Several centuries ago, when brave sailors plied the oceans on sailing fragile boats made of wood, an encounter with an underwater reef did not bode well for travelers.


Today, coral reefs are the most attractive attractions for tourists in many seaside resorts. Many people want to find out what a reef is in the ocean and see with their own eyes the splendor of the underwater kingdom.

What is a reef?

Word "reef", like many other maritime terms, is borrowed from the Dutch language, in which it means "edge" . This word in modern Russian has several meanings:

- a narrow rocky strip, hidden by water or slightly protruding above the surface of the sea, and extremely dangerous for ships sailing near it;

- a sail device that allows you to quickly reduce its volume to control a sailing vessel, using special garters called reef seasons;

- a perennial colony of marine coral polyps, the dead skeletons of which formed a dense limestone massif, which serves as the basis for new corals and a habitat for other marine animals.

Currently the word "reef" usually used in the third meaning, and with an adjective "coral".

Coral reef in the ocean

You shouldn’t be surprised at the ability of coral polyps to form entire large islands. There is nothing surprising in this, especially if we remember the biological productivity of these creatures. The rate of growth of coral biomass remains unchanged throughout the year and is approximately 50-300 grams per day for each square meter reef. They form the basis of an ecosystem in which, in addition to corals, almost all types of living beings participate, from bacterioplankton to mammals.

Coral reefs occupy more than 600 thousand square kilometers on our planet, located mainly in warm seas South-East Asia(about 45% of all reefs), the Atlantic (14%), Indian Ocean (17%), Pacific Ocean (18%), and also in the Red Sea (about 6%). The main habitat of corals is the shelf heated by the sun's rays large islands and continents.


The largest reefs on the planet are the Great Barrier Reef (more than 2 thousand kilometers long, total area about 215 thousand square kilometers), the reef systems of Belize, the Fiji Islands, New Caledonia and Tulear near Madagascar.

Types of coral reefs

Depending on their shape, coral reefs are divided into the following types.

1. Barrier. A wide strip of reef stretches along the coastline, separated from the land by a fairly deep lagoon, the width of which can reach several tens of kilometers and a depth of 70 meters. The width of the reef itself ranges from hundreds of meters to several kilometers. The strip can be single, double and include reef rings.

2. Bordering. The reef runs along the shore and is closely adjacent to it or leaves it between the flat ( top part) and the shore of a narrow lagoon no more than one or two meters deep.

3. Key reef. A flat, small reef is located in shallow water, and its leeward side has been covered with sand by the current until it forms a spit or small island.

4. Platform. The flat reef, in the form of a small island, protrudes from the water at low tide, and its flat, overgrown with algae, is partially covered with debris and sand.

5. Coral jar. A mound of indeterminate shape, created by the activity of coral polyps, is almost or completely hidden by water all the time.

6. Atoll. A ring-shaped calcareous island forms at the top of an underwater volcano. It consists of different types reefs - banks, islets, flat reefs and even small ones. There is at least one lagoon in the center, but most often there are several.

7. Linear. Reef rectilinear ridge 50-100 meters wide, consists of small reefs of all existing species. Even small atolls can be located on its edges, which are often filled with sand and calcareous debris.

How are reefs formed?

Research shows that most reefs existing today are no more than 10 thousand years old. They most likely formed when the level of the World Ocean rose, which was caused by the melting of ice after the end of the Great Ice Age and led to the formation of extensive.

The well-warmed water of the shallow shelf served as an excellent environment for the existence of coral polyps, around which a rich biocenosis soon developed. As the calcareous base grows, the edges of the reef break off during storms and are carried by waves onto its flat, where they accumulate and become compacted. As a result, coral islands of quite large size are formed over thousands of years.


In addition, reefs often form on the tops of underwater volcanoes that have risen close to the sea surface as a result of tectonic activity in the earth's crust. This hypothesis of the origin of reefs was put forward by Charles Darwin. According to his theory, atolls go through three stages of development. Initially, the corals form a colony on the edges of the volcano's summit, grow and form a fringing reef, which subsequently grows to the barrier stage and gradually becomes an atoll.

