial period dropped the water level by 60-70 metres. At that time, temperatures were quite low. Eventually, ice evaporated and the temperature increased. Corals began to grow on the ocean floor’s flat platform and kept pace with rising sea levels to construct enormous reefs.

Threats to Coral Reefs

Coral reefs are threatened by a multitude of problems that threaten their survival. Among the most significant dangers to coral reefs are:

  • Climate change: Increasing sea temperatures, ocean acidification, and alterations in weather patterns are all correlated with climate change and are causing widespread coral bleaching and mortality.
  • Overfishing and harmful fishing methods: Overfishing can lead to ecosystem imbalances, and destructive fishing methods such as blast fishing and cyanide fishing can directly harm or destroy corals.
  • Pollution: Pollution from coastal development, agriculture, and wastewater can lead to algal blooms and other coral-damaging phenomena.
  • Land-use change and coastal development: Erosion, sedimentation, and runoff caused by land-use change and coastal development can harm or kill coral reefs.
  • Invasive species: Invasive species can directly harm or kill corals and compete with native species for resources.
  • Tourism: Tourism can affect coral reefs through activities such as anchor damage, reef walking, and boat-related pollution.
  • Coral mining: Coral is frequently collected for building materials and jewellery, which can lead to the destruction of entire reef systems.

Conservation and management strategies, such as marine protected areas, sustainable fishing techniques, and the reduction of pollution and greenhouse gas emissions, are employed to combat these challenges.

Facts about Corals and coral Reefs

  • Coral reefs are marine ecosystems comprised of colonies of small organisms known as coral polyps.
  • There are around 800 species of coral, the largest of which is the Brain Coral, which can reach a diameter of 3 metres.
  • Less than 1% of the ocean floor is covered by coral reefs, although they are home to more than 25% of all marine species.
  • The fishing industry relies on coral reefs to provide food and money for millions of people throughout the world.
  • In addition to protecting coastlines from storm surges and erosion, coral reefs mitigate the effects of natural disasters on human populations.
  • The tiny algae that live within corals give them their brilliant hues and provide them with food and energy.
  • Certain species of coral reefs can grow up to 20 centimetres each year. Coral reefs can grow up to 1 centimetre every year.
  • There are coral reefs in all of the world’s oceans, from the tropics to the polar regions.
  • The sensitivity of coral reefs to changes in water temperature and acidity makes them susceptible to climate change.
  • Bleaching happens when corals expel their symbiotic algae, causing them to turn white and ultimately perish.
  • Overfishing and pollution are other significant hazards to coral reefs, lowering their resilience to environmental stresses.
  • Some of the oldest coral reefs date back to the Triassic epoch, which occurred millions of years ago (250 million years ago).
  • The Great Barrier Reef is the world’s biggest coral reef system, spanning over 2,400 kilometres off the coast of Australia.
  • The Coral Triangle, the Red Sea Coral Reef, and the Belize Barrier Reef are among notable coral reefs.
  • Millions of tourists visit coral reefs year to snorkel, scuba dive, and explore these extraordinary underwater ecosystems.
  • Coral reefs are significant spiritual and economic resources for numerous indigenous and coastal people across the globe.
  • Certain varieties of coral have substances that can treat cancer, arthritis, and other ailments. Coral reefs have been utilised for medical purposes for centuries.
  • The United Nations has designated 2021-2030 as the Decade of Ocean Research for Sustainable Development, with an emphasis on coral reef preservation and protection.
  • The extinction of coral reefs would have catastrophic effects on the planet’s ecosystems, affecting everything from marine biodiversity to human health.
  • Scientists are investigating new methods for rebuilding damaged coral reefs, including the cultivation of coral in nurseries and its reintroduction into the wild.

MCQ on Corals and coral Reefs

What are coral reefs made of?
a. Rocks
b. Plants
c. Coral polyps
d. Seaweed
Answer: c. Coral polyps

What is the main function of coral reefs?
a. To provide shelter for fish
b. To protect coastlines from erosion
c. To produce oxygen
d. All of the above
Answer: d. All of the above

How do corals get their vibrant colors?
a. They absorb color from the water
b. They produce pigments
c. They host algae that provide color
d. They reflect sunlight
Answer: c. They host algae that provide color

What is coral bleaching?
a. When coral polyps die and fall off the reef
b. When corals lose their color and turn white
c. When algae overgrow and suffocate the coral
d. When coral polyps reproduce and release eggs and sperm into the water
Answer: b. When corals lose their color and turn white

