A coral reef is an underwater ecosystem made up of reef-building corals. Reefs are formed by groups of coral polyps that are connected by a material called calcium carbonate. Most coral reefs are built by stony corals, which have polyps that grow together in clusters.
Coral is part of the class Anthozoa in the animal group Cnidaria, which also includes sea anemones and jellyfish. Unlike sea anemones, corals create hard carbonate exoskeletons that help support and protect the coral. Most coral reefs grow best in warm, shallow, clear, sunny, and wave-swept waters. Coral reefs first appeared 485 million years ago during the Early Ordovician period, replacing the microbial and sponge reefs of the Cambrian period.
Sometimes called the rainforests of the sea, shallow coral reefs are among Earth’s most diverse ecosystems. They cover less than 0.1% of the ocean, about half the area of France. However, they are home to at least 25% of all marine species, including fish, mollusks, worms, crustaceans, echinoderms, sponges, tunicates, and other cnidarians. Coral reefs grow best in waters with few nutrients. They are most commonly found in shallow tropical waters, but smaller coral reefs also exist in deeper, colder areas.
Shallow tropical coral reefs have decreased by 50% since 1950, partly because they are sensitive to changes in water conditions. They are threatened by too many nutrients (like nitrogen and phosphorus), rising ocean temperatures, ocean acidification, overfishing (such as blast fishing, cyanide fishing, and scuba spearfishing), sunscreen use, and harmful land-use practices, including runoff and seeps (like those from injection wells and cesspools).
Coral reefs provide important benefits, such as supporting tourism, fisheries, and protecting shorelines. The yearly global economic value of coral reefs has been estimated to range from $30 billion to $9.9 trillion, depending on the year of the study.
Formation
Most coral reefs were formed after the Last Glacial Period when melting ice caused sea levels to rise and flood continental shelves. Most coral reefs are less than 10,000 years old. As communities established themselves, the reefs grew upward, keeping pace with rising sea levels. Reefs that rose too slowly could become drowned, without sufficient light. Coral reefs are also found in the deep sea away from continental shelves, around oceanic islands and atolls. The majority of these islands are volcanic in origin. Others have tectonic origins where plate movements lifted the deep ocean floor.
In The Structure and Distribution of Coral Reefs, Charles Darwin set out his theory of the formation of atoll reefs, an idea he conceived during the voyage of the Beagle. He theorized that uplift and subsidence of Earth's oceanic crust beneath the oceans formed the atolls. Darwin set out a sequence of three stages in atoll formation. A fringing reef forms around an extinct volcanic island as the island and ocean floor subside. As the subsidence continues, the fringing reef becomes a barrier reef and ultimately an atoll reef.
Darwin predicted that underneath each lagoon would be a bedrock base, the remains of the original volcano. Subsequent research supported this hypothesis. Darwin's theory followed from his understanding that coral polyps thrive in the tropics where the water is agitated, but can only live within a limited depth range, starting just below low tide. Where the underlying earth allows, corals grow along the coast to form fringing reefs, which can eventually become barrier reefs.
Where the bottom is rising, fringing reefs can grow around the coast, but coral raised above sea level dies. If the land subsides slowly, the fringing reefs keep pace by growing upward on a base of older, dead coral, forming a barrier reef that encloses a lagoon between the reef and the land. A barrier reef can encircle an island, and once the island sinks below sea level, a roughly circular atoll of growing coral continues to keep up with the sea level, forming a central lagoon. Barrier reefs and atolls do not usually form complete circles but are broken in places by storms. Like sea level rise, a rapidly subsiding bottom can overwhelm coral growth, killing the coral and the reef, due to what is called coral drowning. Corals that rely on zooxanthellae can die when the water becomes too deep for their symbionts to adequately photosynthesize, due to decreased light exposure.
The two main variables determining the geomorphology, or shape, of coral reefs are the nature of the substrate on which they rest, and the history of the change in sea level relative to that substrate.
The approximately 20,000-year-old Great Barrier Reef offers an example of how coral reefs formed on continental shelves. Sea level was then 120 m (390 ft) lower than in the 21st century. As sea level rose, the water and the corals encroached on what had been hills of the Australian coastal plain. By 13,000 years ago, sea level had risen to 60 m (200 ft) lower than at present, and many hills of the coastal plains had become continental islands. As sea level rise continued, water topped most of the continental islands. The corals could then overgrow the hills, forming cays and reefs. Sea level on the Great Barrier Reef has not changed significantly in the last 6,000 years. The age of living reef structure is estimated to be between 6,000 and 8,000 years. Although the Great Barrier Reef formed along a continental shelf, and not around a volcanic island, Darwin's principles apply. Development stopped at the barrier reef stage, since Australia is not about to submerge. It formed the world's largest barrier reef, 300–1,000 m (980–3,280 ft) from shore, stretching for 2,000 km (1,200 mi).