Coral reefs are calcareous organogenic geological structures. They mainly consist of corals, which are formed with the help of small sea animals. An individual coral, also called a polyp, is cylindrical in shape with an exoskeleton. Exoskeletons give each polyp a hard, rock-like external body. Corals produce calcium carbonate from their bodies. Because the corals remain motionless, individual polyps cluster together. They form colonies that allow them to secrete calcium carbonate and form new reefs.

Corals depend on algae and in turn, the algae find shelter in coral reefs. Live corals and algae form closest to the surface of the water on top of older, dead corals. They excrete limestone during their life cycle, which helps develop reefs. Because corals require algae to survive, they are more common in calm, shallow, clear waters where there is sufficient quantity sunlight.

Coral reefs form in waters dominated by warm ocean currents, which largely limits their distribution to no more than 30° N. w. and Yu. w. It develops rapidly along the reefs, making them one of the most diverse in the world. Overall, coral reefs attract nearly a quarter of the world's living species.

Types of Coral Reefs

Fringing reef off the coast of Eilat (Israel)

Some coral reefs take thousands of years to form. During this time they can develop into several different forms depending on location and surrounding geological features. There are 4 main types of coral reefs:

• Fringing (shore) reefs consist of platform-like coral rocks. They are usually associated with, or are located close to the shore, separated by a semi-enclosed lagoon with deeper water.
• Barrier reefs extend along the island or continental shallows, in the warm waters of the Pacific, Indian and Atlantic oceans, at a distance of several to tens of kilometers from the coast. The width of barrier reefs is hundreds of meters. The distance between the reef and the shore is filled with a lagoon. The largest barrier reef on Earth is about 2000 km long, located off the eastern coast of Australia. Barrier reefs also sometimes extend above the surface of the water.
• Atolls are ring reefs that completely cover the lagoon. The lagoons inside the atolls are more brackish than the surrounding seawaters and often attract fewer species than the surrounding coral reef.
• Intralagoonal reefs(patch reefs) form on shallow areas of the seafloor separated by deeper water from adjacent fringing and barrier reefs.

Functions of coral reefs

Coral reefs have several different functions. They help prevent sediment from washing away and damaging shorelines. Coral reefs act as a physical barrier that helps create healthier, more protected habitats near the coast. They also capture carbon dioxide, which helps create marine conditions. Coral reefs also have economic benefits for nearby settlements. They are collected for the production of medicines and jewelry. Exotic species of fish and marine plants are caught for keeping in aquariums. Also, the magnificent underwater life of coral reefs attracts tourists from all over the world.

Environmental threats to coral reefs

Many coral reefs are experiencing a phenomenon known as bleaching, in which the corals turn white and die after the algae disappear. The bleached coral weakens and eventually dies, leading to the death of the entire reef. The exact cause of the bleaching remains unclear, although scientists speculate that it may be directly related to changes in sea temperatures. Global climate events such as El Niño and climate change have increased the temperature of the world's oceans. Following El Niño events in 1998, approximately 30% of coral reefs were permanently lost by the end of 2000.

Sedimentation also threatens coral reefs around the world. Although they only form in clean waters, soil erosion due to mining and agriculture/forestry causes rivers to carry sediment to the ocean. Natural vegetation, such as growing along waterways, acts as a barrier to precipitation. due to the construction and development of settlements, the amount of precipitation in the sea increases.

Pesticides also enter the ocean through agricultural runoff, which increases the amount of nitrogen in the sea, causing corals to become sick and die. Negligent management practices such as overfishing and uncontrolled coral mining also disrupt fragile marine ecosystems.

Coral reef conservation and restoration

One suggestion for saving coral reefs is to care for them like a garden. Introducing plants to remove sediment and excessive algae growth can help temporarily keep coral reef ecosystems in balance. Reducing pesticide runoff from fields can also help reduce nitrogen levels in the sea. Reducing emissions carbon dioxide from human activities can improve the overall condition of coral reefs.

