Coral reefs are calcareous organogenic geological structures formed by colonial coral polyps and some types of algae that can extract lime from sea water.
Coral reefs are fantastically beautiful and the most “densely populated” ecosystems of the World Ocean. The biomass of benthic invertebrate animals and fish in them is estimated at hundreds of grams per square meter of the seabed. The total number of species on coral reefs can reach or exceed one million.
The reef ecosystem owes such a vibrant life to single-celled algae (symbiont algae) living in corals, whose photosynthetic activity does not stop throughout the year.
The first corals on Earth, tabulates, appeared in the Ordovician period of the Paleozoic era approximately 450 million years ago. Together with stromatoporid sponges, they formed the basis of reef structures.
Throughout the history of their existence, corals have repeatedly experienced periods of decline and mass death - the climate has changed, and the level of the World Ocean has repeatedly risen and fallen. But neither the changing climate, nor the decrease or increase in sea level led to the complete and final death of reef-forming corals - they adapted to new conditions. A balanced reef ecosystem has great restoration potential.
In 1997-1998, there was an anomalous increase in the temperature of the surface waters of the tropical zone of the World Ocean, which led to mass death of corals in vast areas of the Indian and Pacific oceans.
Then, in the coastal areas of Bahrain, the Maldives, Sri Lanka, Singapore and near Tanzania, up to 95% of all corals in shallow reef zones died. In other tropical coastal zones, 20 to 70% of corals suffered the same fate. In the early 2000s, the tragedy repeated itself twice, especially affecting the central and southern parts of the Pacific Ocean, and by the mid-2000s, according to scientists, slightly less than half of the previously existing reefs remained on Earth.
But not in all reef areas the situation is so catastrophic. For example, in the Maldives, coral cover has been completely restored due to the rapid growth of acroporids (acroporal corals), whose growth reaches 20-25 cm per year.
A different picture is observed in the area of Bahrain and Sri Lanka, where the reefs were under very strong anthropogenic pressure.
Thus, it is not sharp climatic fluctuations, but a high level of anthropogenic pressure that leads to fatal consequences for this ecosystem.
What are coral reefs?
Coral reefs are massive deposits of calcite (limestone) formed from the skeletal remains of some colonial marine organisms - plants and animals - among which madrepore corals and coralline algae stand out. In addition to these two dominant groups of reef-forming organisms, the composition of reefs also includes a significant share of other species of animals and plants - mollusks, sponges, foraminifera and some green algae.
There are three main types of interconnected coral reefs: fringing, barrier and atoll.
Fringing or coastal reefs
Located along recently drained or stable coastlines. Such a reef is a terrace-like platform at or slightly below sea level, extending from the shore and often ending in a raised edge, heavily indented by channels, from which the slope of the reef goes quite steeply under the water, and then abruptly ends to great depths. Corals grow fastest on the outer slope of the reef and very slowly on the shallow platform.
Barrier reefs
They often surround individual islands, but are located some distance from the coast, separated from it by a strait or a calm, medium-deep lagoon. The largest and most famous is the Great Barrier Reef, a complex reef system stretching over 1,600 km along the east coast of Australia.
Atolls
Ring-shaped reefs surrounding lagoons, within which there are no land areas. They are ring-shaped with an internal shallow lagoon; ring-shaped with one or more ring breaks through which tidal waters enter the lagoon; in the shape of a half ring, quarter ring or ring-shaped, filled during high tides; or in the form of giant rings consisting of individual small atoll-like reefs fringing huge lagoons.
Types of corals
In general, hard corals that form a reef can be divided into branching, fragile corals (madrepore corals) and massive, rocky corals (brain and mendrin corals). Branching corals are usually found on shallow, flat bottoms. They are colored blue, lavender, purple, red, pink, light green and yellow. Sometimes the tops have a contrasting color, for example, green branches with purple tops.
Brain corals can reach more than 4 meters in diameter. They live at greater depths compared to branched ones. The surface of brain corals is covered with winding crevices. The predominant color is brown, sometimes in combination with green. Dense pores form a kind of bowl, the base of which consists of dead corals, and living ones are located along the edges. The edges grow, increasingly increasing the diameter of the bowl, which can reach 8 m. Living porite colonies are colored pale lilac, the tentacles of the polyps are greenish-gray.
At the bottom of bays, individual mushroom-shaped corals are sometimes found. Their lower flat part fits tightly to the bottom, and the upper part consists of vertical plates converging in the center of the circle. Mushroom coral, unlike branching and massive hard corals, which are colonies, is an independent living organism. Each such coral contains only one polyp, the tentacles of which reach a length of 7.5 cm. Mushroom corals are colored greenish and brownish. The color remains even when the polyp retracts its tentacles.
