What is Coral? Understanding the Foundation of Ocean Ecosystems

Coral reefs represent one of our planet’s most magnificent and vital ecosystems, supporting approximately 25% of all marine life despite covering less than 1% of the ocean floor. These living structures—built by small but mighty coral organisms—face unprecedented threats from climate change, pollution, and human activity, yet they continue to inspire conservation efforts worldwide.

Key Takeaways

• Coral polyps are tiny colonial organisms that secrete calcium carbonate skeletons, creating the massive reef structures we recognize.
• Coral reefs support incredible biodiversity, provide coastal protection, and contribute billions to the global economy through tourism, fisheries, and medical discoveries.
• Climate change is causing widespread coral bleaching, where corals expel their symbiotic algae and lose their primary food source.
• Innovative restoration approaches, including land-based coral farming, are creating climate-resilient corals that can survive in warming oceans.
• Conservation efforts, from reducing carbon emissions to establishing marine protected areas, are essential for the long-term survival of coral ecosystems.

What is Coral

Contrary to common assumption, coral is not a plant but an animal—specifically, a colonial organism composed of thousands of genetically identical polyps. Each coral polyp is a tiny, soft-bodied creature related to sea anemones and jellyfish in the phylum Cnidaria. These remarkable organisms have existed for over 500 million years, adapting and evolving to create the magnificent underwater structures we know today[1]¹.

The basic coral structure consists of a cylindrical body with a central mouth surrounded by stinging tentacles. These stinging cells, called nematocysts, allow coral polyps to capture food particles and microscopic organisms floating in the water. Most corals are sessile, meaning they permanently attach to the ocean floor and do not move, instead relying on their tentacles to bring food to them.

What truly sets corals apart is their ability to secrete calcium carbonate skeletons, which form the physical foundation of coral reefs. As generations of coral polyps live and die, their skeletons remain, creating massive reef structures that serve as habitat for countless marine species[2]². These corallites also provide the framework that helps reefs dissipate wave energy, protecting coastlines from erosion and storm damage.

The extraordinary diversity of coral varieties results in a vast array of growth forms, from branching and table corals to massive boulder-like structures, delicate fan shapes, and intricate brain-like formations. This diversity allows corals to occupy various ecological niches within reef ecosystems, contributing to the overall complexity and resilience of these underwater habitats.

Coral Reefs

Coral reefs are among the most diverse ecosystems on Earth, often called the “rainforests of the sea” due to their incredible biodiversity. These underwater habitats form in shallow, warm waters primarily in tropical and subtropical regions around the world. While coral reefs occupy less than 1% of the ocean floor, they provide a home for approximately 25% of all marine species[3]³.

Reef ecosystems develop through the accumulation of corallites over thousands of years. There are three main types of coral reef formations: fringing reefs that grow near coastlines, barrier reefs that are separated from land by lagoons, and atolls that form circular or oval-shaped reef structures around a central lagoon. Each type represents a different stage in the long-term evolution of reef systems, often associated with geological processes such as island subsidence.

The Great Barrier Reef, located off the coast of Australia, stands as the world’s largest coral reef system, spanning over 1,400 miles and visible from space. This UNESCO World Heritage site contains over 2,900 individual reefs and supports more than 1,500 species of fish, 4,000 types of mollusk, and countless other marine creatures[4]⁴. Its sheer size and complexity make it one of the most remarkable natural structures on our planet.

Beyond their ecological importance, coral reefs provide essential services to human communities worldwide. They support fisheries that feed millions of people, protect coastlines from erosion and storm damage, and generate billions of dollars through tourism. Additionally, compounds derived from coral reef organisms have led to important medical discoveries, including treatments for cancer, arthritis, bacterial infections, and other diseases. The economic value of coral reefs globally is estimated at $375 billion per year, highlighting their immense importance to human societies.

Coral Bleaching

One of the most visible and devastating impacts of climate change on coral reefs is coral bleaching. This phenomenon occurs when corals, under environmental stress, expel the symbiotic algae called zooxanthellae that live within their tissues. Since these algae provide corals with up to 90% of their energy and give them their vibrant colors, their loss leaves the coral’s white skeleton visible through transparent tissue—hence the term “bleaching.”

