Coral reef ecosystems are intricate and diverse collections of species that interact with each other and the physical environment. Coral is a class of colonial animal that is related to hydroids, jellyfish, and sea anemones.

Stony corals, a type of coral characterized by their hard skeleton, are the bedrock of the reef. Stony coral colonies are composed of hundreds of thousands of individual living polyps. Polyps are capable of drawing dissolved calcium from seawater, and solidifying it into a hard mineral (calcium carbonate) structure. That serves as their skeletal support. When you look at a coral colony, only the thin layer on its surface is live coral. The mass beneath is the calcium carbonate skeleton that may be decades old.

The slow growth of polyps and expansion of the hard skeletal structures build up the permanent coral reef structure over time.

In this post, Pritish Kumar Halder takes a brief look at Coral reefs, their location and their conversion process.

Coral underwater

Polyps of reef-building corals contain microscopic algae called zooxanthellae. Which exist with the animal in a symbiotic relationship. The coral polyps (animals) provide the algae (plants) a home. And in exchange the algae provide the polyps with food they generate through photosynthesis. Because photosynthesis requires sunlight, most reef-building corals live in clear, shallow waters that are penetrated by sunlight. The algae also give a coral its color. Coral polyps are actually transparent, so the color of the algae inside the polyps show through.

Types of coral that forms reefs

Coral reefs provide habitat for a large variety of marine life. Including various sponges, oysters, clams, crabs, sea stars, sea urchins, and many species of fish. Coral reefs are also linked ecologically to nearby seagrass, mangrove, and mudflat communities. One of the reasons that coral reefs are so highly valued is because they serve as a center of activity for marine life.

Not all corals on the reef are stony corals.

Hydrocorals, or fire coral, are reef-building hydroids that have a hard calcareous exoskeleton and stinging cells. That can cause a burning sensation when touched.

Octocorals, or ‘soft’ corals, include sea fans and sea whips. Which grow more like fleshy plants and do not form calcium carbonate skeletal structures.

Antipatharians, or black corals, are another type of branching ‘soft’ coral.

Some soft corals have zooxanthellae to acquire food and energy, but others, such as black corals, exist without this symbiotic relationship.

Where are Coral Reefs Found?

Corals can be found throughout the world’s oceans, in both shallow and deep water. However, the reef-building corals that rely on a symbiotic relationship with algae need shallow, clear water allowing light penetration for photosynthesis. Stony corals also require tropical or sub-tropical temperatures, which exist in a band 30 degrees north to 30 degrees south of the equator.

Coral reefs exist in seven U.S. states and territories, including:

  • Florida,
  • Puerto Rico,
  • U.S. Virgin Islands,
  • Hawai, and
  • Guam, American Samoa, and the Commonwealth of the Northern Mariana Islands.
  • There are also coral reefs 100 miles offshore of Texas and Louisiana in the Gulf of Mexico, living on the tops of geologic ‘mesas’.

Coral reefs found in world

Why are Coral Reefs Important?

Coral reefs are among the most biologically diverse and valuable ecosystems on Earth. An estimated 25 percent of all marine life, including over 4,000 species of fish, are dependent on coral reefs at some point in their life cycle. Approximately half a billion people globally depend on coral reef ecosystems for food, coastal protection, and income from tourism and fisheries.

Healthy coral reefs provide:

  • Habitat, feeding, spawning, and nursery grounds for over 1 million aquatic species, including commercially harvested fish species.
  • Food for people living near coral reefs, especially on small islands.
  • Recreation and tourism opportunities, such as fishing, scuba diving, and snorkeling, which contribute billions of dollars to local economies.
  • Protection of coastal infrastructure and prevention of loss of life from storms, tsunamis, floods, and erosion.
  • Sources of new medicines that can be used to treat diseases and other health problems.

All of the services provided by coral reefs translate into tremendous economic worth. By one estimate, the total net benefit per year of the world’s coral reefs is $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, representing the dependence of many different marine species on the reef structure, for $5.5 billion (Cesar, Burke and Pet-Soede, 2003).

In the U.S., the National Marine Fisheries Service estimates the annual commercial value of U.S. fisheries from coral reefs alone to be over $100 million annually (2001). Reef-based recreational fisheries generate another $100 million annually in the U.S.



The greatest threats to reefs are rising water temperatures and ocean acidification linked to rising carbon dioxide levels. High water temperatures cause corals to lose the microscopic algae that produce the food corals need—a condition known as coral bleaching. Severe or prolonged bleaching can kill coral colonies or leave them vulnerable to other threats.

Meanwhile, ocean acidification means more acidic seawater, which makes it more difficult for corals to build their calcium carbonate skeletons. And if acidification gets severe enough, it could even break apart the existing skeletons that already provide the structure for reefs. Scientists predict that by 2085 ocean conditions will be acidic enough for corals around the globe to begin to dissolve. For one reef in Hawaii this is already a reality.


Unfortunately, warming and more acid seas are not the only threats to coral reefs. Overfishing and overharvesting of corals also disrupt reef ecosystems. If care is not taken, boat anchors and divers can scar reefs. Invasive species can also threaten coral reefs. The lionfish, native to Indo-Pacific waters, has a fast-growing population in waters of the Atlantic Ocean. With such large numbers the fish could greatly impact coral reef ecosystems through consumption of, and competition with, native coral reef animals.

Even activities that take place far from reefs can have an impact. Runoff from lawns, sewage, cities, and farms feeds algae that can overwhelm reefs. Deforestation hastens soil erosion, which clouds water—smothering corals.

Coral Bleaching

“Coral bleaching” occurs when coral polyps lose their symbiotic algae, the zooxanthellae. Without their zooxanthellae, the living tissues are nearly transparent, and you can see right through to the stony skeleton, which is white, hence the name coral bleaching. Many different kinds of stressors can cause coral bleaching – water that is too cold or too hot, too much or too little light, or the dilution of seawater by lots of fresh water can all cause coral bleaching.

The biggest cause of bleaching today has been rising temperatures caused by global warming. Temperatures more than 2 degrees F (or 1 degree C) above the normal seasonal maximimum can cause bleaching. Bleached corals do not die right away, but if temperatures are very hot or are too warm for a long time, corals either die from starvation or disease. In 1998, 80 percent of the corals in the Indian Ocean bleached and 20 percent died.