Coral reefs are an attractive sight for tourists and an undesirable sight for passing ships. Many people want to visit resorts in the Pacific and Indian Oceans - the places where they are concentrated.
Definition
Several reef meanings:
- pile of stones Rocky, sandy, coral formations due to the elevation of the bottom and erosion of the banks. Heaps of remains of mollusks, algae and some reef-building organisms;
- a rocky strip hidden or protruding slightly above the water level;
- coral polyps as a perennial colony, serving as a habitat for various animals and the basis for new growths of the next generation;
- in another meaning: a reef is a device for reducing the volume of a sail in case of strong wind.
Reefs form due to the deposition of sand, erosion of certain rocks and other processes. They are formed as a result of the growth of coral polyps and combination with calcareous algae. Coral reefs are shallow, wave-resistant structures built by the release of lime by marine organisms. Polyp formations are located on underlying bases that differ in chemical properties.
Classification
Barrier reefs are located near islands far from the coast and can be separated from it by a strait. Coral is the most common. The most voluminous is the Great Barrier Reef of Australia, which stretches 1600 km along the coast.
Bordering - look like a platform like a terrace, which starts from the shore and extends deep into the ocean. They often end with a raised edge. They can be found along exposed or stable coastlines.
Atolls are ring-shaped structures, sometimes with several ring breaks, with a lagoon in shallow water, in the form of giant rings. They can easily be mistaken for islands, and only by noticing the lack of land inside can one understand that these are reefs.
Transitional types of rocky formations are characterized by similar characteristics to several corals. Some islands are distinguished by the presence of volcanic rocks in the center of the ring. In the case of others, if you examine the submerged platform from below, it may appear that it is an atoll-type reef or an underwater island.
Types of corals
There are hundreds of varieties of corals, differing in color, shape, and size. They can be round and curled, tall, fan-shaped, resembling plants. They are called tropical maritime forests because they are home to over 4,000 species of fish, 700 species of polyps and many thousands of different living creatures. Corals are classified as cnidarians: jellyfish, anemones, and Portuguese man-of-war.
Brainfish are hermaphrodites, they produce female and male cells, and their reproduction occurs as a result of mass spawning, while in other species this phenomenon is observed once a year.
There are two known ways of feeding corals. The first is with the help of stinging tentacles, catching small fish and plankton; the second is a symbiotic relationship in which algae exist inside the polyp and, through the process of photosynthesis, take food for themselves and for the polyp, and receive carbon dioxide and shelter. Most corals do not have their own shade, and zooxanthellae give them specific colors.
Conditions of existence
Corals are the most important component of the internal life of the ocean, depending on which its inhabitants live and develop. They provide economic benefits to humans equivalent to $30 billion a year through fishing, tourism, and food.
The habitat of corals is shallow water, heated to an acceptable temperature (22-27 degrees), with sufficient sunlight so that algae can synthesize nutrients and deliver them to the polyps. The Australian reef is gradually disappearing closer to Antarctica. Water level, salt percentage, temperature - all these conditions must correspond to optimal parameters for the development of polyps.
Currently, corals are threatened by ocean acidification, which is possible through the penetration of CO 2 from the atmosphere. Under the influence of carbon dioxide produced during the combustion of fuel substances, they stop growing.
With global warming of ocean waters, corals are rejecting zooxanthellae algae, which gives them their peculiar brightness. In the absence of vegetation, their interaction with corals and many living organisms ceases. It is known that such reefs become colorless over time, as a result of which they die off.
The crown of thorns starfish is a known reef pest from the South Pacific, eating about a meter of the ecosystem every week. To combat it, chemical methods are excluded so as not to harm other inhabitants, so catching the aquatic inhabitant remains optimal.
The structure of polyps, formed over thousands of years, can be destroyed in a few minutes if fishing is carried out using explosive methods, trawlers or using cyanide.
What is a reef in terms of stability? This is an ecosystem that grows and collapses at the same time: fish and animals feed on it, acids corrode it, and the current erodes it. The visible part of the corals is constantly changing and over time decomposes into grains of sand. But even in unfavorable conditions, the reef increases up to 30 cm per year.
Australian reef
The origin of coral reefs began about 25 million years ago with the displacement of the Australian lithosphere plate. Their total global area is equal to 27 million km2. About 3,000 rocky structures and 900 offshore islands make up Australia's Great Barrier Coral Reef, one of the largest natural structures with living organisms at its core. The park, located underwater, covers an area of more than 344,000 km2.
