Coral Reefs

Coral reefs are a precious resource in the ocean because of their beauty and biodiversity.

Coral reefs also provide shelter for a wide variety of marine life, humans with recreation, a valuable resource of organisms for potential medicines, create sands for beaches, and serve as buffers for shorelines.

The Importance of Coral Reefs

Coral reefs are a precious resource in the ocean because of their beauty and biodiversity. Coral reefs provide shelter for a wide variety of marine life, they provide humans with recreation, they are a valuable source of organisms for potential medicines, they create sand for beaches, and serve as a buffer for shorelines. Coral reefs are built by millions of coral polyps, small colonial animals resembling overturned jellyfish that use excess carbon dioxide in the water from the atmosphere and turn it into limestone.

Coral species distibution map

Corals are in fact animals that fall under the phylum Cnidaria and the class Anthozoa. They are relatives of jellyfish and anemones. Corals can exist as individual polyps, or in colonies and communities that contain hundreds to hundreds of thousands of polyps. For example, brain corals consist of colonies of many individual polyps; each individual polyp averages 1-3 mm in diameter. Corals can be divided into two groups: hard coral and soft coral. Hard corals, also known as stony coral, produce a rigid skeleton made of calcium carbonate (CaCO3) in crystal form called aragonite, with reef-building capabilities. Alternatively, soft corals, including sea fans, do not produce a rigid calcium carbonate skeleton and do not form reefs, though they may be present in a reef ecosystem.

Most reef-building corals have a mutually beneficial relationship with a microscopic unicellular algae called zooxanthellae that lives within the cells of the coral’s gastrodermis. As much as 90% of the organic material the algae manufacture photosynthetically is transferred to the host coral tissue. In addition to the symbiotic relationship with algae, most corals capture and consume live prey ranging from microscopic zooplankton to small fish, depending on coral size. Using its tentacles that extend outside it body, the coral uses its nematocysts, or stinging cells, to stun and kill its prey before passing it to its mouth. Once the food has been digested, the waste is expelled from the same opening.

Corals are unique in that they are capable of reproducing both sexually and asexually. Sexual reproduction is the more common method and can be performed in two ways: broadcast spawning or brooding. Broadcast spawning consists of both male and female coral expelling massive amounts of gametes (eggs and sperm) into the water column during synchronized events. Brooding is similar to broadcast spawning, except only the male gametes are released into the water column. Coral sperm is negatively buoyant once released and hopefully will be carried by ocean currents to female coral where they will fertilize the egg cells of the female coral.

Slow Life from BioQuest Studios

The Variety of Coral Reefs

Coral reefs can be found in both shallow and deep waters and are classified into 2 general categories (hard and soft corals):

Hard corals

Scleractinia, also called stony corals, are exclusively marine animals; they are very similar to sea anemones but generate a hard skeleton. They first appeared in the Middle Triassic and replaced tabulate and rugose corals that went extinct at the end of the Permian. Much of the framework of coral reefs is formed by scleractinians. There are two groups of Scleractinia: Colonial corals found in clear, shallow tropical waters; they are the world’s primary reef-builders (see below for examples), and solitary corals which are found in all regions of the oceans and do not build reefs. Some live in temperate, polar waters, or below the photic zone down to 6,000 m.

Soft corals

The Alcyonacea, or the soft corals, are an order of corals which do not produce calcium carbonate skeletons and so are neither reef-building corals nor do they lay new foundations for future corals. Instead they contain minute, spiny skeletal elements called sclerites. Aside from their scientific utility in species identification, sclerites give these corals some degree of support and give their flesh a spiky, grainy texture that deters predators.

Unlike stony corals, most soft corals thrive in nutrient-rich waters with less light intensity. Almost all utilize zooxanthella as a major energy source. However, most will readily eat any free floating food, such as brine shrimp, out of the water column.

Sea fans

A gorgonian, also known as sea whips or sea fans (soft corals), are an order of sessile colonial cnidarian found throughout the oceans of the world, especially in the tropics and subtropics. Gorgonians are similar to sea pens, another soft coral. Individual tiny polyps form a colonies that are normally erect, flattened, branching, and reminiscent of a fan. Others may be whip-like, bushy, or even encrusting. A colony can be several feet high and across but only a few inches thick. They may be brightly colored, often purple, red, or yellow.

In 1999, a deep coral reef 60 m below the surface was discovered by the United States Geological Survey (USGSCenter for Coastal and Wetland Studies near Pulley Ridge, an underwater barrier island west of the Dry Tortugas National Park off the southern coast of Florida. The Pulley Ridge reef absorbs more light by increasing surface area and growing flat rather than the usual vertical growth seen in shallower coral reefs. Other deep water reefs include the Darwin Mounds and the Mingulay reef complex. More is known about shallow water coral reefs in tropical zones than deep-water reefs discovered recently, however much research into these unique ecosystems is being conducted.