Healthy tropical coral reefs grow horizontally from 1 to 3 cm (0.39 to 1.18 in) per year, and grow vertically anywhere from 1 to 25 cm (0.39 to 9.84 in) per year; however, they grow only at depths shallower than 150 m (490 ft) because of their need for sunlight, and cannot grow above sea level.
As the name implies, coral reefs are made up of coral skeletons from mostly intact coral colonies. As other chemical elements present in corals become incorporated into the calcium carbonate deposits, aragonite is formed. However, shell fragments and the remains of coralline algae such as the green-segmented genus Halimeda can add to the reef's ability to withstand damage from storms and other threats. Such mixtures are visible in structures such as Eniwetok Atoll.
The times of maximum reef development were in the Middle Cambrian (513–501 Ma), Devonian (416–359 Ma) and Carboniferous (359–299 Ma), owing to extinct order Rugosa corals, and Late Cretaceous (100–66 Ma) and Neogene (23 Ma–present), owing to order Scleractinia corals.
Not all reefs in the past were formed by corals: those in the Early Cambrian (542–513 Ma) resulted from calcareous algae and archaeocyathids (small animals with conical shape, probably related to sponges) and in the Late Cretaceous (100–66 Ma), when reefs formed by a group of bivalves called rudists existed; one of the valves formed the main conical structure and the other, much smaller valve acted as a cap.
Measurements of the oxygen isotopic composition of the aragonitic skeleton of coral reefs, such as Porites, can indicate changes in sea surface temperature and sea surface salinity conditions during the growth of the coral. Climate scientists often use this technique to infer a region's paleoclimate
Types
Scientists have identified more than three types of coral reefs since Darwin described the three main kinds: fringing reefs around volcanic islands, which later become barrier reefs and then atolls. Thomas lists four major reef types—fringing reef, barrier reef, atoll, and table reef—based on Stoddart, D.R. (1969). Spalding et al. also list four main reef types: fringing reef, barrier reef, atoll, and "bank or platform reef." They note that other reef structures, such as "patch reefs," do not fit easily into strict definitions.
A fringing reef, also called a shore reef, is directly attached to a shore or borders it with a narrow, shallow channel or lagoon. It is the most common reef type. Fringing reefs follow coastlines and can extend for many kilometers. They are usually less than 100 meters wide, but some are hundreds of meters wide. Fringing reefs form on the shore at the low water level and grow outward as they expand. The final width depends on where the seabed begins to drop steeply. The surface of the fringing reef stays at the same height, just below the waterline. In older fringing reefs, the outer parts extend far into the sea, and the inner parts erode to form a lagoon. These lagoons can be more than 100 meters wide and several meters deep. They run parallel to the coast. Fringing reefs in the Red Sea are among the best developed in the world and occur along all its shores except near sandy bays.
Barrier reefs are separated from a mainland or island shore by a deep channel or lagoon. They resemble the later stages of a fringing reef with its lagoon but differ mainly in size and origin. Their lagoons can be several kilometers wide and 30 to 70 meters deep. Unlike fringing reefs, barrier reefs form in open water, not near a shoreline. Like atolls, they are thought to form when the seabed lowers or sea levels rise. Barrier reefs take much longer to form than fringing reefs, making them much rarer.
The best-known and largest example of a barrier reef is the Australian Great Barrier Reef. Other major examples include the Mesoamerican Barrier Reef System and the New Caledonian Barrier Reef. Barrier reefs are also found on the coasts of Providencia, Mayotte, the Gambier Islands, the southeast coast of Kalimantan, parts of Sulawesi, southeastern New Guinea, and the south coast of the Louisiade Archipelago.
Platform reefs, also called bank or table reefs, form on the continental shelf and in open ocean areas where the seabed rises close enough to the surface for reef-building corals to grow. Platform reefs are found in the southern Great Barrier Reef, the Swain and Capricorn Group on the continental shelf, about 100–200 km from the coast. Some platform reefs in the northern Mascarenes are thousands of kilometers from the mainland. Unlike fringing and barrier reefs, which grow outward, platform reefs grow in all directions. They vary in size, ranging from a few hundred meters to many kilometers across. They are usually oval or elongated in shape. Parts of these reefs may reach the surface, forming sandbanks and small islands around which fringing reefs may develop. A lagoon may form in the center of a platform reef.
Platform reefs are typically found inside atolls, where they are called "patch reefs" and may be only a few dozen meters wide. When platform reefs form along long, old barrier reefs, they often arrange in a line. This is seen, for example, on the east coast of the Red Sea near Jeddah. In older platform reefs, erosion can create a structure that resembles an atoll. These can only be distinguished from real atolls through detailed study, such as core drilling. Some platform reefs in the Laccadives are U-shaped due to wind and water movement.