The primary driver of mass coral bleaching events is elevated ocean temperatures. When water temperatures rise just 1-2°C (1.8-3.6°F) above the maximum summer average for several weeks, the symbiotic relationship between corals and their algae breaks down[5]⁵. This temperature threshold varies by region and coral organisms, but the general trend is clear: as our oceans warm due to climate change, bleaching events become more frequent and severe.

Ocean acidification, resulting as oceans absorb carbon dioxide from the atmosphere, further stresses coral reefs by making it more difficult for corals to build their calcite skeletons. This double impact of warming and acidification represents an existential threat to coral reef ecosystems worldwide.

While bleached corals are not dead and can recover if conditions improve quickly enough, prolonged or repeated bleaching events can lead to mass coral mortality. The frequency and severity of bleaching events have increased dramatically in recent decades. The Great Barrier Reef, for example, experienced major bleaching events in 1998, 2002, 2016, 2017, 2020, and 2022—a stark contrast to the historical pattern of one bleaching event every few decades[6]⁶.

Beyond temperature stress, other factors contributing to coral bleaching include pollution, excessive sunlight, exposure to air during extreme low tides, and changes in salinity. The combined effect of these stressors is pushing many reef systems toward ecological tipping points, beyond which recovery becomes increasingly difficult.

Coral Species

The world’s oceans contain remarkable coral diversity, with over 800 species of reef-building (hermatypic) corals and hundreds more non-reef-building species. This incredible variety ranges from massive boulder corals to delicate branching corals, fast-growing table corals to slow-growing brain corals. Each species has evolved unique adaptations to thrive in specific ecological niches within the reef ecosystem.

Coral organisms generally fall into two main categories:

Soft Corals

Soft corals lack the rigid calcite skeletons of hard corals. Instead, they contain tiny skeletal elements called sclerites that provide support and protection. This group includes:

• Sea fans (Gorgoniidae): Fan-shaped colonies that position themselves perpendicular to water currents to capture food
• Sea whips: Long, slender colonies that sway with ocean currents
• Sea feathers: Feather-like colonies with a central stem and numerous branches
• Leather corals: Fleshy, lobe-shaped colonies with a leathery texture

While soft corals don’t contribute significantly to reef building, they add tremendous diversity and complexity to reef ecosystems. Many soft coral varieties are more tolerant of turbid waters and can thrive in environments where hard corals struggle. Their flexible structures allow them to bend with water currents rather than resist them, enabling them to colonize areas with stronger water flow.

Soft corals often contain chemical compounds that deter predators, making them less vulnerable to grazing fish and other reef inhabitants. These chemical defenses have attracted significant interest from medical researchers seeking new therapeutic compounds.

Hard Corals

Hard corals, also known as stony corals, are the primary reef builders. These corals secrete calcite skeletons that form the foundation of coral reef ecosystems. Some of the most common hard coral families include:

• Acroporidae: Fast-growing branching and table corals, including the iconic staghorn and elkhorn species
• Poritidae: Massive, boulder-shaped corals that can live for centuries
• Merulinidae: Includes brain corals and many other important reef-building species
• Fungiidae: Solitary mushroom corals that, unlike most corals, are not colonial organisms

Hard corals typically have small polyps with six tentacles or multiples of six. These reef-building corals contain endosymbiotic algae and require clear, sunlit waters to thrive, as their algal partners need sunlight for photosynthesis[7]⁷.

The diversity of coral organisms on a reef contributes to its resilience—different species respond differently to environmental stressors, meaning that a diverse reef has a better chance of surviving challenging conditions. This biodiversity also supports a wider range of fish and invertebrate species, creating a more robust and productive ecosystem overall.

Form Coral Reefs

The process of reef formation is a remarkable example of how small organisms can create massive geological structures over time. Reef-building corals form reefs through a continuous cycle of growth, death, and new growth, gradually transforming underwater landscapes over centuries and millennia.