Tropical Coral Reefs

Tropical coral reefs are biotic reefs formed in tropical waters by live organisms such as calcareous algae (including red algae) and corals. In contrast, abiotic reefs are formed by the deposit of sand and other materials in shallow water. Organisms responsible for building tropical (biotic) coral reefs can only grow at 20-28°C, so although coral reefs live in all oceans, most are found between the Tropic of Capricorn and the Tropic of Cancer. The best growing habitat for coral reefs is a clear-water photic zone less than 50 m deep where light shines down and microscopic algae can best provide photosynthesis for the corals.

Corals can be found throughout the oceans, from deep, cold waters to shallow, tropical waters. Shallow coral reefs have optimal growth rates in warm water ranging from 70-85°F (21-29°C). Coral reefs can be found at depths exceeding 91 m (300 ft), but reef-building corals generally grow best at depths shallower than 70 m (230 ft). The most prolific reefs occupy depths of 18-27 m (60-90 ft), though many of these shallow reefs have been degraded. Corals also need salt water to survive, so they also grow poorly near river openings with fresh water runoff. Other factors influencing coral distribution are availability of hard-bottom substrate, the availability of food such as plankton, and the presence of species that help control macroalgae, like urchins and herbivorous fish.

The wide array of coral reef forms includes the Apron reef, the Fringing reef, the Barrier reef, the Patch reef, the Ribbon reef, the Table reef and the Atoll reef. The Apron and Fringe reef both reach down and out from the shore point or peninsula although the Apron reef is typically not as steep as the Fringe reef. Barrier reefs, like the Great Barrier Reef, are separated from the shore by lagoons. An Atoll reef surrounds a lagoon in a circular or uninterrupted fashion and is different from the others because there is no island in the middle.

A Critical Situation

[December 9th, 2022] “The IUCN Red List now includes 150,388 species, of which 42,108 are threatened with extinction. Over 1,550 of the 17,903 marine animals and plants assessed are at risk of extinction, with climate change impacting at least 41% of threatened marine species.” Read more: IUCN Red List: Human activity devastating marine species from mammals to corals

Coral reefs & climate change from Earth Touch

Coral reefs are extremely sensitive to changes in light, temperature (bleaching), overfishing, damaging fishing practices, pollution, and excess sediment from development and erosion. Reefs in Southeast Asia are most at risk of damage due to these factors. Human activity is one of the greatest threats to coral reefs, particularly the destruction of mangrove forests that naturally absorb sediment and nutrients that can suffocate coral reefs with silt and algae blooms.

Bleached corals of the Florida Keys, USA

Former coral reef in the Florida Keys, USA. Destruction most likely due to massive former bleaching events caused by warmer surface waters, nutrient-overload from sewage and overfishing.

Cyanide fishing in the Indonesian and Philippine coral reefs of South Asia stuns and injures valuable fish. Although 85% of the world’s aquarium fish are captured with this destructive method, they suffer a 90% mortality rate usually several weeks after they have been poisoned by cyanide. Fishermen in developing countries depend on reef fish for income to provide for their families; however, illegal fishing practices and overfishing is depleting fish stocks in these areas, rapidly threatening the livelihood of these local populations. Fishermen hit the coral reefs with crowbars to shake out stunned fish and they also even fish with dynamite, which often destroys every living thing on the reef. Many reefs once teeming with life are now wastelands that even the most vigorous conservation efforts can’t begin to restore.
Soft corals closeup
With approximately 85,470 sq km of tropical coral reefs, Indonesia hosts about 33% of the total coral in the world and 25% of all fish species. However, in 2000 it was reported that over 70% of the coral reefs are in bad to fair condition due to fishing practices, out of control tourism, and long periods of bleaching. Coral reefs in the Philippines were found to be 77% less productive from 1966-1986, while the national population doubled in size. If the destruction continues, we will lose about 70% of the world’s reefs within 25-40 years.

The effects of El Niño during 1998 and 2004 are an example of the natural factors that influence the growth of coral reefs. During this El Niño, sea temperatures rose and many coral reefs were bleached or obliterated. Coral bleaching occurs when the single-celled algae vital for coral reef survival and known as symbiotic zooxanthellae are rejected from the coral, soft corals, some sponges and even Tridacna clams. The pigment containing organisms are lost as temperature or stress level due to increased light reaches intolerable levels. As temperatures return to normal, some reefs can recover within several weeks or months. However, equilibrium may not be restored due to global warming and the bleaching effect exposes corals to white and black band diseases. There is some evidence that global warming may actually add to the productivity of an ecosystem through an increase in carbon dioxide and higher temperatures, though the validity of this evidence remains to be seen.