Seas with coral reefs are found in the Pacific, Atlantic and Indian oceans. The most famous among them are the Coral Sea off the coast of Australia, in which the famous Great Barrier Reef is located, and the Caribbean Sea located in the Atlantic Ocean. Corals are quite whimsical living organisms that cannot live in every body of water.

Seas with coral reefs

All seas with coral reefs - the seas are warm. The temperature in them all year round must be in the range from 18 to 30°C, otherwise neither the reef-building corals themselves nor their larvae will survive. That's why most seas with coral reefs located in tropical and equatorial latitudes. But there are exceptions - areas heated by warm currents originating in the tropics. For example, they can be found off the coast of Japan, where they are warmed by currents from the warm China Sea, or off the Bermuda Islands, where the Gulf Stream carries water masses from the Caribbean. Conversely, where cold currents invade tropical seas (for example, off the western African coast), there are no corals.

Warm water - necessary condition for coral growth, but not yet sufficient. Most of their species feed on the activity of microscopic algae – zooxanthellae. Exactly these tiny plants, living right inside the cells of coral polyps, give them the brightest colors of the most unimaginable colors. Without its symbionts, the coral bleaches and dies. And algae, in order to survive themselves and “feed” the coral, need light.

That's why seas with coral reefs mostly not deep and located either on the continental shallows or in areas with pronounced bottom relief - volcanic uplifts and underwater ridges. So, there are a lot of reefs on the border of the Pacific and Indian oceans, because there are enough islands and a large area of ​​shallow waters. Again, there are exceptions to this rule - sometimes there are coral islands - atolls surrounded by reefs in fairly deep water areas far from the coast.

However, you need to understand that the reef grows over millions and billions of years. Once upon a time in ancient times there were warm shallow waters seas with coral reefs. Coral polyps died, having outlived their life, and calcareous skeletons remained from them. Many such skeletons form a dead body, bottom part reef, actually a rock created by tiny animals. And already on the upper, illuminated part live corals live.

Over the course of many years, the reef grew, and with it, due to tectonic movements, the depth increased, and the sea turned into an ocean. Thus, atolls surrounded by reefs arose far from the coast - over several million years, the corals themselves created the shallow water they needed. Due to the light requirements of corals and their symbionts, reefs, as a rule, are not found where the water is cloudy, for example, due to suspended silt or proliferating plankton.

Another feature seas with coral reefs – high salinity of water. They do not tolerate desalination, and even the Great Barrier Reef is cut by rivers flowing into the sea, and the mouth of the Amazon prevents the spread of corals from the Caribbean further along the South American coast.

Coral seas located in the Atlantic and Indo-Pacific (water areas of the Indian and Pacific oceans) differ greatly in species composition reef-building corals. Most of them belong to the same genera of madrepores - Acropora and Porites; there are over 400 in the Indo-Pacific various types reef builders, and in the Atlantic there are another 75 unique to this ocean. Although Atlantic corals are a different species, they often look almost identical to their Pacific counterparts and lead similar lifestyles.

This is due to the fact that they most likely have common ancestors. Previously, about 50 million years ago, the Atlantic and Indo-Pacific were connected, but then due to plate drift Atlantic Ocean was separated from the Pacific by the Isthmus of Panama, and from the Indian by the African continent. The straits that remained after this, which still connect these oceans, are too cold for the spread of heat-loving larvae of colorful marine inhabitants. Therefore, their Atlantic group turned out to be isolated and developed independently, leading in the course of evolution to the emergence of species unique to this ocean.

The richest species diversity corals in the waters of the Atlantic are in the Caribbean Sea, and in the Indo-Pacific - the area between the Philippines and the northern Australian coast. As a rule, the more species there are in one sea ​​with coral reefs, the larger these reefs are and the faster they grow. If it contains less than 50 species of corals, it is usually small and poorly developed.

