Coral reef. Great Coral Reef. Underwater world of coral reefs

Oceans and seas are the property of mankind, as in them not only most of all known (and unknown) species of living beings live. In addition, only in the gloomy depths of sea water can sometimes see such pictures, the beauty of which sometimes can simply stun even the most indifferent person. Look at the 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 called coral colonies, which sometimes form really giant formations, similar in size to rocks.

Note that the real corals that can form reefs are Scleractinia, belonging to the class Anthozoa, type 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, and not just in shallow water. So, the beautiful black coral lives at such depth, on which no sunlight penetrates.

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

What reefs exist?

There are three main varieties of them: fringing, barrier and atolls. As you can guess, the fringing variety occurs in shallow water near the coast. The most impressive formations are barrier reefs that look like a breakwater. They are located along the coast of the continents or large islands. As a rule, they are very important. Firstly, millions of species of living beings are taking refuge there, and secondly, these data play an important role in shaping the climate of the region, preventing 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" are located along the coast of the Bahamas, as well as in the western part of the Atlantic.

Atolls are small islands of annular form. Their coast is protected by coral reefs, forming a natural barrier that does not allow strong tides and ocean currents to flush the fertile layer from the land surface. Where do reefs come from, what is the mechanism of their formation?

The emergence of coral reefs

Since most polyps need a relatively shallow water, ideal for them is the presence of a small and flat base, preferably located near the coast. However, many scientists believe that the conditions under which a polyp colony can be formed are much more diverse.

Thus, many atolls had to appear on the tops of old volcanoes by all indications, but traces of really high lava formations that could fully confirm this theory are not everywhere. The famous scientist Charles Darwin, traveling on the no less famous ship "Beagle", 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 the explanation of how the world of coral reefs arose.

Darwin's "Reef" Theory

Suppose that the volcano, which originated in antiquity, gradually increased due to lava, which fell into the external environment as a result of numerous eruptions. As soon as the surface of the ocean remains about 20 meters, there will be optimal conditions for the settlement of the top of the seamount with coral. They begin to rapidly build up the colony, gradually completely modifying the primary relief that arose after eruptions.

When the young coral reef reaches a critical mass, the volcano, the upper part of which by that time has almost collapsed, begins to gradually dive back into the ocean. As you dive, the corals begin to grow more intensively, and therefore the reef begins to become even more massive, remaining approximately at the same level with respect to the surface of the water.

Dynamic theory of formation

Near the reef, sand begins to accumulate, most of which are the skeletons of the corals themselves, ground by erosion and some species of marine creatures. The sink becomes more and more, the reef begins to appear over the surface of the ocean, gradually forming an atoll. The dynamic model assumes that the elevation of the colony of polyps above the surface of the water is due to a constant change 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 carry in itself at least some remains of the volcanic core.

Is the volcanic theory of the origin of reefs true?

To test this, in 1904 trial drilling was conducted on the island of Funafuti in the Pacific Ocean . Alas, the technologies that existed at that time allowed to reach a depth of only 352 meters, after which the work was stopped, and scientists could not reach the supposed nucleus.

In 1952, 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 was proved that the coral reef was formed more than 60 million years ago, when the colony of polyps settled on the top of an extinct volcano. Darwin was once again right.

How the reefs changed during periods of declining levels of the world's oceans

It is known that the amplitude of ocean oscillations in different periods reached one hundred meters. The modern level has stabilized only six thousand years ago. Scientists believe that 15 thousand years ago the ocean level was at least 100-150 meters below the modern level. Thus, all the coral reefs that were formed at that time are now 200 to 250 meters below the modern edge. After this mark, the formation of colonies of polyps becomes impossible.

In addition, often the former coral reefs (photo is in the article), which were formed in even more ancient periods, are found on the present land. They were formed at a time when the ocean level was as high as possible, and there were still no ice caps on the Earth's poles. Note that between the glacial periods polyps actually did not form any more significant colonies, as the water level changed too quickly.

Egypt is particularly indicative in this respect. Coral reefs in the Red Sea are sometimes found at great depths, which several millions of years ago were the bottoms of ordinary shallow seas.

The main components of the coral reef

To understand how a colony of polyps is constructed, for example, consider the coast of Jamaica. In any photo of the classic atoll, one can first see a steeply rising sand spit from the depths. Dark stripes located parallel to the atoll are traces of the destruction of corals that occurred in different geological periods due to fluctuations in the ocean level.

Sailors define this zone according to breakers: even at night, the sound of the surf, which is heard long before the appearance of the shore, warns of the presence of reefs. After the protected zone, a plateau begins, on which corals are opened at low tide. Strangely enough, but in the water area of the lagoon the depth increases sharply, the colonies of polyps in this area are not so developed, at low tide they continue to remain under water. The site near the shore, which is constantly opened during low tides, is called the littoral. There are few corals there.

The largest and branched corals grow on the outer edges, which look into the open ocean. The greatest concentration of marine inhabitants is observed in the littoral area. By the way, who can be found visiting a coral reef? The underwater world of Egypt and other popular tourist countries is so rich that your eyes will scatter! Yes, in the richness of the fauna these places can not be denied.

Underwater world of coral reefs

As the scientists say, only on one Great Barrier Reef (which we already mentioned) is inhabited by 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 fish of coral reefs - parrots. They got their name for a specific kind of "beak", which is a modified jaw plate. The jaws of these "parrots" are so strong that they can easily tear and grind whole blocks of corals.

Since polyps are not very caloric, these fish have to constantly eat. Over a year, one population can destroy several tons of coral. The digested residues are thrown into the external environment in the form of sand. 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 of sea urchins. Their natural enemies - starfish - sometimes become the culprits of destruction and the reefs themselves. So, the star The crown of thorns, which arrived on the Australian coast from the other hemisphere, already destroyed almost 10% of the entire Barrier Reef! Because of this, oceanologists and ichthyologists of the whole world have declared to her a real war: the stars are caught and destroyed.

The conducted events still give a certain effect, but because today the underwater world of Australia begins to recover.