The reef-building process begins when free-swimming coral larvae, called planulae, attach to hard surfaces on the ocean floor. Once settled, these larvae transform into polyps and begin to secrete calcite skeletons. As polyps divide and multiply, they form coral colonies of genetically identical polyps that continue to grow and expand[8]⁸.

The calcite skeletons secreted by coral polyps provide the structural foundation for reef development. Over time, as generations of corals build upon the skeletons of their predecessors, these structures can grow into massive reef systems. The Great Barrier Reef, for example, has developed over approximately 500,000 years, with some individual reefs estimated to be 6,000-8,000 years old.

Different types of coral reefs form based on their relationship to land masses:

1. Fringing reefs grow directly from the shoreline of islands or continents, with little to no separation from land.
2. Barrier reefs run parallel to shorelines but are separated from land by lagoons, which can be quite deep and wide.
3. Atolls are circular or oval-shaped reefs that surround a central lagoon, typically forming when islands with fringing reefs sink beneath the ocean surface over time, while the coral continues to grow upward.

Coral reef formation is not solely the work of corals themselves. Coralline algae, which also deposit calcium carbonate, help cement coral fragments together, strengthening the reef structure. Additionally, other organisms like mollusks, foraminifera, and certain types of algae contribute calcium carbonate to the reef framework.

The rate of reef growth varies considerably depending on coral species, environmental conditions, and geographic location. Under optimal conditions, fast-growing branching corals can extend by 10-20 centimeters per year, while massive boulder corals might grow only 0.3-2 centimeters annually[9]⁹.

Hard Corals

Hard corals, also known as stony corals or scleractinian corals, form the foundation of coral reef ecosystems through their production of calcite skeletons. These skeletal structures accumulate over generations, creating the complex three-dimensional habitats that support the incredible biodiversity of coral reefs.

Most hard corals maintain a symbiotic relationship with microscopic algae called zooxanthellae, which live within the coral’s tissue. Through photosynthesis, these algae convert sunlight and carbon dioxide into organic compounds, providing up to 90% of the coral’s energy needs. In return, the coral provides protection and access to sunlight for the algae. This mutually beneficial relationship explains why reef-building corals are primarily found in clear, shallow waters where sunlight can penetrate.

Hard corals grow in various morphologies or shapes, each adapted to specific environmental conditions:

• Branching corals create complex habitats with numerous branches that provide refuge for small fish and invertebrates
• Massive corals form large, boulder-like structures that can withstand strong wave action
• Plate or table corals grow horizontally to maximize sunlight exposure
• Encrusting corals spread across the substrate in thin layers
• Columnar corals develop vertical, column-like structures
• Brain corals feature winding, maze-like patterns that resemble brain tissue

The diversity of growth forms allows different coral species to occupy various ecological niches within the reef. For example, branching corals often dominate in protected areas with moderate water flow, while massive corals tend to thrive in areas exposed to stronger wave action.

Hard corals reproduce through both sexual and asexual methods. Sexual reproduction occurs primarily through mass spawning events, where multiple coral species release eggs and sperm into the water column simultaneously. These spectacular events, often timed with lunar phases, create a soup of reproductive material that facilitates genetic diversity. Asexual reproduction occurs through fragmentation, where broken coral pieces can reattach and grow into new colonies, or through budding, where new polyps develop from existing ones[10]¹⁰.

Coral Reefs Found

Coral reefs are found throughout tropical and subtropical oceans, generally between 30° North and 30° South latitudes. These ecosystems require specific environmental conditions to thrive, including:

• Water temperatures between 23-29°C (73-84°F)
• Clear, shallow waters allowing sunlight penetration
• Good water circulation
• Stable salinity levels
• Low nutrient concentrations

The global distribution of coral reefs is not uniform but concentrated in certain regions:

The Indo-Pacific region, particularly the area known as the Coral Triangle (encompassing Indonesia, Malaysia, the Philippines, Papua New Guinea, the Solomon Islands, and Timor-Leste), contains the highest coral diversity on Earth. This region, often called the “Amazon of the seas,” hosts over 600 coral species and thousands of fish species[11]¹¹.