Atolls are circular or continuous barrier reefs that surround a lagoon without a central island. They usually form from fringing reefs around volcanic islands. Over time, the island erodes and sinks below sea level. Atolls may also form when the seabed sinks or sea levels rise. This creates a ring of reefs enclosing a lagoon. Atolls are common in the South Pacific, such as in the Caroline Islands, Cook Islands, French Polynesia, Marshall Islands, and Micronesia.
Atolls are also found in the Indian Ocean, including the Maldives, Chagos Islands, Seychelles, and around Cocos Island. The entire Maldives consists of 26 atolls.
Zones
Coral reef ecosystems have different areas, or zones, that support various types of habitats. Scientists usually identify three main zones: the fore reef, the reef crest, and the back reef (also known as the reef lagoon).
These zones are connected physically and ecologically. Life on the reef and ocean processes allow seawater, sediment, nutrients, and marine animals to move between them.
Most coral reefs are found in waters less than 50 meters deep. Some grow on tropical continental shelves where cool, nutrient-rich upwelling does not happen, like the Great Barrier Reef. Others are located in deep ocean areas around islands or as atolls, such as in the Maldives. Reefs near islands form when islands sink into the ocean, and atolls form when an island sinks below the sea.
Some scientists, like Moyle and Cech, describe six zones, but most reefs have only some of these zones.
The reef surface is the shallowest part of the reef. It is affected by waves and tides. When waves move over shallow areas, they become shallower, causing the water to be rough. These conditions are ideal for corals to grow. The light is enough for photosynthesis by the symbiotic zooxanthellae, and the rough water brings plankton to feed the coral.
The off-reef floor is the shallow sea floor near a reef. This zone is found on continental shelves next to reefs. Reefs around tropical islands and atolls drop suddenly to deep water and do not have this floor. Usually sandy, the floor often supports seagrass meadows, which are important feeding areas for reef fish.
The reef drop-off is the area where the reef slopes downward for the first 50 meters. This zone is home to reef fish that find shelter on the cliff face and plankton in the nearby water. The drop-off is mainly found around oceanic islands and atolls.
The reef face is the area above the reef floor or drop-off. This zone is often the most diverse part of the reef. Coral and calcareous algae create complex habitats with cracks and crevices that provide protection. Invertebrates and epiphytic algae supply much of the food for other organisms. A common feature of this forereef zone is spur and groove formations that help move sediment downslope.
The reef flat is a sandy area behind the main reef that may contain broken pieces of coral. This zone may border a lagoon and act as a protective area, or it may lie between the reef and the shore, where it is a flat, rocky area. Fish often prefer this zone when it is present.
The reef lagoon is a completely enclosed area that is less affected by wave action and may contain small reef patches.
However, the shape of coral reefs is always changing. Each reef is made up of irregular patches of algae, attached invertebrates, and bare rock and sand. The size, shape, and number of these patches change yearly based on factors that favor one type of patch over another. Growing coral causes constant changes in reef structure. On a larger scale, tropical storms can destroy large parts of a reef and move boulders in sandy areas.
Locations
Coral reefs are estimated to cover 284,300 square kilometers (109,800 square miles), which is less than 0.1% of the oceans' surface area. The Indo-Pacific region, which includes the Red Sea, Indian Ocean, Southeast Asia, and the Pacific, makes up 91.9% of all coral reefs. Southeast Asia accounts for 32.3% of this total, and the Pacific, including Australia, covers 40.8%. The Atlantic and Caribbean regions together account for 7.6% of coral reefs.
Although corals live in both temperate and tropical waters, shallow-water reefs are found only between about 30° N and 30° S of the equator. Most tropical corals do not grow below 50 meters (160 feet) in depth. The best temperature range for most coral reefs is 26–27°C (79–81°F), and few reefs exist in waters colder than 18°C (64°F). A Darwin Point occurs when reef-building corals can no longer grow fast enough to keep up with rising sea levels, causing the reef to be permanently submerged. One such point is located at the northwestern end of the Hawaiian Archipelago.
Some coral reefs in the Persian Gulf have adapted to extreme temperatures, including 13°C (55°F) in winter and 38°C (100°F) in summer. Thirty-seven species of scleractinian corals live in these conditions around Larak Island.
Deep-water corals live in colder waters at greater depths and higher latitudes, as far north as Norway. While deep-water corals can form reefs, not much is known about them.
The northernmost coral reef on Earth is near Eilat, Israel. Coral reefs are rare along the west coasts of the Americas and Africa because upwelling and strong cold coastal currents lower water temperatures in these areas (the Humboldt, Benguela, and Canary Currents, respectively). Corals are also rarely found along the coasts of South Asia, from the eastern tip of India (Chennai) to the borders of Bangladesh and Myanmar, as well as along the coasts of northeastern South America and Bangladesh. This is due to the release of freshwater from the Amazon and Ganges Rivers, which affects water conditions.