What is the largest coral reef system in the world?
a. The Great Barrier Reef
b. The Coral Triangle
c. The Red Sea Coral Reef
d. The Belize Barrier Reef
Answer: a. The Great Barrier Reef

What are some of the major threats to coral reefs?
a. Overfishing
b. Pollution
c. Climate change
d. All of the above
Answer: d. All of the above

What is coral mining?
a. The practice of removing coral from reefs for use in construction
b. The process of extracting minerals from coral reefs
c. The practice of catching and selling live coral
d. None of the above
Answer: a. The practice of removing coral from reefs for use in construction

What is coral fragmentation?
a. The process of breaking apart coral colonies for scientific study
b. The process of growing new coral colonies from fragments of existing ones
c. The process of removing coral from reefs for use in aquariums
d. None of the above
Answer: b. The process of growing new coral colonies from fragments of existing ones

What is the role of algae in coral reefs?
a. Algae provide food and energy for coral polyps
b. Algae compete with coral for space on the reef
c. Algae can harm coral by releasing toxins
d. None of the above
Answer: a. Algae provide food and energy for coral polyps

What is the process of coral spawning?
a. When coral polyps release eggs and sperm into the water for fertilization
b. When coral polyps release toxins to defend against predators
c. When coral polyps reproduce asexually
d. None of the above
Answer: a. When coral polyps release eggs and sperm into the water for fertilization

What is the significance of the Coral Triangle?
a. It is the largest coral reef system in the world
b. It has the highest marine biodiversity in the world
c. It is the most heavily fished coral reef system in the world
d. None of the above
Answer: b. It has the highest marine biodiversity in the world

What is the role of mangroves in coral reefs?
a. Mangroves provide food for coral polyps
b. Mangroves protect coral reefs from storms and erosion
c. Mangroves compete with coral for space on the reef
d. None of the above
Answer: b. Mangroves protect coral reefs from storms and erosion


What is Coral Bleaching?

The phenomenon known as coral bleaching happens when coral polyps eject the coloured algae (zooxanthellae) that reside within their tissues. These algae nourish the coral and furnish it with its brilliant colours. The coral appears white or “bleached” as the algae are evacuated or die.
The natural occurrence of coral bleaching has increased in frequency and severity due to human-induced climate change. When the water temperature is too high or other stressors, such as pollution or overfishing, are present, the coral may become stressed and expel its algae. Coral may die if the stress is persistent or severe.
Bleaching is a major hazard to the world’s coral reefs. As coral dies, it affects the entire marine environment, which includes fish, sea turtles, and other marine species. Moreover, coral reefs provide coastal protection and support economies through tourism and fishing. Conservation and climate change mitigation initiatives, such as cutting carbon emissions and establishing marine protected zones, are being implemented to alleviate the effects of coral bleaching.

What are corals?

Corals are marine invertebrates that belong to the phylum Cnidaria. They are colonial animals that secrete a hard exoskeleton made of calcium carbonate.

What is a coral reef?

A coral reef is a diverse underwater ecosystem made up of coral polyps, algae, fish, and other marine organisms. Coral reefs are found in warm, shallow waters in the tropics.

How are coral reefs formed?

Coral reefs are formed when coral polyps secrete calcium carbonate, which forms the hard exoskeleton that creates the structure of the reef. Over time, as more polyps settle and grow, the reef grows larger.

What is the importance of coral reefs?

Coral reefs are important because they provide habitat and shelter for a wide variety of marine organisms. They also protect coastlines from storms and erosion and support local economies through tourism and fishing.

Why are coral reefs in danger?

Coral reefs are in danger due to a variety of human-induced factors, including climate change, overfishing, pollution, and coastal development. These factors can lead to coral bleaching, disease, and destruction of the reef ecosystem.

How can we protect coral reefs?

There are many ways to protect coral reefs, including reducing carbon emissions to combat climate change, reducing pollution and runoff from coastal development, and implementing sustainable fishing practices. Individuals can also support coral reef conservation efforts through donations and responsible tourism.

How long do coral reefs live?

Coral reefs can live for hundreds of years or even thousands of years, depending on the species of coral and the conditions of the ecosystem in which they live.

What is coral spawning?

Coral spawning is a natural process in which corals release eggs and sperm into the water column, allowing fertilization and the development of new coral colonies. This usually occurs once a year, during a specific phase of the lunar cycle.

Can coral reefs recover from damage?

Yes, coral reefs can recover from damage if the conditions are right. If the damage is not severe and the water quality and temperature are conducive to coral growth, new coral polyps can settle and grow, eventually restoring the damaged reef.

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