CORAL REEFS

 

CORAL REEFS

Overview of Corals

• Corals are animals, not plants, and are related to jellyfish and anemones.
• They form complex marine ecosystems known as coral reefs, which are the largest biogenic reefs on Earth.
• Coral reefs are home to over 4000 species of reef fish, representing one-fourth of all fish species on Earth.
• Often called the "tropical rainforests of the sea," coral reefs are:
• Diverse and productive
• Beautiful marine ecosystems

Similarities to Tropical Rainforests

• Both thrive in nutrient-poor conditions while supporting rich communities through efficient recycling.
• High levels of species diversity.
• Coral reefs have high total (gross) productivity:
•  2500   grams of carbon per square meter per year  for coral reefs
•  2200   grams of carbon per square meter per year  for tropical forests
•  125    grams of carbon per square meter per year  in the open ocean

Structure of Coral Colonies

• Coral colonies consist of many small, jelly-like animals called polyps.
• Most corals are colonial, while some species live as solitary individuals.
• Coral reefs are sensitive to environmental changes; stressed corals lose algae, leading to bleaching (whitening).

Scientific Classification

• Kingdom: Animalia
• Phylum: Cnidaria
• Class: Anthozoa (corals and anemones)
• Subclass: Hexacorallia (hard corals)
• Order: Scleractinia (stony corals)
• Subclass: Octocorallia (soft corals)
• Order: Alcyonacea (soft corals)

Distribution of Coral Reefs

• Coral reefs are found in tropical regions near the equator.
• The central Indo-Pacific and Caribbean have the greatest marine life diversity.
• The most diverse regions include:
• Australia
• Philippines
• Indonesia
• Malaysia
• Papua New Guinea (500-600 species of corals)
• India has four major reef ecosystems (atoll, fringing, and barrier) covering about 2375km square   less than 1% of global coral reef areas.

Types of Corals

1. Non-reef builders (Ahermatypic)
• Solitary hard corals
• Lack symbiotic algae (zooxanthellae)
• Do not form reefs
2. Reef builders (Hermatypic)
• Have zooxanthellae in their body wall
• Form large colonies from connected polyps
• Use zooxanthellae for energy and skeleton building

Characteristics of Coral Polyps

• Both hard and soft coral polyps have soft bodies with a mouth and multiple tentacles.
• Hard corals have 6 tentacles; soft corals have 8 tentacles.
• Hard corals have a hard cup base, while soft corals have a soft base.

Examples of Corals

• Hard Corals:
• Staghorn coral - Acropora cervicornis
• Table coral – Acropora sp
• Brain coral
• Elkhorn coral - Acropora palmata
• Soft Corals:
• Sea fan – Gorgonia ventalina
• Toadstool coral
• Tree coral
• Sea pens – Pennatula  phosphorea

Formation of Coral Reefs

• There are three major types of reefs:

Coral Reefs

Types of Coral Reefs

1. Fringing Reefs
• Located close to the shore.
• Extend out to sea but remain in shallow waters.
2. Barrier Reefs
• Found farther offshore, typically 10-100 kilometers from the coast.
• Form massive walls of coral, separated from the coast by a large channel or lagoon.
3. Atolls
• Circular reefs surrounding a lagoon.
• Form when volcanic islands sink into the ocean over millions of years.

Feeding of Corals

• Coral polyps extend their tentacles to catch food in the water.
• They primarily feed on microscopic zooplankton and other small particles.

Reproduction in Corals

Corals can reproduce both sexually and asexually.

a) Sexual Reproduction – Broadcast Spawning

• A coral colony can start from a single polyp.
• On specific nights, many corals release eggs and sperm into the water in a mass spawning event.
• The fertilized eggs float to the surface, forming new coral larvae called planulae.

Steps:

1. Mature corals synchronize to release gamete bundles (sperm and eggs).
2. Sperm fertilizes eggs.
3. Fertilized eggs undergo cell divisions.
4. Planulae drift with currents for days to weeks, seeking a suitable substrate.
5. Larvae develop mouths and tentacles.
6. Polyps grow colonies through budding.

b) Asexual Reproduction – Budding

• Coral planulae can also reproduce asexually by budding.
• A polyp divides to create a nearly identical copy that remains attached to the parent.
• Over time, this leads to a colony of hundreds or thousands of polyps.

c) Asexual Reproduction – Fragmentation

• Portions of a colony can break off and establish new colonies.
• Success depends on favorable growth conditions; strong wave action can hinder settlement.