Significant coral reefs include:
Coral
When alive, corals are groups of small animals living inside hard shells made of calcium carbonate. Coral heads are made up of many individual animals called polyps, which form different shapes. Polyps are usually very small, but they can be as large as 12 inches (30 cm) across.
Reef-building corals live only in the photic zone, the area where sunlight can reach the water. This zone is above 70 meters deep.
Coral polyps cannot make their own food through photosynthesis. Instead, they have a special partnership with tiny algae called dinoflagellates, specifically those in the group Symbiodinium, often called zooxanthellae. These algae live inside the polyps and provide nutrients like glucose, glycerol, and amino acids. Because of this partnership, coral reefs grow faster in clear water, which allows more sunlight to pass through. Without the algae, coral would grow too slowly to form reefs. Corals get about 90% of their nutrients from the algae. In return, the corals protect the algae, which live in large numbers inside each cubic centimeter of coral, and provide the carbon dioxide the algae need for photosynthesis.
The different colors of zooxanthellae come from pigments they produce, giving corals a brown or golden-brown appearance. Other colors, like red, blue, and green, come from proteins in the coral itself. If a coral loses most of its zooxanthellae, it turns white (or sometimes pale colors if it has its own pigments) and is called "bleached." This condition can kill the coral if it is not fixed.
There are eight groups of Symbiodinium. Most research has focused on groups A–D. Each group helps its coral host in different ways but also has traits that may make survival harder. Each photosynthetic organism has a specific sensitivity to damage from sunlight, such as damage to proteins. How quickly they can repair themselves and reproduce affects their survival. Group A is found more often in shallow water. It produces substances that protect against ultraviolet (UV) light, using a compound from glycerin to absorb UV radiation. This helps corals survive in warmer water. If UV or heat damage occurs and is repaired, it increases the chances of survival for both the coral and the algae. This suggests that group A is more resistant to UV and heat than other groups.
Groups B and C are found more often in deeper water, which may explain why they are more vulnerable to rising temperatures. Like plants in shaded areas on land, these groups need to absorb more light to make energy. Because they absorb more UV light, they are more likely to bleach than group A.
Group D has been observed to survive better in high temperatures and has a higher survival rate than groups B and C during recent bleaching events.
Reefs grow as polyps and other organisms leave behind calcium carbonate, forming the structure of the reef. This material builds up around and under the coral, making the reef taller and wider. Waves, fish like parrotfish, sea urchins, sponges, and other forces break down coral skeletons into pieces. These pieces settle into the reef or form sand in nearby lagoons.
Coral shapes are often named after objects on land, such as brains, cabbages, table tops, antlers, wires, and pillars. These shapes depend on factors like how much light the coral receives, wave activity, and events like breakages.
Corals reproduce both sexually and asexually. A single polyp can use both methods during its lifetime. Sexually, corals reproduce through internal or external fertilization. Reproductive cells are found on the mesenteries, which are membranes inside the polyp’s stomach cavity. Some corals are hermaphroditic (both male and female), while others are only male or female. A few species change sex as they grow.
Internally fertilized eggs develop inside the polyp for days or weeks before becoming a tiny larva called a planula. Externally fertilized eggs develop during synchronized spawning, when many polyps release eggs and sperm into the water at the same time. Spawning happens at specific times of the year, depending on water temperature, tides, and the moon. It is most successful when tides are not too different and water movement is low. Eggs or planulae are usually released at night and sometimes follow the lunar cycle (three to six days after a full moon).
After being released, planulae settle in a few days or may float for weeks. They use clues like sounds from existing reefs and chemical signals to find a place to attach. Many planulae are eaten or fail to survive, but those that attach compete for food and space.
Other reef builders
Corals are the most successful reef-builders. However, many other organisms that live in reef communities also add skeletal calcium carbonate in the same way as corals. These include coralline algae, some sponges, and bivalves. Reefs are always built by the combined efforts of these different groups of organisms, with other organisms being the main reef-builders during different time periods in Earth's history.
Coralline algae are important for building reef structures. Although they add minerals more slowly than corals, they are better at surviving strong waves. This helps them form a protective layer over parts of the reef that experience the most force from waves, such as the front of the reef facing the open ocean. They also strengthen the reef by adding layers of limestone across its surface. In areas where corals cannot grow well, coralline algae may be the main builders of a reef made of algae.
Sponge reefs are formed by sea sponges. Hexactinellid sponges create reefs near the coasts of British Columbia, southeast Alaska, and Washington state. Reefs found in Hecate Strait, British Columbia, have reached lengths of up to 7 kilometers and heights of 20 meters. Hexactinellid sponge reefs were first discovered in the Middle Triassic (245–208 million years ago). These sponges were most widespread during the late Jurassic (201–145 million years ago), when a reef system 7,000 kilometers long stretched across the northern Tethys and North Atlantic basins. However, these reefs later declined and were thought to be extinct until existing reefs were found in 1987–1988.
