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Coral: Characteristics, Diet, Facts & More [Fact Sheet]

Coral is not just an underwater ornament but a vital part of the marine ecosystem. These living organisms, often mistaken as rocks or plants, are actually colonies of tiny animals known as polyps.

Coral reefs, formed by the accumulation of coral over thousands of years, are among the most diverse and valuable ecosystems on Earth.

In this article, we will dive into the fascinating world of coral, exploring their unique biology, diverse species, and the critical role they play in the ocean’s health.

Coral at a Glance


Phylum:Cnidaria (Cnidarians)
Order:Various (e.g., Scleractinia for hard corals)
Family:Various (e.g., Acroporidae, Faviidae)
Genus:Various (e.g., Acropora, Montipora)
Species:Various (e.g., Acropora millepora, Montipora digitata)

Essential Information

Average Size:Varies greatly; from a few millimeters to several meters in diameter
Average Weight:Depends on the size and type of coral
Average Lifespan:20-50 years; some can live for over a century
Geographical Range:Mostly in shallow tropical and subtropical oceans
Conservation Status:Ranges from Least Concern to Critically Endangered

Species and Subspecies

Corals are broadly categorized into two main types: hard (Scleractinian) and soft corals (Alcyonacea). Key differences include:

  • Hard Corals (Scleractinia):
    • Build the backbone of coral reefs.
    • Secretes a calcium carbonate skeleton.
    • Includes species like Staghorn Coral (Acropora cervicornis) and Brain Coral (Diploria labyrinthiformis).
  • Soft Corals (Alcyonacea):
    • Do not produce a rigid calcium carbonate skeleton.
    • Includes species like Sea Fans (Gorgonia) and Sea Whips (Leptogorgia).

Each species has its unique structure, color, and ecological role. The diversity of coral species is crucial for the health and resilience of coral reef ecosystems.



Corals are incredibly diverse in their physical appearance. Hard corals typically exhibit rigid, calcium carbonate exoskeletons that form various structures, such as branching, plate-like, or mound shapes.

Soft corals, on the other hand, have flexible, often tree-like forms. The colors of corals range from vibrant reds, yellows, and blues to more subdued browns and greens, depending on the species and the zooxanthellae (symbiotic algae) present in their tissues.

Each coral polyp, the basic unit of a coral colony, is a small, anemone-like animal with a central mouth surrounded by a ring of tentacles.

These tentacles contain stinging cells called nematocysts, used for capturing prey. Hard corals have a calcium carbonate base, while soft corals are supported by tiny, spiny skeletal elements called sclerites, which give them a more flexible structure.

Corals do not exhibit sexual dimorphism in the traditional sense, as they are sessile animals. However, coral colonies can reproduce sexually or asexually, depending on the species and environmental conditions.

Habitat and Distribution

Corals are predominantly found in shallow, tropical, and subtropical waters around the world. The most extensive coral reefs are in the Indo-Pacific region and the Caribbean Sea.

Coral habitats vary widely:

  • Hard corals are typically found in shallow, clear, and sunlit waters where they form coral reefs.
  • Soft corals are found in a range of habitats, from shallow to deep waters, often in areas with strong currents.
Gorgonian coral
Gorgonian coral


Corals are sessile organisms, meaning they are permanently attached to the ocean floor. Their behavior revolves around feeding, reproducing, and maintaining their symbiotic relationship with zooxanthellae, which provide nutrients to the coral through photosynthesis.

Corals exist in colonies that can range from a few individuals to thousands, forming large structures like reefs that provide habitats for a multitude of marine species. Most corals feed at night, extending their tentacles to capture plankton and small organisms.

Corals do not communicate in the conventional sense. However, they can release chemical signals into the water, which can influence the behavior of other nearby coral colonies, particularly during spawning events.

Hard corals play a crucial role in building and maintaining the structure of coral reefs, which are essential habitats for diverse marine life.

