What is the news?

• Rising sea temperatures have triggered severe bleaching events, leading to significant coral mortality in Gulf of Mannar.
• Annual surveys show a decline in live coral cover from 37% in 2005 to 27.3% in 2021.

About Corals

• Coral are made up of genetically identical organisms called polyps.
• These polyps have microscopic algae called zooxanthellae living within their tissues in a mutualistic relationship.
• The coral provides the zooxanthellae with the compounds necessary for photosynthesis.
• In return, the zooxanthellae supply the coral with organic products of photosynthesis, like carbohydrates, which are utilized by the coral polyps for synthesis of their calcium carbonate skeletons.
• Corals are marine invertebrates or animals which do not possess a spine.
• They are the largest living structures on the planet.
• Each coral is called a polyp and thousands of such polyps live together to form a colony, which grow when polyps multiply to make copies of themselves.
• Coral reefs are also called the “rainforests of the seas”.

Types of Coral

Corals are of two types — hard corals and soft corals:

1. Hard corals extract calcium carbonate from seawater to build hard, white coral exoskeletons. Hard corals are in a way the engineers of reef ecosystems and measuring the extent of hard coral is a widely-accepted metric for measuring the condition of coral reefs.
2. Soft corals attach themselves to such skeletons and older skeletons built by their ancestors. Soft corals also add their own skeletons to the hard structure over the years. These growing multiplying structures gradually form coral reefs.

Conditions needed for Corals to Flourish

• Extensive submarine platforms for the formation of colonies by the coral polyps (not more than 90m below sea level).
• High mean annual temperature ranging 20-21 degree Celsius.
• Clean sediment-free water because muddy water or turbid water clogs the mouths of coral polyps resulting into their death.
• Oceanic salinity ranging between 27-30 ppt.
• Ocean currents and waves, as they bring food supply for the polyps.

How do corals bleach?

• When exposed to conditions like heat stress, pollution, or high levels of ocean acidity, the zooxanthellae start producing reactive oxygen species not beneficial to the corals.
• So, the corals kick out the colour-giving algae from their polyps, exposing their pale white exoskeleton and leading to coral starvation as corals cannot produce their own food.
• Severe bleaching and prolonged stress in the external environment can lead to coral death.

Reasons for the Decline

• Climatic Vagaries: Predicted coral bleaching events pose a dire threat to the already fragile ecosystem of the Gulf of Mannar. The IPCC predicts a decline of 70-90% in global coral reefs with a warming of 1.5°C.
• Ocean Acidification: Increased acidity levels in the ocean due to carbon dioxide absorption exacerbate coral stress and hinder their ability to calcify.

PYQ:

2014: Which of the following have coral reefs?

1. Andaman and Nicobar Islands
2. Gulf of Kachchh
3. Gulf of Mannar
4. Sunderbans

Select the correct answer using the code given below.

1. 1, 2 and 3 only
2. 2 and 4 only
3. 1 and 3 only
4. 1, 2, 3 and 4

2018: Consider the following statements

1. Most of the world’s coral reefs are in tropical waters.
2. More than one-third of the world’s coral reefs are located in the territories of Australia, Indonesia and Philippines.
3. Coral reefs host far more number of animal phyla than those hosted by tropical rainforests.

Which of the statements given above is/are correct?

1. 1 and 2 only
2. 3 only
3. 1and 3 only
4. 1, 2 and 3

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