Archaeocyatha, an extinct group of sponges, were the first organisms on Earth to build reefs. They are an important fossil for identifying the Lower Cambrian period worldwide. Similarly, Stromatoporoidea was another extinct group of reef-building sponges. Unlike corals, stromatoporoids usually attached themselves to soft surfaces, so their "reefs" covered only one level instead of forming tall, layered structures of skeletons.
Oyster reefs are groups of oysters that live together in colonies. These structures are also called oyster beds or oyster banks. Oyster larvae need a hard surface to attach to, such as the shells of older or dead oysters. This allows reefs to grow over time as new larvae settle on older oysters. Crassostrea virginica were very common in Chesapeake Bay and along the Atlantic coastal plain until the late 1800s. Ostrea angasi is a species of flat oyster that has also formed large reefs in South Australia.
Hippuritida, an extinct group of bivalves called rudists, were major reef-builders during the Cretaceous period. By the mid-Cretaceous, rudists became the main reef-builders in tropical areas, outnumbering scleractinian corals. During this time, ocean temperatures and salt levels were higher than they are today, which may have helped rudist reefs thrive.
Some snails, like the Vermetidae family, stay in one place and attach themselves to the reef. This helps them contribute to reef building.
Darwin's paradox
Coral reefs grow best in warm, clear, and agitated ocean waters, a fact noted by Charles Darwin during his visit to Tahiti in 1842. This creates a puzzle: how can coral reefs thrive in waters with very few nutrients? Darwin observed that the strongest corals grow in areas hit by strong waves, while weaker or absent corals are found where loose sediment builds up.
Tropical waters have few nutrients, yet coral reefs flourish like "oases in the desert," leading to a scientific question known as "Darwin's paradox": how can such rich ecosystems exist in nutrient-poor conditions? Coral reefs support over one-quarter of all marine species, forming complex food webs. Large predator fish eat smaller fish, which eat zooplankton, and so on. All food webs depend on plants, which are the primary producers. Coral reefs typically produce 5–10 grams of carbon per square meter per day.
Tropical waters are clear because they lack nutrients and drifting plankton. The sun warms the surface layer, making it less dense than deeper water. A stable layer called the thermocline separates warm surface water from cooler, deeper water. This layer prevents nutrients from sinking water—like nitrogen, phosphorus, and potassium—from returning to the surface.
In the ocean, microscopic phytoplankton are the main plants that need sunlight and nutrients to grow. They live near the surface but use up nutrients quickly. In tropical waters, these nutrients are not replaced because of the thermocline.
Around coral reefs, lagoons fill with material from the reef and island, creating safe areas for marine life. Reefs recycle nutrients more than the open ocean. Producers like phytoplankton, seaweed, and algae transfer nutrients to corals. Phytoplankton are eaten by fish and crustaceans, reducing the need for new nutrients.
Corals absorb nutrients like nitrogen and phosphorus directly from water. At night, they extend their tentacles to catch zooplankton, which provide nitrogen to the coral and its symbiotic algae. Sponges in reef crevices filter water and consume phytoplankton, later excreting nutrients corals can use.
The rough surfaces of corals help them survive in agitated waters. Smooth water layers around objects usually block nutrients, but coral surfaces disrupt these layers, allowing nutrients to reach the coral. Turbulent water promotes reef growth.
Deep, nutrient-rich water entering reefs can affect temperature and nutrient systems. This water disrupts the thermocline, causing changes in temperature and nutrient levels. Water moves through reefs via currents, waves, and tides. Internal waves, which move along density layers, can mix water and bring nutrients to the surface.
Coral reef structures may enhance mixing, creating pockets of cooler, nutrient-rich water. Cool water from depth, brought by internal waves and tides, supports the growth of suspension feeders, algae, plankton, and larvae. Seaweed like Codium isthmocladum reacts to deep water nutrients, as do eggs and larvae on reefs.
Cyanobacteria help by fixing nitrogen into soluble nitrates. Coral reefs rely on nearby habitats like seagrass meadows and mangrove forests for nutrients. These areas provide dead plants and animals rich in nitrogen, feeding reef animals. Reefs protect mangroves and seagrass from waves and provide sediment for their roots.
Biodiversity
Coral reefs are among the most productive ecosystems on Earth. They create complex and varied underwater habitats that support many different types of marine life. Fringing reefs near the water's edge work together with mangrove forests and sea grass meadows. The reefs protect the mangroves and sea grass from strong waves and currents that could damage them or wash away the soil they grow in. In return, the mangroves and sea grass help protect the coral from too much silt, fresh water, and pollution. This balance helps many reef animals, such as those that eat sea grass or use the reef for shelter and breeding.
