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Coral reefs are the ocean’s most diverse and complex ecosystems, supporting 25% of all marine life, including 800 species of reef-building corals and more than one million animal and plant species. They are close relatives of sea anemones and jellyfish, as each coral is a colony consisting of many individual sea anemone-like polyps that are all interconnected.

Tropical coral reefs, found in warm, clear water at relatively shallow depths, are intricately patterned carpets of life growing on foundations formed primarily by calcium carbonate exoskeletons and coralline algae. These structures fuse over time, enlarging the reef and creating countless nooks and crannies. As the reef grows, species from nearly every major taxonomic group cover every square inch of these tightly integrated systems, providing food and shelter to a spectacular variety of fish and invertebrate species, including many of commercial value.

‘Hard’ corals use calcium carbonate from seawater to synthesize a hard, mineral protective shell around each polyp. These exoskeletons, along with shells formed by coralline algae, mollusks and tubeworms, spicules made by sponges, and shells of other calcifying species form the structural foundation of coral reefs. Corals catch plankton with their tentacles, but most of their nutrition comes from photosynthetic algae that live in their tissues, using the coral’s waste products for their own nutrition and feeding the corals with sugars and other nutritious compounds that leak through their cell membranes.

Deep water reefs, formed by large, long-lived but fragile, soft corals are also architecturally and ecologically complex and teem with life, but lack a calcium carbonate foundation. Though beyond the reach of sunlight, underwater lights reveal them to be nearly as beautiful and colorful as their tropical counterparts.

The condition of coral reefs is important to the Ocean Health Index because healthy reefs provide many benefits to people, including food, natural products, coastal protection from storms, jobs and revenue, tourism and recreation, biodiversity and others.

60% of reefs are already seriously damaged by local sources such as overfishing, destructive fishing, anchor damage, coral bleaching, coral mining, sedimentation, pollution, and disease. When these types of human threats are combined with the influence of rising ocean temperatures, 75% of reefs are threatened (Burke et al. 2011).

How Was It Measured?

The extent of coral reefs was derived from the 500m resolution dataset developed for Reefs at Risk Revisited (Burke et al. 2011), in conjunction with a re-sampled version of the Ocean Health Index EEZ regions. The condition of reefs was estimated using data for percentage cover by live coral determined from 12,634 surveys conducted from 1975-2006 and summarized by Bruno and Selig (2007) and Schutte et al. (2010).

The reference point for coral reefs is the percent cover of coral reefs in 1975. The current Status is reported as: Current precent cover of coral reefs ÷ percent cover of coral reefs in 1975.

Like all of the habitats used in the OHI, coral reefs are used as a component in calculating scores for many of the different goals. However, it is used differently depending on the goal in question. Although habitats such as coral reefs are used in calculating these goal scores, countries are not penalized for not having a certain habitat type. Calculations are based on the rank of existing habitats, as opposed to using all possible habitat types. The condition and extent of coral reefs can either be used as a direct measure, or indirectly in a supporting capacity.

Coral reefs is used directly as a component in calculating Coastal Protection (Corals) and Biodiversity (Habitat: Coral). For these goals, the extent and condition of coral habitat factors directly into score calculations. Coral is also used as a component for the Natural Products goal, but is measured in a different way. 

Coral reefs is also used indirectly in calculating scores for Artisanal Fishing, Natural Products (Ornamental Fish), and Livelihoods and Economies (Aquarium Trade). For example, Artisanal Fishing: High Bycatch was assessed by looking at the presence of blast and poison fishing, practices that both degrade coral reefs.

Major Sources Of Damage To Coral Reefs

What Are The Impacts?


Corals that are exposed to elevated sea surface temperatures expel the symbiotic photosynthetic algae responsible for their nutrition and coloration [zooxanthellae] in a process known as coral bleaching. Corals can recover from occasional bleaching, but not from repeated bleaching. Increases in sea-surface temperature of about 1-3 °C are projected to result in more frequent coral bleaching events and widespread mortality.

Elevated sea surface temperatures cause increased damage to reefs from breakage as storm frequency and intensity increase.

Increasing amounts of carbon dioxide in the atmosphere cause increased amounts in surface waters, leading to ocean acidification (lowered pH). Acidification decreases the availability of calcium carbonate, making it harder for corals and other calcifying organisms to form their shells; it also dissolves existing shells.

By the end of the century, it is predicted that ocean pH will drop from its current value of about 8.1 by as much as 0.4 units; by 2050, conditions will not be sufficient for the formation of calcium carbonate (Hoegh-Guldberg et al. 2007).