Factors Limiting the Distribution of Coral Reefs

Coral reefs are sensitive to environmental changes. Six major physical factors limit their development:

1. Temperature
• Ideal range: 25 – 29°C
• Growth occurs in waters between 18 – 36°C
• Development halts if the annual mean minimum temperature drops below 18°C
2. Depth
• Reefs do not develop in water deeper than50 - 70 m.
• Most grow in depths of 25 m or less.
3. Light
• Essential for the survival of symbiotic algae (zooxanthellae) in coral tissues.
• Insufficient light due to sediment can lead to coral death.
4. Salinity
• Hermatypic corals thrive in normal seawater salinity (32 - 35 ppt).
• Low salinity from freshwater influx can prevent reef development.
5. Sedimentation
• Caused by freshwater runoff, sediment can smother corals and block light.
• High turbidity reduces coral reef development.
6. Wave Action
• Coral colonies are resistant to damage from wave action due to their dense calcium carbonate skeletons.

Mining

• Live coral is removed from reefs for various uses:
• Bricks
• Road-fill
• Cement for new buildings
• Corals are also sold as souvenirs to tourists and exporters who may not be aware of the long-term damage caused by harvesting for the live rock trade.

Climate Change

• Corals cannot survive if water temperatures are too high.
• Global warming has led to increased coral bleaching, which is expected to become more frequent and severe in the coming decades.
• An integrated worldwide approach is urgently needed to prevent global warming and protect coral reefs.

Coral Diseases

• Common diseases observed in the Indian Ocean include:
1. White Band Disease
• Caused by Rickettsia bacteria and Vibrio charchari.
2. Black Band Disease
• Characterized by a black band of thickness 0.5 - cm
• Caused by cyanobacteria, fungus, sulfate-reducing bacteria, and sulfide-oxidizing bacteria.
3. Pink Spot Disease
• Features a pink band of 3 - 10  mm
• Caused by fungi and cyanobacteria.

Conservation and Management Measures

• The global community has recognized the severity of the problem and is taking steps to halt the degradation of coral ecosystems.
• The Convention on International Trade in Endangered Species of Wild Fauna and Flora (CITES) has classified many corals as threatened species.
• Establishing marine sanctuaries or reserves may help protect these ecosystems in the future.

Integrated Measures to Undertake

• Creation of awareness and environmental education.
• Providing alternative/additional livelihood options.
• Community-based reef ecosystem protection and management.
• Coral reef restoration and rehabilitation.
• Enforcement mechanisms.
• Establishing baseline data.
• Long-term reef monitoring.
• Sea ranching for stock enhancement.
• Marine biodiversity data management.

Wave Action

• Wave action provides:
• A constant source of fresh, oxygenated seawater.
• Prevention of sediment from settling on the coral colony.
• Renewal of plankton, which serves as food for the coral colony.

Exposure of Air

• Coral reefs are limited in upward growth by exposure to air.
• Mucus secretion can prevent dehydration temporarily.
• Long exposure (over an hour or two) typically kills most corals.

Nutrient Loading and Contaminant Inputs

• Corals thrive in nutrient-poor "marine deserts" due to their symbiotic relationship with zooxanthellae.
• Excess nutrients can harm corals by:
1. Direct toxicity at high concentrations.
2. Promoting phytoplankton growth, reducing water clarity and light.
3. Encouraging macroalgae growth, which competes with corals for space.
• Contaminants include:

• Heavy metals, pesticides, herbicides, solvents, and fuels.
• These can come from waste streams or sediment absorption.

Animals Associated with Coral Reefs

• Coral reefs provide habitats for diverse organisms that rely on corals for food and shelter.
• Key taxonomic groups include:
• Porifera: Sponges that inhabit coral cavities and cause bioerosion.
• Polychaeta: Worms like Hermodice carunculata that depend on corals for food.
• Mollusca: Includes giant clams (Tridacna sp), which feed on corals.
• Crustacea: Shrimps and crabs that seek shelter in corals.
• Echinodermata: Starfish like Acanthaster planci (crown of thorns) that are coral predators.
• Pisces: Various fish species that depend on corals for protection.

Importance of Corals

• Corals are crucial for:
• Ecosystem Formation: They help form islands and coastal forests.
• Coastal Protection: Reefs absorb wave forces, reducing coastal erosion.
• Food Resources: Provide about 10% of the world's fish catch.
• Carbon Sequestration: Act as a reservoir of carbon, aiding the carbon cycle.
• Medicine: Coral organisms are researched for cancer treatments and bone regeneration.
• Economic Value: Estimated annual net benefit of 29.8 billion from reef-related tourism and marine exports.

Threats to Coral Reefs

• Approximately 25% of coral reefs are damaged beyond repair, with 66% under serious threat.
• Major threats include:
1. Destructive Fishing Practices: Cyanide fishing, blast fishing, and bottom trawling.
2. Overfishing: Disrupts ecological balance and food chains.
3. Careless Tourism: Activities that damage reefs, such as touching corals and improper waste disposal.
4. Pollution: Urban waste, sewage, and agrochemicals poisoning reefs.
5. Sedimentation: Erosion from construction and farming leads to sediment smothering corals.
6. Coral Bleaching: A phenomenon exacerbated by climate change and pollution.

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