Coral reefs are home to many animals, including fish, seabirds, sponges, cnidarians (like corals and jellyfish), worms, crustaceans (such as shrimp, cleaner shrimp, spiny lobsters, and crabs), mollusks (like cephalopods), echinoderms (such as starfish, sea urchins, and sea cucumbers), sea squirts, sea turtles, and sea snakes. Mammals other than humans are rare on reefs, with dolphins being a notable exception. Some animals eat corals directly, while others feed on algae growing on the reef. The number of living things on a reef is closely linked to the variety of species present.
Different animals may share the same hiding spots on a reef at different times of day. For example, nighttime predators like cardinalfish and squirrelfish hide during the day, while damselfish, surgeonfish, triggerfish, wrasses, and parrotfish hide from predators like eels and sharks.
The many hiding places, or refuges, in coral reefs are the most important reason for the high number of species and the large amount of living material found there. Coral reefs also have a high level of microscopic life compared to other environments.
Coral reefs are often at risk of being taken over by algae. Overfishing and too much nutrients from land, such as from sewage or fertilizer runoff, can allow algae to grow too much and harm coral. Runoff can carry nitrogen and phosphorus, which help algae grow. Algae can outcompete coral for space and smother it by reducing oxygen levels. Lower oxygen levels can slow coral growth and make it more likely to get sick or break down. Algae are found in many areas of coral reefs, including turf algae, coralline algae, and macro algae. Some sea urchins, like Diadema antillarum, eat algae and can help reduce the risk of algae taking over reefs.
Sea sponges are an important part of coral reef communities. There are 420 sponge species in reefs near Indonesia, 486 in Indian waters, and 1,500 in the Great Barrier Reef in Australia. Sponges help recycle waste in reef food webs by turning it into food for other animals. Some sponges that have special algae inside them produce more oxygen and organic matter than they use. These contributions are especially important in the Great Barrier Reef but less so in the Caribbean. Sponges also provide small homes for some invertebrates and fish.
More than 4,000 fish species live in coral reefs. Scientists are still trying to understand why so many species live there. Possible explanations include theories like the "lottery," where the first fish to claim a spot can keep it, "competition," where stronger fish take better spots, and "predation," where the number of fish depends on how many are eaten by predators. Healthy reefs can produce up to 35 tons of fish per square kilometer each year, but damaged reefs produce much less.
Sea urchins, sea slugs, and some other animals eat seaweed. Certain sea urchins, like Diadema antillarum, can help prevent algae from taking over reefs. Scientists are studying native sea urchins, like Tripneustes gratilla, to see if they can control invasive algae. Nudibranchia and sea anemones eat sponges.
Many invertebrates, called "cryptofauna," live in the hard parts of coral skeletons. Some drill into the coral (a process called bioerosion), while others live in existing holes and cracks. Animals that drill into coral include sponges, bivalve mollusks, and sipunculans. Other animals that settle on reefs include many crustaceans and polychaete worms.
Coral reefs are important homes for seabirds, many of which are endangered. For example, Midway Atoll in Hawaii supports nearly three million seabirds, including two-thirds of the world’s Laysan albatross population and one-third of the black-footed albatross population. Seventeen seabird species live on Midway, with the short-tailed albatross being the rarest, as its population was greatly reduced by feather hunting in the 19th century.
Sea snakes eat only fish and their eggs. Birds like herons, gannets, pelicans, and boobies also eat reef fish. Some land animals, such as monitor lizards, marine crocodiles, and semiaquatic snakes, sometimes live near reefs. Sea turtles, especially hawksbill turtles, eat sponges.
Ecosystem services
Coral reefs provide important benefits to tourism, fishing, and protection of coastlines. The total economic value of coral reefs worldwide is estimated to be between $29.8 billion and $375 billion each year. Approximately 500 million people receive benefits from these services.
Destroying one square kilometer of coral reef over 25 years is estimated to cost between $137,000 and $1,200,000.
To help manage coastal coral reefs better, the World Resources Institute (WRI) created tools to measure the value of tourism, shoreline protection, and fishing related to coral reefs. These tools were developed with five Caribbean countries. By April 2011, studies had been published for St. Lucia, Tobago, Belize, and the Dominican Republic. The WRI aimed to ensure that the study results helped improve coastal policies and planning. A study in Belize estimated that reef and mangrove services are worth $395–559 million each year.
In Bermuda, coral reefs provide economic benefits worth about $722 million per year, based on six key ecosystem services, according to Sarkis et al. (2010).