The Great Barrier Reef off Australia’s northeastern coast represents the world’s largest coral reef system, spanning approximately 2,300 kilometers (1,400 miles) and comprising nearly 3,000 individual reefs and 900 islands.

The Caribbean Sea supports extensive reef development, though with lower species diversity than Indo-Pacific reefs. Important reef systems in this region include the Mesoamerican Barrier Reef System (the second-largest barrier reef in the world), the Bahamas, and the reefs surrounding Cuba.

The Red Sea contains some of the northernmost coral reefs, with over 200 coral species adapted to higher salinity and temperature fluctuations than corals in other regions.

The western Indian Ocean, including the reefs of Madagascar, the Seychelles, the Maldives, and along the East African coast, supports diverse coral communities despite facing significant threats from climate change and human activities.

Beyond these major regions, isolated reefs exist in the Eastern Pacific around the Galápagos Islands, Hawaii, and along parts of the Central American coast, though these tend to have lower coral diversity due to their isolation.

Marine protected areas (MPAs) play a crucial role in preserving coral reef ecosystems around the world. Currently, about 27% of coral reefs fall within MPAs, though the level of protection varies considerably[12]¹².

Coral Eat

Despite their plant-like appearance, corals are animals that must capture food to survive. Their feeding methods vary between different coral types, with many species utilizing multiple nutritional strategies to obtain the energy and nutrients they need.

The primary feeding mechanism for most coral polyps involves extending their tentacles to capture zooplankton and other small organisms from the surrounding water. These tentacles contain specialized stinging cells called nematocysts, which immobilize prey upon contact. Once captured, the prey is transferred to the polyp’s mouth and digested in the gastrovascular cavity. This direct feeding method provides coral with essential nutrients, particularly nitrogen and phosphorus, which may be limited in their symbiotic algal partners.

For reef-building corals that host endosymbiotic algae (zooxanthellae), photosynthesis represents a critical nutritional pathway. These microscopic algae living within coral tissues convert sunlight into organic compounds, providing their coral hosts with up to 90% of their energy requirements. What the coral consumes directly through its tentacles supplements this photosynthetically derived nutrition.

Beyond these primary methods, corals employ several other feeding strategies:

• Mucus nets: Many corals secrete mucus that traps organic particles, bacteria, and plankton
• Absorption: Corals can directly absorb dissolved organic matter from seawater
• Extracellular digestion: Some corals release digestive enzymes into the surrounding water

Different coral species have evolved specialized feeding adaptations based on their environmental conditions. For example, deep-water corals that live beyond the reach of sunlight rely entirely on capturing food particles, as they lack endosymbiotic algae. These corals often have more pronounced feeding structures and may position themselves to maximize contact with nutrient-rich currents.

Coral feeding activity typically increases at night when many zooplankton species migrate toward the surface and when there’s less predation risk for the extended coral polyps. During these periods, reef visitors can observe the transformation as normally unobtrusive corals extend their colorful tentacles to feed[13]¹³.

Coral Mining

Coral mining represents one of the most direct human threats to coral reef ecosystems worldwide. This destructive practice involves the removal of live coral colonies or coral limestone from reefs for various commercial purposes. The impact of coral mining extends far beyond the immediate physical damage, creating cascading effects that can permanently alter reef ecosystems.

Historically, coral extraction has been conducted for several purposes:

• Construction material: Coral limestone has been used as building material in many coastal communities due to its availability and structural properties.
• Calcite production: Corals are processed to extract calcite for use in cement, road construction, and as soil neutralizers in agriculture.
• Coral collection: Live corals are harvested for the aquarium trade, souvenir production, and jewelry making.

The immediate consequences of coral mining are devastating. The physical removal of coral structures destroys habitats that have taken centuries or millennia to develop. When mining operations break apart reef structures, they kill coral polyps directly and create vast fields of coral rubble that are unsuitable for new coral settlement. The resulting sedimentation smothers nearby corals and blocks sunlight required by their endosymbiotic algae.