Diet and Feeding Behavior

Corals have a mixed diet:

  • Zooxanthellae Symbiosis: Most of their energy and nutrient requirements are met through a symbiotic relationship with zooxanthellae, microscopic algae that live within the coral’s tissues. These algae perform photosynthesis, converting sunlight into energy that the coral uses.
  • Plankton and Small Organisms: Corals also capture small marine organisms like plankton. At night, corals extend their tentacles to catch these tiny creatures, using their nematocysts to stun or kill the prey.

Corals are passive hunters. Their feeding behavior involves waiting for plankton and other small organisms to drift by, at which point they use their stinging tentacles to capture and consume them.


Corals face predation from a variety of marine organisms:

  • Parrotfish and Butterflyfish: These fish species feed on coral polyps.
  • Crown-of-Thorns Starfish: This starfish is a well-known predator of corals, capable of devastating large sections of reef.
  • Snails and Worms: Certain species of snails and marine worms also prey on coral polyps.

Reproduction and Life Cycle

Corals reproduce in two main ways:

  • Sexual Reproduction: Many corals engage in a synchronized spawning event, where they release eggs and sperm into the water column simultaneously. This increases the chance of fertilization.
  • Asexual Reproduction: Corals can also reproduce asexually through budding, where new polyps bud off from parent polyps to form new colonies.

In sexual reproduction, once the eggs are fertilized, they develop into free-swimming larvae called planulae, which eventually settle on a suitable substrate and develop into new polyps.

The number of offspring produced can be in the millions for large coral colonies. There is no parental care; once the planulae settle, they must fend for themselves. The survival rate of these larvae is low, but those that do survive can grow and develop into new coral colonies.

Conservation and Threats

The conservation status of coral varies widely among species. Some are listed as “Critically Endangered” or “Endangered” on the IUCN Red List due to their declining populations and vulnerability to environmental threats.

Corals face numerous threats, including:

  • Climate Change: Rising sea temperatures can lead to coral bleaching, where corals expel the zooxanthellae, losing their color and main source of energy.
  • Ocean Acidification: Increased levels of CO2 in the atmosphere lower the pH of ocean water, affecting the corals’ ability to produce their calcium carbonate skeletons.
  • Pollution: Pollution from land, including agricultural runoff, chemicals, and plastics, can damage coral reefs.
  • Overfishing and Destructive Fishing Practices: These can disrupt the ecological balance of reef systems.
  • Physical Damage: From activities like anchoring, coastal development, and irresponsible tourism.

Efforts to conserve and protect coral reefs include:

  • Establishing marine protected areas.
  • Regulating fishing practices.
  • Reducing carbon emissions to mitigate climate change.
  • Restoring damaged reefs through coral gardening and other restoration projects.
  • Raising public awareness about the importance of coral reefs.

Fun Facts

  1. Ancient Architects: Some coral reefs, like the Great Barrier Reef, are so large they can be seen from space and are thousands of years old.
  2. Hotspot of Biodiversity: Coral reefs support an estimated 25% of all marine species despite covering less than 1% of the ocean floor.
  3. Coral Colors: The vibrant colors of coral come from the zooxanthellae living within their tissues.
  4. Sensitivity to Light: Corals can sense changes in light intensity and adjust their symbiotic algae accordingly.
  5. Chemical Warfare: Corals can engage in chemical warfare, releasing toxins to ward off competitors for space.

Frequently Asked Questions

Can coral reefs recover from bleaching?

Yes, corals can recover from bleaching if the stressors are removed and conditions improve, but repeated bleaching events can lead to permanent damage or death.

How can I help protect coral reefs?

You can help by reducing your carbon footprint, using reef-safe sunscreens, supporting sustainable fishing practices, and spreading awareness about the importance of coral conservation.

Are all corals hard and stony?

No, there are also soft corals, which do not have a rigid calcium carbonate skeleton like hard corals.

How do corals grow?

Corals grow through both asexual reproduction (budding off new polyps) and sexual reproduction (spawning), adding new layers to their calcium carbonate structure.

Why are coral reefs important?

Coral reefs are crucial for marine biodiversity, protect coastlines from erosion, support fisheries and tourism industries, and are a source of new medicines.

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