Coral reefs protect shorelines by absorbing wave energy. Many small islands would not exist without reefs. Reefs can reduce wave energy by 97%, helping to prevent loss of life and damage to property. Coastlines protected by reefs are more stable against erosion than those without reefs. Reefs can absorb waves as effectively as, or more effectively than, artificial structures like breakwaters. About 197 million people who live below 10 meters in elevation and within 50 kilometers of a reef may benefit from reduced risks due to reefs. Restoring reefs is much cheaper than building artificial breakwaters in tropical areas. Without the top meter of reefs, flood damages would double, and storm-related costs would triple. For 100-year storm events, flood damages would increase by 91% to $272 billion without the top meter of reefs.
About six million tons of fish are caught each year from coral reefs. Well-managed reefs produce about 15 tons of seafood per square kilometer each year. Fisheries in Southeast Asia’s coral reefs alone produce about $2.4 billion worth of seafood annually.
Threats
Coral reefs have existed for 485 million years and have faced many dangers, such as disease, being eaten by animals, invasive species, damage from fish eating algae, too much algae growing, and natural events like earthquakes. In recent years, human actions have added new threats. Between 2009 and 2018, coral reefs around the world lost 14% of their health.
Human activities that harm coral include removing coral for building materials, dragging heavy nets along the ocean floor, and digging canals or paths into islands and bays. These actions can harm marine life if not managed carefully. Other local dangers include using explosives to fish, catching too many fish, removing too much coral, and pollution, including a banned chemical called tributyltin. These activities are more common in areas with weak environmental rules. Chemicals in some sunscreens may trigger hidden viruses in coral’s algae partners, which can affect reproduction. However, grouping tourists on floating platforms has been shown to reduce the spread of coral disease by visitors.
Greenhouse gas emissions increase ocean temperatures and sea levels, leading to coral bleaching and loss of coral coverage. Climate change also causes stronger and more frequent storms, changes ocean currents, and harms coral reefs. Ocean acidification, caused by higher carbon dioxide levels, makes it harder for corals to build their skeletons and increases the rate at which they break down. Corals can adjust their body fluids to reduce these effects. Pollution from volcanoes and human-made particles can also influence ocean temperatures.
In 2011, two scientists found that modern marine animals face similar challenges from multiple dangers as those during a major extinction event 252 million years ago. Corals, which have soft bodies and hard shells, are especially at risk.
When stressed, corals "bleach," meaning they expel the algae that give them color. Corals with Clade C algae are more likely to bleach from heat, while those with Clade A or D algae are more resistant. Stronger coral types, like Porites and Montipora, are also more resilient.
Every 4–7 years, an El Niño event causes some reefs with heat-sensitive corals to bleach. Major bleaching events happened in 1998 and 2010. However, reefs that survive severe bleaching often become more resistant to future heat stress due to natural selection. Similar adaptation might help corals survive global warming.
A study of coral on Jarvis Island, which experienced 10 major bleaching events between 1960 and 2016, found that the reef recovered from near-total death after severe events.
Protection
Marine protected areas (MPAs) are places set aside to protect ocean and estuarine environments. They help manage fishing practices and protect habitats. MPAs can also support goals like restoring coral reefs, preserving biodiversity, and providing economic benefits to communities.
The success of MPAs is still being studied. For example, a study in Indonesia, the Philippines, and Papua New Guinea found little difference in fish populations between protected and unprotected areas. In some cases, MPAs may cause local conflicts due to poor community involvement, disagreements between governments and fishers, or lack of funding. However, in places like the Phoenix Islands Protected Area, MPAs can help local communities earn income similar to what they would have made without protections. Overall, MPAs can help protect coral reefs, but they require clear management and enough funding to work effectively.
The Caribbean Coral Reefs – Status Report 1970–2012 says that coral decline might be reduced or reversed if overfishing stops, especially of species important to reefs, like parrotfish. Human activities, such as sewage discharge, should also be reduced. This can be done by limiting coastal development and tourism. Healthier reefs are found in countries that protect parrotfish and sea urchins, and that ban certain fishing methods like trapping and spearfishing. These actions help create reefs that are more likely to survive challenges.
Protecting a variety of healthy reefs, including areas that act as climate refuges, helps maintain genetic diversity. This diversity is important for corals to adapt to changing climates. Using different conservation methods in both marine and land areas increases the chance of successful coral adaptation.
Designating reefs as biosphere reserves, marine parks, national monuments, or world heritage sites can help protect them. Examples include Belize’s barrier reef, Sian Ka’an, the Galapagos Islands, the Great Barrier Reef, Henderson Island, Palau, and the Papahānaumokuākea Marine National Monument.
In Australia, the Great Barrier Reef is managed by the Great Barrier Reef Marine Park Authority. It follows laws, including a biodiversity action plan and a Coral Reef Resilience Action Plan. This plan includes strategies to reduce carbon emissions and educate the public about protecting coral reefs.