The long-term ecological impacts extend throughout the reef ecosystem. The destruction of coral habitats leads to decreased fish populations, affecting both reef biodiversity and the livelihoods of local fishing communities. Coastal protection is compromised as damaged reefs lose their ability to buffer shorelines from wave energy, leading to increased coastal erosion and vulnerability to storm damage[14]¹⁴.

In recognition of these severe impacts, many countries have implemented regulations against coral mining. International agreements like the Convention on International Trade in Endangered Species of Wild Fauna and Flora (CITES) restrict the trade of coral species. However, enforcement remains challenging in remote areas and developing nations where economic pressures drive continued extraction.

Sea Fans

Sea fans, with their elegant, fan-shaped colonies swaying gently in ocean currents, represent some of the ocean’s most visually striking coral relatives. These organisms belong to the subclass Octocorallia (also called Alcyonacea), a group of cnidarians characterized by polyps with eight tentacles.

Unlike the hard, reef-building corals that create calcite skeletons, sea fans feature a flexible internal skeleton composed of a protein called gorgonin, reinforced with embedded calcite spicules. This unique structure allows sea fans to bend and sway with water movements rather than resisting them, an adaptation that helps these colonies thrive in areas with strong currents.

The distinctive fan shape of these organisms isn’t merely aesthetic—it’s a highly effective adaptation for filter feeding. By growing perpendicular to prevailing water currents, sea fans maximize their exposure to flowing water, allowing their polyps to capture passing plankton and organic particles efficiently. Some species can grow to impressive dimensions, reaching heights of over 2 meters (6.5 feet) in favorable conditions.

Sea fans display remarkable biodiversity, with hundreds of species exhibiting various colors ranging from purple and red to yellow, orange, and brown. These colors derive from both the pigmentation of the sea fan itself and the presence of endosymbiotic algae living within some species’ tissues. The red coral (Corallium rubrum), prized for centuries in jewelry making, is a well-known member of this group.

Beyond their visual appeal, sea fans serve crucial ecological functions within marine ecosystems. They provide complex habitat structures that offer shelter to numerous small invertebrates and juvenile fish. Some specialized fish and invertebrate species have evolved to mimic sea fan branches or colors as camouflage against predators[15]¹⁵.

Great Barrier Reef

The Great Barrier Reef stands as one of our planet’s most extraordinary natural wonders—a living structure so vast it’s visible from space and the only living organism that can make that claim. Stretching approximately 2,300 kilometers (1,400 miles) along Australia’s northeastern coast, this UNESCO World Heritage site comprises nearly 3,000 individual reefs and 900 islands, covering an area of about 344,000 square kilometers (133,000 square miles)[16]¹⁶.

This remarkable ecosystem began forming approximately 500,000 years ago, with the current reef structure developing after the last ice age, about 8,000 years ago. The reef’s rich biodiversity supports:

• Over 600 types of hard and soft corals
• More than 1,600 species of fish
• 133 varieties of sharks and rays
• 30 species of whales and dolphins
• 6 of the world’s 7 species of threatened marine turtles
• 3,000 varieties of mollusks
• 500 species of seaweed

Beyond its ecological significance, the Great Barrier Reef generates approximately AU$6.4 billion annually for the Australian economy through tourism, fishing, and other activities, supporting nearly 64,000 jobs.

Despite its size and resilience, the Great Barrier Reef faces unprecedented challenges. Climate change represents the most serious threat, with rising ocean temperatures triggering mass coral bleaching events in 1998, 2002, 2016, 2017, 2020, and 2022. The 2016 and 2017 back-to-back bleaching events were particularly devastating, affecting approximately two-thirds of the reef’s corals.

Ocean acidification, resulting from increased carbon dioxide absorption, reduces coral’s ability to build calcite skeletons. Additional pressures include agricultural runoff delivering sediments, nutrients, and pesticides to reef waters; crown-of-thorns starfish outbreaks that consume coral tissue; and coastal development impacting water quality.

