Coral Reefs – Complete GK Capsule for Railway Exams
1. What is a Coral Reef?
- Definition: Underwater ecosystem built by colonies of tiny animals (coral polyps) that secrete calcium carbonate (CaCO₃) skeletons.
- Called: “Rainforests of the Sea” – support 25 % of all marine species while occupying <1 % ocean floor.
- Symbiosis: Polyps + Zooxanthellae (algae) → algae provide 90 % food via photosynthesis; polyps give shelter & CO₂.
| Factor |
Ideal Range / Fact |
| Water temperature |
23–29 °C (survival limit 20–32 °C) |
| Salinity |
27–40 ppt (best 34–36 ppt) |
| Depth |
0–30 m (photic zone) |
| Sunlight |
Minimum 6 hrs day⁻¹ |
| pH |
8.0–8.3 (alkaline) |
| Wave action |
Moderate – supplies O₂ & nutrients |
3. Types of Coral Reefs – Darwin Classification (1842)
| Type |
Location Example (India) |
Key Feature |
| Fringing |
Andaman-Nicobar, Gulf of Mannar |
Direct attachment to shore; no lagoon |
| Barrier |
Andaman (east coast) |
Parallel to shore; wide lagoon in-between |
| Atoll |
Lakshadweep (36 atolls) |
Ring-shaped; central lagoon; built on subsiding volcanic islands |
4. Coral Reef Distribution – Global & India
- Global:
- Total ocean area covered: ~2,84,300 km² (≈ 0.1 % ocean area)
- Indo-Pacific region hosts 91 % of reef area.
- Great Barrier Reef (GBR), Australia: world’s largest (2,300 km length; listed 1981 as UNESCO World Heritage).
- India:
- Reef area: ~5,790 km² (Gulf of Mannar 2,467 km² largest)
- Four Coral Reef Regions:
- Gulf of Mannar (Tamil Nadu) – 21 islands; Biosphere Reserve 1989
- Andaman-Nicobar – 500+ species; 2004 tsunami damaged ~34 % reefs
- Lakshadweep – 36 islands; all atolls
- Gulf of Kachchh (Gujarat) – only marginal/fringing patch reefs; 2002 declared Marine National Park
5. Biodiversity Hotspot Statistics
- Species supported: >4,000 fish spp., 800 coral spp. (world), 200+ soft corals.
- India coral spp.: ~470 hard coral species (37 genera).
- Marine National Park, Gulf of Kachchh: 70 hard & soft coral species.
6. Threats & Status
| Threat |
Data Point / Event |
| Coral bleaching |
1998 El-Niño: lost 42 % GBR corals |
| Climate change |
1 °C rise → 82 % world reefs bleached (IPCC 2022) |
| Ocean acidification |
pH drop 0.1 since pre-industrial → 30 % slower calcification |
| Human local |
Overfishing, dredging, coastal tourism |
| Indian reef loss |
Gulf of Mannar: 16 % live coral (2021) vs 36 % (2005) |
7. Conservation Efforts & Dates to Remember
- International Year of the Reef (IYOR): 1997 & 2008 (ICOADS)
- International Coral Reef Initiative (ICRI): founded 1994; India member since 1996
- Coral Triangle Day: 9 June (since 2012) – awareness in Indo-Pacific
- India projects:
- Coral Transplantation: Zoological Survey of India (ZSI) & National Institute of Oceanography (NIO) started 2010
- Ministry of Environment “Coral Reef Recovery Programme”: ₹290 cr (2021-26)
- UNESCO Biosphere: Gulf of Mannar (1989), Great Nicobar (2013 buffer zone)
- Ramsar Reef Sites (India): no exclusive reef Ramsar; but Sundarbans & Chilika have patchy corals
8. Quick Reference Table – Indian Reefs
| Region |
Reef Type |
No. of Islands |
Notable Island / Feature |
Declared Marine Park / BR |
| Gulf of Mannar |
Fringing |
21 |
Kurusadai Island |
BR 1989; NP 1986 |
| Andaman-Nicobar |
Fringing & Barrier |
572 |
Jolly Buoy (MBNP) Red Skin (MBNP) |
Mahatma Gandhi MNP 1983 |
| Lakshadweep |
Atoll |
36 |
Kavaratti (capital) |
No NP; entire UT protected |
| Gulf of Kachchh |
Patch |
42 reefs |
Pirotan Island |
MNP 1982 |
9. One-liner Revision Facts
- Corals are animals, not plants; belong to phylum Cnidaria.