On Ahus Island, Papua New Guinea, people follow a traditional practice of limiting fishing in six areas of their reef lagoon. Their rules allow line fishing but ban netting and spearfishing. This has led to larger fish populations and bigger fish compared to areas with no fishing restrictions.
Higher levels of carbon dioxide in the atmosphere cause ocean acidification, which harms coral reefs. To reduce this, countries have passed laws to cut greenhouse gas emissions, like carbon dioxide. Land use laws also aim to reduce deforestation, which releases carbon dioxide. Deforestation can also cause erosion, which harms coral reefs. Programs that encourage fewer car trips and better land management help reduce carbon emissions. Advanced satellite technology can monitor reef health.
The United States Clean Water Act requires states to monitor and limit polluted water runoff into the ocean.
Restoration
Coral reef restoration has become more common in recent decades because many coral reefs around the world are dying. Problems that harm corals include pollution, rising ocean temperatures, extreme weather, and overfishing. As reefs decline, fish habitats, biodiversity, coastal areas, jobs, and natural beauty are also at risk. In the 1970s–1980s, scientists began developing new methods to help restore coral reefs.
Coral aquaculture, also called coral farming or coral gardening, is a method that helps grow corals again. This process skips the early, most dangerous stages of coral growth. Small pieces of coral, called "seeds," are grown in nurseries and then planted back on reefs. People who farm corals have different goals, such as protecting reefs or earning income. Because this method is simple and has shown good results, coral nurseries are now the most widely used and effective way to restore reefs.
Coral farming works by using a coral’s natural ability to grow from small pieces. If these pieces attach to new surfaces, they can grow into full corals. This idea was first tested in 1995 by Baruch Rinkevich, who found it successful. Today, coral farming has changed into many forms, but it still focuses on growing corals. This method replaced older ways of moving corals, such as transplanting. While transplanting has worked in the past and has a high survival rate, it involves taking corals from existing reefs. Because many reefs are already damaged, this method should be avoided if possible. However, saving healthy corals from areas where reefs are dying might still be useful.
Coral farming usually starts with creating a nursery where coral fragments can grow safely. Nurseries should be placed in areas that help corals grow and reduce the chance of them dying. These nurseries can be in floating structures, aquariums, or other safe locations. Once a place is chosen, corals are collected and grown.
One major advantage of coral farming is that it helps corals survive when they are young. By removing dangers like predators and obstacles, corals can grow without many problems. However, farming cannot stop large threats like rising ocean temperatures or hurricanes, which can still harm corals in nurseries.
New technology is being used to improve coral farming. Scientists from the Reef Restoration and Adaptation Program (RRAP) tested a robotic camera that counts coral babies and tracks their growth. This camera uses computer programs to analyze coral health in real time. This tool, developed by QUT, is used during coral spawning events to help researchers manage large numbers of corals more effectively.
Restoring reefs often involves adding materials for corals to grow on. These materials can include old tires, ship parts, subway cars, and special concrete shapes called reef balls. Corals can also grow naturally on structures like oil rigs. In big projects, corals are attached to these materials using tools like metal pins, superglue, or milliput. Some corals can also be tied to surfaces with thread.
Biorock is a special material made by running a small electrical current through seawater. This process causes minerals to form a white rock called aragonite, which is the same as natural coral reefs. Corals grow quickly on Biorock, and the electricity also helps corals and other sea creatures grow faster. The area around the electrical current is more acidic, which stops harmful algae from growing. The speed of coral growth depends on how much the current helps the minerals form.
Just having many structures on the ocean floor is not enough to create reefs. Scientists tested different types of structures near Ticao Island, Philippines, in 2013. They used smooth and rough rocks, fences, and no structures at all. After one month, they found that rough rocks helped more coral larvae settle. After a year, the areas with medium complexity (just rocks) supported local fish populations. The study showed that restoring reefs can be done affordably if protected properly.
A study on Oahu, Hawaii, showed how coral can be moved safely. The University of Hawaii helped relocate corals damaged by dredging, which covers corals in sand. Corals can only grow on hard surfaces, so scientists moved them to a nearby site with rock. Divers from the U.S. Army helped transport the corals, and they found little damage during the process. Corals grew well on the rock and wires at the new site. No harm to the environment or nearby activities was reported.
Another method to help reefs is using sound. In parts of the Great Barrier Reef, scientists played recordings of healthy reef sounds. This attracted twice as many fish to these areas compared to areas with no sound. It also increased the variety of fish species by 50%.
A new idea for restoring reefs is using gene therapy. Scientists are testing ways to give corals special bacteria or naturally heat-tolerant algae. These changes could help corals survive warmer ocean temperatures. Some research already shows that genetically modified corals might be able to live in hotter waters. Scientists like Madeleine J. H. van Oppen and James K. Oliver are studying these possibilities.