In response to these threats, the Australian government has implemented the Reef 2050 Long-Term Sustainability Plan, a comprehensive strategy for protecting and managing the Great Barrier Reef through 2050[17]¹⁷.

Conclusion

Coral reefs stand at a critical crossroads, facing unprecedented threats while simultaneously inspiring innovative solutions. These living structures—built by tiny coral polyps over thousands of years—represent some of Earth’s oldest and most complex ecosystems, supporting biodiversity, protecting coastlines, and sustaining millions of people worldwide through fisheries, tourism, and ecosystem services.

The challenges facing coral reefs are daunting. Climate change drives mass bleaching events with increasing frequency and severity. Ocean acidification weakens coral skeletons. Pollution, overfishing, and direct physical damage compound these stresses. Scientists estimate that without significant intervention, we could lose up to 90% of the world’s coral reefs by 2050.

Yet amid these sobering projections, hope remains. Breakthrough approaches to coral restoration, including land-based coral farming with enhanced climate resilience, are demonstrating that we can actively rebuild reef ecosystems. Marine protected areas, when properly managed, show that corals can recover when given the chance. Communities worldwide are recognizing the value of healthy reefs and taking action to protect them.

The story of coral is ultimately a human story—one where our actions directly determine the fate of these ancient, living structures. By understanding what coral is, how it functions, and why it matters, we take the first step toward ensuring that coral reefs continue to thrive for generations to come. The solutions exist, from reducing carbon emissions to supporting restoration efforts, and the time to implement them is now.

About Coral Vita

Coral Vita is a mission-driven company dedicated to restoring our world’s dying and damaged reefs. Using innovative land-based farming techniques, Coral Vita grows diverse and resilient corals in months instead of the decades they take in nature. These corals are then transplanted into threatened reefs, helping to preserve ocean biodiversity while protecting coastal communities that depend on healthy reefs for protection, food, and income].

Founded by environmental entrepreneurs Sam Teicher and Gator Halpern, Coral Vita’s high-tech coral farms incorporate breakthrough methods to restore reefs in the most effective way possible. In 2021, the company was recognized as the inaugural winner of Prince’s William’s Revive Our Oceans Earthshot Prize Winner for their pioneering work in coral restoration.

To learn more about Coral Vita’s work or to get involved in coral reef conservation efforts, visit their website at www.coralvita.co or contact them directly through their Contact Us page.

Frequently Asked Questions

References

1. https://oceanservice.noaa.gov/education/tutorial_corals/coral01_intro.html ↩︎
2. https://coral.org/en/coral-reefs-101/coral-reef-ecology/coral-polyps/ ↩︎
3. https://www.iucn.org/resources/issues-briefs/coral-reefs-and-climate-change ↩︎
4. https://www.gbrmpa.gov.au/the-reef/about-the-reef ↩︎
5. https://www.noaa.gov/education/resource-collections/marine-life/coral-reef-ecosystems ↩︎
6. https://www.aims.gov.au/research-topics/coral-reefs/coral-bleaching ↩︎
7. https://www.coraldigest.org/index.php/coral-species/ ↩︎
8. https://oceanservice.noaa.gov/education/tutorial_corals/coral04_reefs.html ↩︎
9. https://coralreef.noaa.gov/education/coralfacts.html ↩︎
10. https://oceanservice.noaa.gov/education/tutorial_corals/coral05_reproduction.html ↩︎
11. https://www.worldwildlife.org/places/coral-triangle ↩︎
12. https://www.unep.org/news-and-stories/story/coral-reefs-we-continue-take-more-they-can-give ↩︎
13. https://oceanservice.noaa.gov/facts/coral_feed.html ↩︎
14. https://www.sciencedirect.com/science/article/abs/pii/S0025326X99000971 ↩︎
15. https://sanctuaries.noaa.gov/news/feb17/sea-fans-forests-of-the-coral-reef.html ↩︎
16. https://www.gbrmpa.gov.au/the-reef/reef-facts ↩︎
17. https://www.science.org/content/article/great-barrier-reef-hit-another-major-bleaching-event ↩︎