But the best way is to see the beauty of coral reefs with your own eyes. And I offer you the most interesting film “Coral Paradise” by Leni Riefenstahl, which will introduce you to the beauty of one of the seas with coral reefs:

Coral reefs are large underwater formations consisting of the skeletons of coral polyps, which are marine invertebrate animals. The order of reef-building coral polyps called madreps or stony corals—they extract calcium carbonate from seawater and create a hard, durable exoskeleton that protects their soft, baggy bodies.

Each individual coral is called a polyp. New coral polyps live on the calcium carbonate exoskeletons of their ancestors, and after death they add another exoskeleton to the existing structure. Over many centuries, the coral reef grows with each new polyp and eventually becomes a large and visible structure underwater.

Corals can be seen everywhere - from the Aleutian Islands off the coast of Alaska to the warm tropical waters of the Caribbean. The largest coral reefs can be seen in the clear, shallow subtropical and tropical ocean waters, where they grow rapidly. The largest of the coral reef systems, the Great Barrier Reef of Australia, is more than 2,400 kilometers long.

Coral life

According to the non-profit environmental organization Coral Reef Alliance (CORAL), there are hundreds of species of corals in the world. Corals come in a variety of shapes and colors, from round and curled brain corals, which resemble a human brain, to tall, elegant eight-rayed Muricea corals (sea whips) and sea fans, which have intricate, brightly colored appearances. trees or plants.

In photos: Stunning new coral species in Polynesia.

Corals belong to the phylum Cnidaria. This group also includes jellyfish, anemones, Portuguese man o' war and other marine animals. Although each individual animal is considered a polyp, corals are often described as colonies consisting of thousands of polyps.

Corals feed on two different ways: Some species can catch small marine organisms such as fish and plankton using stinging tentacles on the outer edges of their bodies. However, according to the conservation agency environment USA (US Environmental Protection Agency (EPA)) most corals maintain a symbiotic (mutually beneficial) relationship with seaweed, known as zooxanthellae.

These algae live inside the body of the coral polyp and, through the process of photosynthesis, produce food for themselves and for the polyp. The polyps, in turn, provide the algae with a home and carbon dioxide. In addition, zooxanthellae give corals their vibrant colors—most coral polyp bodies are transparent and colorless.

Some coral species, such as brain corals, are hermaphrodites and produce both eggs and sperm at the same time. They reproduce through mass coral spawning, which in some species occurs only once a year on a specific night.

Other species, such as elkhorn coral, are dioecious and form colonies with only females or only males. Among these coral colonies, all the polyps of one particular colony produce only sperm. To continue the breeding process, they rely on a neighboring colony, which produces only eggs.

Photo of corals taken on a reef located behind the island of Ofu. national park American Samoa.

World of Coral Reefs

Most of the significant coral reefs existing on this moment began to form 5,000-10,000 years ago, according to information from the CORAL organization. These formations primarily exist in warm, shallow waters that provide sufficient sunlight for the algae that provide food for the coral polyps.

Coral reefs cover less than 1 percent of the ocean floor—collectively, they would cover an area of ​​approximately 285,000 square kilometers, which is close to the size of the state of Nevada. However, they are among the most productive and diverse ecosystems on Earth.

Approximately 25 percent of all known marine species depend on coral reefs for food, habitat and reproduction. Corals are sometimes called the "rainforests of the sea" because of their biodiversity. They are habitat for more than 4,000 species of fish, 700 species of coral, and many thousands of other plants and animals.

However, according to experts, their lives are in imminent danger.

Coral reefs are in danger

According to scientists from the Hopkins Marine Station at Stanford University, coral reefs are vital marine habitats on which many ocean species depend. In addition, they provide direct economic benefits to people totaling approximately 30 billion annually through food, fisheries and tourism.

However, coral reefs face several threats. The first threat is increasing ocean acidification, caused by the oceans absorbing huge amounts of carbon dioxide (CO2) released into the atmosphere by burning fossil fuels. This limits the corals' ability to produce the calcium carbonate exoskeletons they depend on for habitat.

Water pollution also negatively affects corals. Agricultural pesticides and fertilizers, oil and gasoline, discharge Wastewater and the release of soil rocks from eroded land areas into the ocean, disrupting the delicate relationships that exist between the plants, corals and other animals that are part of the reef ecosystem.

