About Coral reefs

• Coral reefs are large underwater structures composed of the skeletons of colonial marine invertebrates called coral.
• The coral species that build reefs are known as hermatypic, or “hard,” corals because they extract calcium carbonate (CaCO3) from seawater to create a hard, durable exoskeleton that protects their soft, sac-like bodies.
• Other species of corals that are not involved in reef building are known as “soft” corals. These types of corals are flexible organisms often resembling plants and trees and include species such as sea fans and sea whips.

Symbiotic relationship

• Most reef-building corals contain photosynthetic algae, called zooxanthellae, that live in their tissues. The corals and algae have a symbiotic relationship.
• The coral provides the algae with a protected environment and compounds they need for photosynthesis. In return, the algae produce oxygen and help the coral to remove wastes. The presence of the zooxanthellae also provides colored pigments to help protect the coral’s white skeleton from sunlight.

Significance of Coral reefs

• Coral reefs only occupy 0.1% of the area of the ocean but they support 25% of all marine species on the planet.
• Because of the diversity of life found in the habitats created by corals, reefs are often called the “rainforests of the sea.”

Geographical distribution

• • The reef-building corals prefer to grow at
    • depths shallower than 30 m (100 ft), or
    • where the temperature range is between 16-32°C, and

• light levels are high

• The majority of reef building corals are found within tropical and subtropical waters. These typically occur between 30°N and 30°S of the equator.
• The largest of these coral reef systems, the Great Barrier Reef in Australia, is more than 1,500 miles long (2,400 kilometers).

Coral reefs in India

• Coral reefs are present in the areas of Gulf of Kutch, Gulf of Mannar, Andaman & Nicobar, Lakshadweep Islands and Malvan coast of Maharashtra.

Main threats to coral reefs

• • Climate change: Warmer water temperatures can result in coral bleaching. When water is too warm, corals expel the algae (zooxanthellae) living in their tissues causing the coral to turn completely white. This is called coral bleaching. When a coral bleaches, it is not dead. Corals can survive a bleaching event, but they are under more stress and are subject to mortality.
  • Ocean acidification: Ocean acidification is caused by rising levels of CO2 in the atmosphere. Oceans absorb some of the CO2. As CO2 enters the ocean, it reacts with water increasing hydrogen ion concentration (thus decreasing ocean pH) and decreasing the carbonate ion concentration. A reduction in carbonate ions affects the Calcification process of Corals.

• Calcification is the process by which corals form their skeletons by combining calcium ions and carbonate ions to create calcium carbonate.

• Pollution: Urban and industrial waste, plastics, sewage, agrochemicals, and oil pollution are poisoning reefs. Some pollutants, such as sewage and runoff from farming, increase the level of nitrogen in seawater, causing an overgrowth of algae.
• Sedimentation: Erosion caused by construction, mining, logging, and farming is leading to increased sediment in rivers. This ends up in the ocean, where it can smother corals by depriving them of the light needed to survive.
• Destructive fishing practices: These include cyanide fishing, blast or dynamite fishing, bottom trawling, and muro-ami (banging on the reef with sticks). Bottom-trawling is one of the greatest threats to cold-water coral reefs.

Why in news?

• Recently, scientists have recorded four species of azooxanthellate corals for the first time from Indian waters. These new corals were found from the waters of Andaman and Nicobar Islands.

What are Azooxanthellate corals?

• Azooxanthellate corals are a group of corals that do not contain zooxanthellae and derive nourishment not from the sun but from capturing different forms of plankton.
• These groups of corals are deep-sea representatives, with the majority of species reporting from between 200 m to 1000 m.
• Meanwhile, Zooxanthellate corals are restricted to shallow waters.

News in detail

• According to some scientists from ZSI, all the four groups of corals recorded are from the same family Flabellidae.
• Truncatoflabellum crassum, T. incrustatum, T. aculeatum and T. irregulare under the family Flabellidae were previously found from Japan to the Philippines and Australian waters while only T. crassum was reported within the range of Indo-West Pacific distribution including the Gulf of Aden and the Persian Gulf.
• There are about 570 species of hard corals found in India and almost 90% of them are found in the waters surrounding Andaman and Nicobar Islands. The pristine and oldest ecosystem of corals share less than 1% of the earth’s surface but they provide a home to nearly 25% of marine life.
• According to ZSI, coral reefs are one of the most productive, sustainable and pristine ecosystems of the world’s oceans, especially in shallow coastal waters. These habitats contribute several services associated with human needs and existence. Hard corals are the prime and intrinsic part of the coral reef ecosystem.

References:

1. https://www.thehindu.com/sci-tech/science/four-new-corals-recorded-from-indian-waters/article65553530.ece
2. www.livescience.com/40276-coral-reefs.html