The NOAA Coral Reef Watch satellite

Massive coral bleaching occurred in the Great Barrier Reef of Australia between 1998 and 2002 and in reefs in the Indian Ocean, the MaldivesSri LankaKenyaTanzania, and the Seychelles. Most areas in the Great Barrier Reef rebounded with little damage but in some areas approximately 90% of the coral has vanished. The reefs in the Indian Ocean suffered the most damage and 90% of the coral reefs were lost in the remaining five locations. In Indonesia, the damage is less extensive but more diversity is lost in an area significantly more difficult to restore.

Conservation & Restoration

The fish that grow and live on coral reefs are a significant food source for over a billion people worldwide—many of whom live far from the reefs that feed them. Approximately half of all federally managed fisheries in the United States depend on coral reefs and related habitats for a portion of their life cycles. The NOAA National Marine Fisheries Service estimates the annual commercial value of US fisheries from coral reefs to be over $100 million. Reef-based recreational fisheries generate over $100 million annually in the US. Globally, one estimate shows fisheries benefits account for $5.7 billion of the total $29.8 billion global net benefit provided by coral reefs. Sustainable coral reef fisheries in Southeast Asia alone are valued at $2.4 billion per year. These numbers do not take into account the value of deep-sea corals, which are themselves home for many commercially valuable species and thus additional fisheries value.

Part of the problem with the coral reefs in Indonesia was the move made in 1991 to delocalize power in the Indonesian and Philippine governments. The result was a lack of funding and national support for protection of the South Asian reefs. More recently, conservation efforts have included roping off Marine Protected Areas (MPAs), research and implementation of electrolysis as stimulant for growth, moving reefs to new places and cutting back on harmful fishing practices—all expensive and time consuming endeavors estimated to cost over $100 million dollars. MPAs have been established in regions like Indonesia so that sustainable fisheries can be managed and ecologically important habitats will be protected with a social and biological objective. Laws similar to those found in national parks have been developed to prohibit illegal harvesting of fishes. The hope is that by designating MPAs, coral reefs will be restored, areas will become more beautiful, diversity of life will not be lost and communities will have a sustainable source of income in fishing and tourism. Work is being done to effectively manage MPAs and scientists have found that co-management, the collaboration of local, provincial and national parties, is an effective management strategy. As with many organizations, MPAs will have to overcome challenges that include finding participants, streamlining viewpoints about how effective certain ideas will be and raising enough money to implement change.

School of fish around soft corals

An international and non-profit organization called the Marine Aquarium Council or MAC was created to make the aquarium fish trade more responsible and sustainable through education and to limit harmful fishing practices. By avoiding stock depletion, adding more governmental regulation of reefs, managing reefs better, learning how to take care of fish and food once it is caught and creating a reliable data record, the MAC hopes to avoid a ban on the aquarium industry with a loss of income to the locals and a boom in illegal fishing. Among those involved in the project are researchers, conservationists and industry operators, all who would like to find a sustainable way to meet industry demands through education in the form of international standards and certification plans. MAC hopes that consumers, collectors and retailers will begin to realize how important it is to them and others to sustain their most valuable natural resource—the coral reef.

Other conservation efforts by various organizations include the intricate process of growing coral and coral reefs, a fragile organism that is sensitive to any environmental or biological change. Coral can be grown using a process known as mineral accretion where limestone is stimulated to collect on metal by a safe low voltage current, providing a nice place for baby coral to latch on and grow. The voltage itself can be provided using solar panels or energy from wave action. Scientists active in the Global Coral Reef Alliance (GCRA) grow coral reefs and will even show others the technique. To learn more and view pictures of the restoration effort visit


Many species found in coral ecosystems produce chemical compounds for defense or attack, particularly the slow-moving or stationary species like nudibranchs and sponges. Searching for potential new pharmaceuticals, termed bioprospecting, has been common in terrestrial environments for decades. However, bioprospecting is relatively new in the marine environment and is nowhere close to realizing its full potential. Creatures found in coral ecosystems are important sources of new medicines being developed to induce and ease labor; treat cancer, arthritis, asthma, ulcers, human bacterial infections, heart disease, viruses, and other diseases; as well as sources of nutritional supplements, enzymes, and cosmetics. The medicines and other potentially useful compounds identified to date have led to coral ecosystems being referred to as the medicine cabinets of the 21st century by some, and the list of approved and potential new drugs is ever growing.

Tourism and Recreation

Every year, millions of scuba divers and snorkelers visit coral reefs to enjoy their abundant sea life. Even more tourists visit the beaches protected by these reefs. Local economies receive billions of dollars from these visitors to reef regions through diving tours, recreational fishing trips, hotels, restaurants, and other businesses based near reef ecosystems. One estimate places the total global value of coral-reef based recreation and tourism at $9.6 billion of the total global net benefit of coral reefs.

Center for Biological Diversity: Saving Staghorn corals
Center for Biological Diversity: Saving Elkhorn corals

Mote’s restored coral spawns on Florida’s Coral Reef! | Mote Marine Laboratory & Aquarium

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