Coral Reefs
What are Corals?
- Coral is actually a living animal.
- Corals have a symbiotic relationship with microscopic algae called zooxanthellae.
- Zooxanthellae live within coral tissues and get light and carbon dioxide from the coral.
- Corals get nutrients, energy, and color from the algae.
- The tissues of corals themselves are actually transparent (white) — the beautiful colors of coral reefs come from zooxanthellae.
- Coral polyps are tiny animals that form colonies. As polyps grow and die, they leave behind their limestone (calcium carbonate) skeletons, which are colonized by new polyps, building up the reef over time.
- Coral reefs can form a variety of shapes: Branched, Table-like, Cups, boulders, or knobs.
Types of Corals
- Hard corals: Build reefs (reef-building or hermatypic corals).
- Soft corals: e.g., sea fans, gorgonians — do not build reefs.
Features
- Found in shallow tropical areas where seawater is clean, clear, and warm.
- Estimated coral reef cover in Indian waters: ~18,000 sq km.
- Coral reefs are among the most productive and complex ecosystems, with high biological diversity — often called the Tropical Rainforests of the Oceans.
- High productivity comes from their primary production and tight recycling of scarce nutrients.
Ecological Importance
- Provide habitat for invertebrates, vertebrates, plants, and birds.
- Serve as natural protective barriers against coastal erosion and storm surges.
- Largest biogenic calcium carbonate producers.
- Provide substrate for mangroves and seagrasses.
- Highly adapted to capture plankton and recycle nutrients efficiently.
Types of Coral Reefs
Coral reefs are classified by their location:
| Type | Description | Examples in India |
|---|---|---|
| Fringing reefs | Contiguous with the shoreline, most common. | Andaman Islands |
| Patch reefs | Isolated patches inside lagoons or bays. | Palk Bay, Gulf of Mannar, Gulf of Kachchh |
| Barrier reefs | Linear offshore reefs parallel to shore, separated by a lagoon. | Nicobar, Lakshadweep |
| Atolls | Circular or semi-circular reefs around subsiding seafloor platforms. | Lakshadweep, Nicobar |
- When coral growth does not keep up with subsidence, the reefs become submerged banks (seen in Lakshadweep).
Coral Reef Ecosystem in India
- Seagrasses grow on Kavaratti Atoll.
- Mangroves are prevalent on Andaman & Nicobar reefs.
- Major reef areas of India (in order of species diversity):
- Andaman & Nicobar Islands
- Lakshadweep Islands
- Gulf of Mannar
- Gulf of Kachchh
Threats to Coral Reefs
Natural Causes
- Outbreak of reef-destroying organisms (e.g., predators, diseases).
- Bleaching: Loss of symbiotic zooxanthellae due to stress.
- Depletion of essential symbionts.
- Violent storms and flooding.
- Extreme temperatures (both high and low).
- El Niño Southern Oscillation (ENSO) events.
- Subaerial exposure (exposure of corals above water during low tides).
- Predatory outbreaks (e.g., Crown-of-thorns starfish).
- Epidemics (epizootics) affecting coral health.
Anthropogenic (Human-Induced) Causes
- Chemical pollution: pesticides, cosmetics, toxic runoff.
- Industrial pollution: heavy metals, effluents.
- Mechanical damage: from dredging, anchoring, and shipping.
- Nutrient and sediment loading: from land-based runoff.
- Tourism and recreational activities: trampling, souvenir collection.
- Mining or coral collection.
- Thermal pollution: from coastal power plants and industrial discharges.
- Intensive fishing practices: destructive techniques like blast fishing and overfishing.