The Formation of Coral and Coral Reefs

UGC Four Quadrant E-Content

The Formation of Coral and Coral Reefs

B.Sc. Zoology (Hons.)  |  Non-Chordate Zoology / Marine Biology  |  Phylum Cnidaria — Class Anthozoa

Dr Chandralekha Deka PDUAM, Amjonga, Goalpara, Assam UG Level

1. Introduction to Coral Organisms

Corals are marine invertebrates (Phylum Cnidaria, Class Anthozoa) living in compact colonies of polyps. Called the "rainforests of the sea" for extraordinary biodiversity.

Phylum

Cnidaria

Class

Anthozoa

Skeleton

Aragonite (CaCO₃)

Habitat

Euphotic zone (0–60 m)

Stony (Scleractinian)

Possess CaCO₃ exoskeleton (aragonite). Primary reef-builders. Order Scleractinia.

Soft Corals

Lack rigid skeletons; contain calcium carbonate spicules. Do not build reefs.

Coral Polyp Anatomy

1

Oral disc

Upper surface bearing the central mouth.

2

Tentacles

In multiples of six; contain cnidocytes with nematocysts for prey capture and defence.

3

Gastrovascular cavity

Central digestive and circulatory chamber.

4

Basal disc

Lower surface anchoring polyp to substrate; where CaCO₃ is secreted.

5

Mesoglea

Gelatinous layer between epidermis and gastrodermis.

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2. Zooxanthellae Symbiosis

Mutualistic symbiosis with zooxanthellae (Symbiodiniaceae) provides ~90% of coral's energy through photosynthesis — restricting reef corals to shallow euphotic zones.

Component	Coral Provides	Zooxanthellae Provide
Nutrients	Nitrogenous waste (NH₃, NO₃⁻)	Fixed organic carbon (glucose)
Environment	Protected intracellular location	Photosynthetic products
CO₂	Metabolic CO₂	Enhanced calcification substrate

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3. Biomineralization — Skeleton Formation

Ca²⁺ + 2HCO₃⁻ → CaCO₃↓ + H₂O + CO₂

1

Ion Transport

Ca²⁺ and HCO₃⁻ actively transported into calcifying fluid (CF).

2

pH Elevation

CF maintains pH 8.2–8.5, higher than seawater (~8.1), promoting CaCO₃ precipitation.

3

Nucleation

Organic matrix proteins rich in aspartate and glycine nucleate aragonite crystals.

4

Crystal Growth

Organized aragonite arrays form skeleton at 1–10 mm linear extension per year.

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4. Reproduction & Larval Settlement

Broadcast Spawning

Synchronised mass spawning 3–6 nights after full moon; external fertilization in water column.

Brooding

Internal fertilization; larvae retained in gastrovascular cavity until competency.

Budding

New polyps from basal region; genetically identical clones for rapid colony expansion.

Planula Settlement

Ciliated larva settles after 3–14 days; metamorphosis triggered by chemical cues from coralline algae.

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5. Five Types of Coral Reefs

1. Fringing Reefs

Directly attached to shore. Most common type. Example: Red Sea coast.

2. Barrier Reefs

Separated from shore by deep lagoon; parallel to coastline. Example: Great Barrier Reef, Australia.

3. Atolls

Ring-shaped; formed on sinking volcanic islands. Example: Bikini Atoll, Marshall Islands.

4. Patch Reefs

Small, isolated; grow on continental shelf or inside atolls. Example: Florida Keys.

5. Ribbon Reefs

Long, narrow, winding; on outer continental shelf. Example: Northern Great Barrier Reef.

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6. Environmental Factors

Temperature

Optimal 23–29°C. Above 30°C causes bleaching — zooxanthellae expelled; coral starves.

Light

Euphotic zone required (<60 m depth). Photosynthesis drives 90% of energy input.

Salinity

Optimal 34–36 ppt. Freshwater influx or hypersalinity (>40 ppt) inhibits calcification.

Water Chemistry

Ocean acidification reduces CO₃²⁻, lowering calcification rates and aragonite saturation.

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7. Bioerosion & Net Reef Production

Organism	Mechanism	Rate
Parrotfish	Grazing; skeletal maceration	0.5–5 kg m⁻² yr⁻¹
Sea Urchins	Mechanical abrasion	0.1–1 kg m⁻² yr⁻¹
Boring Sponges	Chemical & mechanical dissolution	0.1–2 kg m⁻² yr⁻¹
Boring Bivalves	Mechanical boring	0.01–0.1 kg m⁻² yr⁻¹

Net Reef Production = Gross Calcification − Bioerosion

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8. Adaptive Significance & Threats

Habitat & Shelter

Home to ~25% of all marine species despite covering <1% of the ocean floor.

Coastal Protection

Natural wave barriers protecting coastlines from storm surge and erosion.

Fisheries Support

Support commercial fisheries; food security for hundreds of millions of people.

Biomedical Value

Source of novel biochemical compounds for drug discovery and cancer research.

Climate changeOcean acidificationCoral bleachingOverfishingEutrophicationBlack band diseaseWhite syndrome

Concept Map — Coral Reef Formation Pathway

Click any node for a detailed explanation of that stage.