- Zooxanthellae is a dinoflagellate algae (genus Symbiodinium).
- Calcification rate: 0.3–2 cm/year (branching corals fastest).
- Coral spawning occurs once a year (Oct-Nov GBR; mass event).
- Coral polyps are radially symmetrical; have nematocysts.
- First coral reef atlas of India: 2018 by Space Applications Centre (ISRO) using Resourcesat-2 LISS-IV.
- Coral reefs protect coastline: reduce wave energy up to 97 %.
- Coral medicine: anti-HIV (bryostatin), anti-cancer (didemnin).
- “Coral bleaching” first widely reported during 1997-98 El-Niño.
- Coral reefs contribute $2.7 trillion per decade in ecosystem services (WWF 2021).
15+ MCQs for Railway Exams
Answer
1. Which is the largest coral reef system in the world?
A. Andaman reef
B. Maldives reef
C. Great Barrier Reef
D. Red Sea reef
Answer
C. Great Barrier Reef
2. The term “Rainforests of the Sea” refers to
A. Mangroves
B. Seagrass beds
C. Coral reefs
D. Estuaries
Answer
C. Coral reefs
3. Which algae live symbiotically inside coral polyps?
A. Kelp
B. Zooxanthellae
C. Ulva
D. Chlorella
Answer
B. Zooxanthellae
4. Which of the following is NOT a type of coral reef according to Darwin?
A. Fringing
B. Barrier
C. Atoll
D. Lagoonal
Answer
D. Lagoonal
5. The Gulf of Kachchh coral reefs are classified as
A. Atolls
B. Barrier reefs
C. Patch / Fringing
D. Submerged banks
Answer
C. Patch / Fringing
6. In which year was the Gulf of Mannar declared a Biosphere Reserve?
A. 1986
B. 1989
C. 1991
D. 1999
Answer
B. 1989
7. Which Union Territory of India is entirely made up of atoll reefs?
A. Puducherry
B. Lakshadweep
C. Andaman
D. Diu
Answer
B. Lakshadweep
8. Coral polyps belong to which phylum?
A. Porifera
B. Cnidaria
C. Mollusca
D. Echinodermata
Answer
B. Cnidaria
9. The ideal sea-surface temperature range for coral survival is
A. 10–15 °C
B. 15–20 °C
C. 23–29 °C
D. 30–35 °C
Answer
C. 23–29 °C
10. Which event caused massive coral bleaching in 1998?
A. La-Nina
B. El-Niño
C. Indian Ocean Dipole
D. Cyclone Hudhud
Answer
B. El-Niño
11. The Coral Triangle Day is observed every year on
A. 1 June
B. 9 June
C. 22 May
D. 8 June (World Oceans Day)
Answer
B. 9 June
12. Which Indian organization released the first-ever Coral Reef Atlas of India?
A. NIO
B. ZSI
C. ISRO
D. NIOT
Answer
C. ISRO
13. The main compound that constitutes coral skeleton is
A. Calcium phosphate
B. Calcium carbonate
C. Silica
D. Chitin
Answer
B. Calcium carbonate
14. Which state has the highest coral reef area in India?
A. Gujarat
B. Tamil Nadu
C. Andaman-Nicobar
D. Lakshadweep
Answer
B. Tamil Nadu (Gulf of Mannar reef area)
15. Ocean acidification is primarily caused by the absorption of
A. Oxygen
B. Carbon dioxide
C. Sulphur dioxide
D. Nitrogen
Answer
B. Carbon dioxide
16. Which project was launched by India in 2021 with ₹290 crore for coral restoration?
A. Project Coral
B. Blue Flag
C. Coral Reef Recovery Programme
D. Sagarmala
Answer
C. Coral Reef Recovery Programme
17. Which island of Lakshadweep is known for its coral nursery project?
A. Kiltan
B. Kavaratti
C. Agatti
D. Bitra
Answer
B. Kavaratti
Revise daily: 1 hour → 10 one-liners + 15 MCQs → Score full in “Coral Reefs” under Railway GK section.
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