Overfishing can seriously degrade the structure and health of coral reefs. If populations of algal grazers are reduced, aggressive algae can overgrow the reef. Reefs can also decline if overfishing reduces populations of fish that normally keep coral predators in check.   

Overfishing threatens more than 70% of coral reefs in the Caribbean (Burke et al. 2011).

Most hard corals develop and grow very slowly, so recovery from damage caused by hurricanes, shipwrecks or anchors may take many years . Branching hard corals break more easily in storms or from physical contact, but may recover more quickly because they grow faster and each fragment can potentially form a new coral. 


Corals and affiliated sponges contain bioactive chemical compounds that can be useful as cancer and virus-fighting drugs. For example, AZT, a compound generated by a Caribbean reef sponge, is an antiretroviral drug that effectively slows the spread of the HIV virus. Read more here about medicines from coral reefs and other marine organisms. 


Almost 500 million people depend on coral reefs for coastal protection, food, and tourism income (Wilkinson, 2008).

Coral reefs help protect shorelines from storm damage and can absorb 70-90% of wave energy.

The total net benefit per year of the world’s coral reefs was estimated in 2003 to be $29.8 billion. Tourism and recreation account for $9.6 billion of this amount, coastal protection for $9.0 billion, fisheries for $5.7 billion, and biodiversity for $5.5 billion. Costs of coral bleaching to tourism expressed in Net Present Value (NPV) were estimated at $10-$40 billion (Cesar, Burke and Pet-Soede, 2003, cited in Conservation International 2008).

What Has Been Done?

Coral reefs are globally widespread and many are threatened by climate change, ocean acidification, pollution.and other pressures. Scientists would be hard pressed to study them all, but ordinary citizens--including you-- can help. A recent study by the European Commission found that volunteers in the Reef Check program identified decreases in live coral cover and increases in the cover of rubble and sand about as well as did scientists from the University of Rhode Island, though their identifications and counts of fishes differed from the professional assessments. The study showed that citizen scientists can make valuable contributions to some aspects of long-term marine environmental monitoring.


In 2010, the not-for-profit organization Nature Seychelles launched its Reef Rescuers Project. This project aims to restore coral reefs around the Seychelles. These reefs were damaged in a mass coral bleaching event caused by an El Niño event in 1998. Surviving coral fragments were selected and used to create an underwater nursery. These corals were raised on ropes for about a year, a method known as “coral gardening”, then transplanted to reefs off of Praslin. By 2015, Nature Seychelles had transplanted more than 24,000 corals.

Now, Nature Seychelles has launched a program called the Coral Reef Rescuers Training Program. The program will provide  scientific and practical knowledge about reef gardening and how to  transplant the corals from underwater nurseries to sites that need restoration. In passing on their knowledge, Nature Seychelles hopes to give people the tools they need for large-scale  restoration projects wherever coral reefs need help.


Get More Information

Coral Reef Alliance [CORAL]

An international NGO founded to support local projects that benefit coral reefs and surrounding communities.

International Coral Reef Initiative [ICRI]

A partnership between non-government organizations, governments and other international organizations that works to implement international conventions and agreements.

Reef Base: A Global Information System for Coral Reefs

A source for coral reef data, publications, maps, and other resources from around the world hosted by World Fish Center

Science to Action: Coral Health Index: Measuring Coral Community Health 

A guidebook to evaluating coral health and understanding impacts

World Resources Institute [WRI]: Reefs at Risk Revisited

A booklet detailing spatial and statistical data on current threats to coral reefs. 


Bruno, J. F. and E.R. Selig. (2007). Regional Decline of Coral Cover in the Indo-Pacific: Timing, Extent, and Subregional Comparisons. PLoS ONE 2, e711.

Burke, L., K. Reytar, M. Spalding and A. Perry. (2011). Reefs at Risk Revisited. World Resources Institute: Washington D.C.

Hoegh-Guldberg, O. et al. (2007). Coral reefs under rapid climate change and ocean acidification. Science 318, 1737–1742 (2007).

Schutte, V. G. W., E.R. Selig and J.F. Bruno. (2010). Regional spatio-temporal trends in Caribbean coral reef benthic communities. Mar Ecol Prog Ser 402, 115–122.

Spalding, M., C. Ravilious and E.P. Green. (2001). World Atlas of Coral Reefs.Prepared at the UNEP World Conservation Monitoring Centre. Berkeley,CA: University of California Press

Wilkinson, C. (2008). Status of Coral Reefs of the World: 2008. Global Coral Reef Monitoring Network and Reef and Rainforest Research Center, Townsville, Australia.

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