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Reef Zonation — Depth Profile

Zone	Depth	Characteristics	Dominant Organisms
Back-reef Lagoon	0–10 m	Sheltered; sediment accumulation	Seagrass, patch reefs
Reef Flat	0–2 m	High wave energy; shallow	Branching & massive corals
Reef Crest	0–5 m	Highest wave energy; wave barrier	Acropora palmata, Porites
Fore-reef Slope	2–50 m	Max biodiversity at 10–20 m	Plate-like, branching, massive forms

Fringing vs Barrier vs Atoll

Feature	Fringing	Barrier	Atoll
Shore attachment	Direct	Lagoon-separated	Encircles lagoon
Lagoon	Absent	Deep lagoon	Central lagoon
Origin	Shore bedrock	Darwin subsidence	Volcanic subsidence
Example	Red Sea	Great Barrier Reef	Bikini Atoll

Self-Assessment Quiz

10 questions on coral biology, reef types, biomineralization, and ecology. One correct answer per question.

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Critical Thinking Challenges

1. A reef has net negative production (bioerosion > calcification). Propose three management interventions to restore positive production.

Consider: (1) Reduce fishing pressure to allow herbivore recovery (parrotfish regulate bioerosion); (2) Control nutrient runoff to reduce algal overgrowth suppressing coral recruitment; (3) Coral restoration programmes — coral gardening and transplantation to accelerate calcification recovery. Also reduce direct physical damage from anchoring and tourism.

2. Zooxanthellae are expelled in bleaching. Why does this not immediately kill the coral but becomes fatal if prolonged?

Coral can capture zooplankton with cnidocytes for short-term energy. However zooxanthellae provide ~90% of energy. Without them, coral runs severe energy deficit. After 4–8 weeks, metabolic reserves are exhausted — starvation and death follow, with the skeleton colonised by algae.

3. How does ocean acidification reduce coral calcification rates? Use the carbonate chemistry equation.

CO₂ + H₂O → H₂CO₃ → H⁺ + HCO₃⁻. This lowers pH and depletes CO₃²⁻. Aragonite saturation Ω = [Ca²⁺][CO₃²⁻]/Ksp; lower [CO₃²⁻] reduces Ω making CaCO₃ precipitation thermodynamically unfavourable. Corals expend more energy on ion pumping to maintain elevated pH in calcifying fluid — net calcification falls.

4. Compare broadcast spawning vs. brooding. Under what environmental conditions might each be favoured by natural selection?

Broadcast: vast larvae, dilution reduces predation, synchrony boosts fertilisation, wide dispersal aids genetic connectivity — favoured in stable, clear-water environments. Brooding: lower fecundity but higher larval survival and settlement success — favoured when local habitat is suitable and dispersal is less important. High-turbidity, disturbed reefs may favour brooding species.

Web Resources for Further Learning

Authoritative sources for deeper understanding of coral reef formation and marine ecology.

Smithsonian Ocean — Corals: understanding the reef-builders

ocean.si.edu

NOAA — Coral Reef Education Resources

noaa.gov

NOAA Coral Reef Watch — Global bleaching alerts & thermal stress maps

coralreefwatch.noaa.gov

AIMS — Australian Institute of Marine Science: Reef Monitoring

aims.gov.au

ICRI — International Coral Reef Initiative

icriforum.org

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Recommended Textbooks

Invertebrate Zoology — R.D. Barnes (1987)

Saunders College Publishing. Comprehensive Cnidaria biology and coral ecology.

Invertebrate Zoology: A Functional Evolutionary Approach — Ruppert, Fox & Barnes (2004)

Brooks/Cole. Functional anatomy of cnidarians; coral reproduction and skeletal biology.

Marine Biology: An Ecological Approach — J.W. Nybakken (1993)

Harper Collins. Coral reef ecology, zonation, and trophic dynamics.

Corals of the World — J.E.N. Veron (2000)

Australian Institute of Marine Science. Definitive illustrated guide to coral taxonomy.

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Key Glossary

Aragonite

Orthorhombic polymorph of CaCO₃; primary component of coral skeleton.

Bleaching

Loss of zooxanthellae; coral turns white; fatal if prolonged beyond 4–8 weeks.

Calcification

Biological deposition of CaCO₃ to form skeletal structures; rate 1–10 mm/year.

Cnidocyte

Specialized cell with nematocyst; used for prey capture and defence.

Euphotic Zone

Water layer allowing photosynthesis; typically 0–200 m; reef corals restricted to 0–60 m.

Holobiont

Coral + zooxanthellae + microbiota as an integrated functional unit.

Mesoglea

Gelatinous layer between epidermis and gastrodermis in cnidarians.

Planula

Ciliated larva of cnidarians; settles after 3–14 days of planktonic life.

Scleractinia

Order of stony corals; primary reef-builders with CaCO₃ exoskeletons.

Zooxanthellae

Symbiodiniaceae dinoflagellates in mutualistic symbiosis with corals.

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Learning Outcomes Checklist

Define corals; state taxonomic classification (Phylum Cnidaria, Class Anthozoa, Order Scleractinia).

Describe all five components of coral polyp anatomy.

Explain zooxanthellae symbiosis and its disruption during bleaching.

Describe the 4-step biomineralization process and write the calcification equation.

Compare asexual (budding, fragmentation) and sexual (broadcast spawning, brooding) reproduction.

Distinguish all 5 reef types (fringing, barrier, atoll, patch, ribbon) with global examples.

Evaluate six environmental factors: temperature, light, salinity, chemistry, sedimentation, wave energy.

Discuss ecological services and major threats to coral reef ecosystems.

Author

Dr Chandralekha Deka

Institution

PDUAM, Amjonga, Goalpara

E-Content as per UGC Four Quadrant Model  |  Non-Chordate Zoology — Coral Reef Formation  |  zoologys.co.in