As ocean temperatures rise due to global warming, coral polyps reject the zooxanthellae they depend on for food. Once the zooxanthellae disappear, the corals also lose their bright colors, and all that remains is a white exoskeleton. This process is called coral bleaching. According to the non-profit environmental organization Coral Reef Alliance (CORAL), corals that experience bleaching usually die off.

Additionally, forms of fishing such as cyanide fishing (in which cyanide is used to make fish easier to catch), "blast fishing" using explosives, and overfishing by trawlers can destroy thousands of years old coral within minutes. reef.

“Overfishing, ocean acidification and ocean pollution are gradually killing coral reefs,” said Roger Bradbury, an ecologist at the Australian national university(Australian National University), located in Canberra, in an interview with the New York Times. “Each of these factors individually is quite capable of causing the destruction of coral reefs around the world, but taken together this destruction is guaranteed.”

The oceans and seas are the heritage of humanity, since not only do most of all species of living beings known (and unknown) to science live in them. In addition, only in the gloomy depths of sea waters can you sometimes see such pictures, the beauty of which can sometimes simply stun even the most indifferent person. Look at a coral reef and you will see that nature is many times greater than the creation of any talented artist.

What it is?

Coral reefs are colonies of corals that sometimes form truly gigantic formations, similar in size to rocks.

Note that true corals that can form reefs are Scleractinia, belonging to the class Anthozoa, phylum Cnidaria. Single individuals form giant colonies of polyps, and calcareous colonies of older individuals provide support for the development and growth of young animals. Contrary to popular belief, polyps are found at all depths, not just in shallow water. Thus, the most beautiful black coral lives at such a depth that not a single ray of sunlight penetrates.

But a true coral reef can only be formed by those species that live in the shallow waters of tropical seas.

What reefs are there?

There are three main types of them: fringing, barrier and atoll. As you might guess, the fringing species is found in shallow waters near the coast. The most impressive formations are the barrier reefs, which look like a breakwater. They are located along the coasts of continents or large islands. As a rule, they are very important. Firstly, millions of species of living beings find refuge there, and secondly, these formations play an important role in shaping the climate of the region, obstructing ocean currents.

The largest and most famous is the Great Barrier Reef, which stretches for 2000 km, forming the eastern edge of the Australian mainland. Other not so significant and large “relatives” of it are located along the coast Bahamas, as well as in the western Atlantic.

Atolls are small ring-shaped islands. Their coastline is protected by coral reefs, forming a natural barrier that prevents strong tides and ocean currents from washing away fertile layer from the surface of the land. Where do reefs come from, what is the mechanism of their formation?

The emergence of coral reefs

Since most polyps require a relatively shallow water environment, the ideal location for them is to have a small and flat base, preferably located near the coast. However, many scientists believe that the conditions under which the formation of a colony of polyps is possible are much more diverse.

Thus, many atolls, by all indications, should have arisen on the tops of old volcanoes, but traces of really high lava formations that could fully confirm this theory have not been found everywhere. The famous scientist Charles Darwin, traveling on the equally famous Beagle ship, was engaged not only in the formation of an evolutionary view of the development of mankind. Along the way, he managed to make many discoveries, one of which was an explanation of how the world of coral reefs arose.

"Reef" theory of Charles Darwin

Let us assume that a volcano that arose in ancient times gradually increased due to lava, which entered the external environment as a result of numerous eruptions. As soon as about 20 meters remain from the ocean surface, optimal conditions will arise for the top of the underwater mountain to be populated by corals. They begin to quickly expand the colony, gradually completely modifying the primary relief that arose after the eruptions.

When a young coral reef reaches a volcano, top part which by that time had already practically collapsed, begins to gradually sink back into the ocean. As you dive, the corals begin to grow more intensely, and therefore the reef begins to become even more massive, remaining at approximately the same level in relation to the surface of the water.