To date, only ten percent has been studied. What a reef is is known, but the features of its coexistence with all its inhabitants and components are being studied. Eyewitnesses claim that no office equipment is able to display the natural beauty and silence of the Australian Nature Reserve.
If you dive below ten meters deep, you can see the glow of the corals and admire the number of their shades: red, green, purple, black, yellow, brown, white. The reef is about 400,000 years old and its growth has accelerated as ocean levels have risen. The new areas are located above the old ones at a depth of about twenty meters. Thousands of generations of its colonies will pass before the reef takes on new shapes and sizes.
Discoverer
In 1770, the navigator James Cook, sailing over a rocky strip of coral, unexpectedly learned what reefs were when their edges stuck into the ship and pierced its hull. The crew had to drop tons of cargo, including a heavy firearm, to lighten the ship and bring it to the shallows to repair numerous holes. The cannons, recovered from the bottom by tourists a century later, became museum exhibits, and one of them was left on Green Island.
About five hundred ships were wrecked in the area of the reef, and some researchers tried to map it. In 1970, an oil tanker sank to the bottom in this place. Torres Strait Islanders and Aboriginal Australians fished here, so Cook was not the only one to discover the reef.
Inhabitants
Six of the seven rare species of turtles live here, the most ancient sharks, including whale sharks, butterfly fish, batfish, crocodiles and many other inhabitants of amazing shape and beauty. You can find a huge flounder weighing 200 kg and 2.5 m long. Dolphins, whales, and killer whales raise their offspring near the reef. Many crustaceans build their homes here: lobsters, crabs, lobsters, shrimp. There are also deadly animals here, including blue-ringed octopuses.
Birds nest on the surface of rocky formations: petrels, frigatebirds, terns, eagles and others, up to two hundred species. Of plants, unfortunately, only 40 species are available.
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 should be in the range from 18 to 30°C, otherwise neither the reef-forming 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 is a necessary condition for coral growth, but not yet sufficient. Most of their species feed on the activity of microscopic algae – zooxanthellae. It is these tiny plants that live right inside the cells of coral polyps that 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 of these skeletons form the dead, lower part of the reef, essentially 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.
The coral seas located in the Atlantic and Indo-Pacific (water areas of the Indian and Pacific oceans) differ greatly in the species composition of reef-forming corals. Most of them belong to the same genera of madrepores - Acropora and Porites; in the Indo-Pacific there are over 400 different species of 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, the 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 of corals in the waters of the Atlantic is 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 the equivalent of underwater forests. Twenty-five percent of all marine life lives in coral reefs. The combination of plant life and colorful marine fish makes these spots a favorite dive spot for scuba divers. Large coral reefs, such as the Great Barrier Reef, are actually made up of many smaller reefs that are connected into a single ecosystem.
Reed Bank
This reef is located in the Philippines and covers an area of 8,866 square kilometers. This natural wonder is located in the South China Sea.
Chagos Archipelago
The Chagos Archipelago in the Maldives covers an area of 12,000 square meters. km. It is the second largest atoll in the world.
Saya de Mala
Saya de Mala in the Indian Ocean covers an area of 40,000 square kilometers. These are the largest flooded banks in the world. This ridge connects the Seychelles and Mauritius islands along the Mascarene Plateau. Along with its coral reefs, the marine habitat includes grasslands for the green turtle and breeding grounds for blue whales. 
Zongsha Islands
Located in the South China Sea. It is an 80 kilometer long atoll that covers an area of 6,448 square kilometers. This atoll is a disputed territory between the People's Republic of China, Taiwan and the Philippines. 
Andros Reef
Andros is a barrier coral reef in the Bahamas that stretches for more than 200 kilometers. The island lies along the edge of an ocean chasm known as the Tongue of the Ocean. The reef expands down along the chasm to a depth of 6,000 feet instead of lying flat along the ocean floor. 
Florida Reef
This is a reef system around the coast of Florida that extends from the Atlantic Ocean into the Gulf of Mexico near Key West. This reef system is approximately 7,000 years old and is 322 kilometers long. 
Mesoamerican reef
The Mesoamerican barrier coral reef system extends along the eastern coast of Central America. From its northern point, the Yucatan Peninsula in Mexico, to the southern coast of Honduras, the reef reaches a total of 943 kilometers. 
Barrier Reef of New Caledonia
This coral reef is almost 1,500 kilometers in length and is located near the former French colony of New Caledonia in the Pacific Ocean. Some parts of the reef have been damaged by nickel mining, but overall the health of this reef is quite good. 