Dynamic theory of formation

Sand begins to accumulate near the reef, most of which are the skeletons of the corals themselves, ground up by erosion and some species of sea creatures. There are more and more shallows, and the reef eventually begins to protrude above the surface of the ocean, gradually forming an atoll. suggests that the rise of a colony of polyps above the surface of the water occurs due to constant changes in the level of the World Ocean.

Many geologists and geographers of that time immediately became interested in this theory. If it is correct, then every large coral reef should have contained at least some remnants of a volcanic core.

Is the volcanic theory of the origin of reefs true?

To test this, a test drilling was organized on the island of Funafuti in 1904. Alas, the technologies that existed at that time made it possible to reach a depth of only 352 meters, after which work was stopped, and scientists were never able to reach the supposed core.

In 1952, the Americans began drilling in the Marshall Islands for the same purpose. At a depth of about 1.5 kilometers, scientists found a layer of volcanic basalt. It has been proven that the coral reef was formed more than 60 million years ago, when a colony of polyps settled on the top of an extinct volcano. Darwin was right once again.

How reefs changed during periods of falling sea levels

It is known that the ocean at different periods reached one hundred meters. The current level stabilized only six thousand years ago. Scientists believe that 15 thousand years ago the sea level was at least 100-150 meters lower than today. Thus, all the coral reefs that formed at that time are now located 200-250 meters below the modern edge. After this mark, the formation of polyp colonies becomes impossible.

In addition, often former coral reefs (photos are in the article), which were formed in even more ancient periods, are also found on the current land. They were formed at a time when the ocean level was at its highest, and there were no ice caps at the earth's poles yet. Note that between the ice ages, polyps did not actually form any significant colonies, since the water level changed too quickly.

Egypt is especially indicative in this regard. Coral reefs in the Red Sea are sometimes found at enormous depths, which several million years ago were the bottom of ordinary shallow seas.

Main components of a coral reef

To understand exactly how a polyp colony works, let’s take the coast of Jamaica as an example. In any photo of a classic atoll, you can first see a sand spit rising steeply from the depths. The dark stripes located parallel to the atoll are traces of coral destruction that occurred at different times due to fluctuations in ocean levels.

Sailors determine this zone by the breakers: even at night, the sound of the surf, which is heard long before the shore appears, warns of the presence of reefs. After the protected area there is a plateau where the corals are exposed at low tide. Oddly enough, in the water area of ​​the lagoon the depth increases sharply, colonies of polyps in this area are not so developed, and during low tide they continue to remain under water. The area near the shore that is constantly exposed during low tides is called the intertidal zone. There are few corals there.

The largest and most branched corals grow on the outer edges that face the open ocean. The highest concentration of marine life is observed in the littoral area. By the way, who can you meet when visiting a coral reef? The underwater world of Egypt and other popular tourist countries is so rich that your eyes will run wild! Yes, you cannot deny the richness of fauna in these places.

The underwater world of coral reefs

As scientists say, the Great Barrier Reef alone (which we have already talked about) is home to almost two thousand species of fish! Can you imagine how many worms, sponges and other invertebrates live there?

The most colorful inhabitants are the amazing coral reef fish - parrots. They got their name from a specific type of “beak”, which is a modified jaw plate. The jaws of these “parrots” are so strong that they can easily tear off and grind entire blocks of coral.

Since polyps are not very high in calories, these fish have to constantly eat. In a year, one population can destroy several tons of corals. Their digested remains are released into the external environment in the form of sand. Yes, yes, “parrots” play an important role in the formation of amazingly beautiful beaches of snow-white coral sand.

Recognizable and colorful inhabitants of these places are also hundreds of species. Their natural enemies sometimes become the culprits in the destruction of the reefs themselves. Thus, the Crown of Thorns star, which arrived on the Australian coast from another hemisphere, has already destroyed almost 10% of the entire Barrier Reef! Because of this, oceanographers and ichthyologists around the world have declared a real war on it: the stars are caught and destroyed.

The measures taken still have a certain effect, and therefore today undersea world Australia is beginning to recover.