Red Sea Reef
The Red Sea coral reef off the coasts of Egypt, Israel and Saudi Arabia is approximately 5,000 to 7,000 years old. Ten percent of the 1,200 species found in this reef live only in this area. The Red Sea coral reef includes Dahab's Blue Hole, one of the most popular and dangerous dive sites in the world. 
Great Barrier Reef
The largest and most famous coral reef is the Great Barrier Reef. Over 2,500 kilometers long, it covers an area of 348,000 square kilometers and is home to more than 400 marine species. It is also one of Australia's most prominent landmarks. Unfortunately, the reef is under threat from pollution and fishing. 
A coral reef is a huge colony consisting of billions of tiny creatures called coral polyps. Their length is only a few millimeters. They absorb calcium dissolved in seawater, forming a calcareous colony skeleton from it.
The most famous and longest coral reef stretches for almost two thousand kilometers along the east coast of Australia - this is. It consists of many coral islets. It consists of about 2,900 individual reefs and 71 islands. Some call this reef the eighth wonder of the world, and it is the most popular place among scuba divers. Its age is at least ten thousand years. Reefs grow at different rates. Some manage to grow by 20 centimeters from year to year, while others barely add 20 millimeters.
Coral reefs are home to about 4,000 species of mollusks, about 2,000 species of fish, 1,000 different sponges, 350 species of starfish and other echinoderms. There are almost two and a half thousand reef-forming corals in total.
Depressions are visible on the surface of the reefs: polyps hide in them and wait for their prey. Polyps catch plankton. They are helped to hunt by their stinging capsules, whose touch is unpleasant for humans.
German journalist Alfrem Brem cites the story of a scientist who studied corals: “I try to break off a twig from another coral, but again unsuccessfully: the coral turns out to be burning and at the first touch my hand begins to burn terribly, as if it had been burned.” Polyps feed on single-celled organisms, as well as worm larvae and tiny crustaceans that infest the plankton.
In addition, they absorb nutrients contained in the water. However, this diet is not enough for most corals. Therefore, they live in symbiosis with unicellular algae. Algae help corals by supplying them with oxygen, and in return they receive minerals. It is algae that colors tropical coral reefs in amazing colors.
And so, all this is good, and coral reefs have many advantages for the animal world, as we see. But, in 1998, the death of coastal corals began to be noticed in the Pacific and Indian oceans. And it turned out that a quarter of the entire Great Barrier Reef was damaged. A decline in the number of reefs is still observed today; scientists are working on this problem. They argue that if things continue like this, then humanity will also have problems, and not just the fish world.
Corals are dying primarily because the water in the World Ocean is becoming warmer due to the greenhouse effect. The staff of the US National Oceanographic Institute came to this conclusion: Corals react very sensitively to all temperature changes. In tropical seas, the water is usually heated to 26 - 28 degrees. If it rises by just one degree for several days, then the corals become uneasy: they experience real stress. And a state occurs in which they are forced to reject the algae with which they live in symbiosis. The reefs are bleached, their colorful colors fading. The lush underwater forest turns into a dull, whitish frame made up of limestone skeletons.
Australian biologist Professor Ove Hegh-Guldberg from the University of Sydney confirmed this guess with the help of simple experiments (their results were published in the summer of 1999). He placed the corals in a simple aquarium and heated the water in it, after some time their fertility dropped by 40 percent. If the extinction of coral reefs continues, the seas will become completely different, their aquatic world will become poorer.
This is what the famous 19th century zoologist Alfred Edmund Brehm wrote about coral reefs - “the wonderful coral thickets are superior to the mythical gardens of the Hesperides.” Coral reefs provide a huge service to cities by serving as natural breakwaters. In addition, some coral species are used in medicine, for example, Eleutherobia. This species produces the protein eleutherobin, which stops the development of cancer cells.
In general, coral reefs feed many and are useful to many, including humans. But there is a way out; it is possible to restore the dead areas of the reef. Many reef species reproduce only once a year, releasing their reproductive cells during the full moon in the spring. For several days, the sea near the reef is covered with a thick mucous mass. Then the larvae sink to the bottom, forming a new colony or merging with the parent community.
In this way, this building material can be transferred to problem areas.
Or there is another, more interesting way. Take a wire dipped in water and pass a small current through it. Soon the metal becomes covered with a crust: brucite and calcium carbonate, substances containing magnesium and calcium, settle on it. This crust serves as a shelter for corals and mollusks. Over time, the reef grows. This way you can control the overall appearance of the reef. For example, you can extend a line along the beach.
Coral reefs photos
