Coral reefs

Most of the coral reefs that we observe today formed after the ice age , when ice melting led to sea level rise and flooding of the continental shelf . This means that their age does not exceed 10,000 years. Based on the shelf, the colonies began to grow up and reached the surface of the sea. Coral reefs are also found far from the continental shelf around the islands and in the form of atolls. Most of these islands are of volcanic origin. Rare exceptions have arisen as a result of tectonic shifts. In 1842, Charles Darwin, in his first monograph, The Structure and Distribution of Coral Reefs ^[16], formulated an immersion theory that explained the formation of atolls by raising and settling earth’s crust under the oceans ^[17] . According to this theory, the process of formation of the atoll goes through three successive stages. First, after the volcano damps and the bottom settles, a fringing reef develops around the formed volcanic island . With further subsidence, the reef becomes a barrier and, finally, turns into an atoll .

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  According to Darwin's theory, a volcanic island first appears

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  As the bottom settles, a fringing reef forms around the island, often with a shallow intermediate lagoon

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  During subsidence, the fringing reef grows and becomes a large barrier reef with a large and deeper lagoon.

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  Finally, the island hides under water, and the barrier reef turns into an atoll enclosing an open lagoon

According to Darwin's theory, coral polyps thrive only in the clear tropical seas of the tropics, where water is actively mixed, but can exist only in a limited range of depths, starting just below low tide . Where the level of the underlying land permits, corals grow around the coast, forming coastal reefs that can eventually become a barrier reef .

Darwin predicted that under each lagoon there should be a stone base, which is the remains of a primary volcano. Subsequent drilling confirmed his hypothesis. In 1840, on the Hao Atoll ( Tuamotu Island), using primitive drill at a depth of 14 m, exclusively corals were discovered. In 1896-1898, while trying to drill a well to the base of Funafuti Atoll ( Tuvalu Island ), the drill sank to a depth of 340 m in a homogeneous thickness of coral limestone. The 432 m deep well on the elevated atoll of Quito-Daito-Shima ( Ryukyu Island) also did not reach the bedrock of the atoll. In 1947, a well with a depth of 779 m was drilled on Bikini , reaching the Early Miocene deposits, about 25 million years old ^[3] . In 1951, two wells 1266 and 1389 m deep on the Envetok Atoll ( Marshall Islands ) passed Eocene limestones about 50 million years old and reached indigenous basalts of volcanic origin. These findings indicate the volcanic genesis of the base of the atoll ^[18] .

Where the bottom rises, coastal reefs can grow along the coast, but, rising above sea level, corals die and become limestone . If the land settles slowly, the rate of growth of fringing reefs over old, dead corals is sufficient to form a barrier reef surrounding the lagoon between the corals and the ground. Further lowering of the ocean floor leads to the fact that the island is completely hidden under water, and on the surface there remains only a reef ring - an atoll ^[20] . Barrier reefs and atolls do not always form a closed ring, sometimes storms break walls. A rapid rise in sea level and subsidence of the bottom can suppress coral growth, then coral polyps will die and the reef will die ^[21] . Corals living in symbiosis with zooxanthellae may die due to the fact that enough light ceases to penetrate to the depth for photosynthesis of their symbionts ^[22] .

If the bottom of the sea under the atoll rises, an island atoll will arise. An annular barrier reef will become an island with several shallow passages. With a further rise in the bottom, the passages will dry out and the lagoon will turn into a relict lake ^[20] .

The coral growth rate depends on the species and varies from a few millimeters to 10 cm per year ^[23] , although under favorable conditions it can reach 25 cm ( acropores ) ^[1] .

The first corals on Earth appeared about 450 million years ago. The extinct tabuli along with stromatoporid sponges formed the basis of reef structures. Later (416 ~ 416–359 million years ago) four-rayed corals of rugosa appeared; the reef area reached hundreds of square kilometers. 246–229 million years ago, the first corals appeared, living in symbiosis with algae, and in the Cenozoic era (about 50 million years ago), madreporic corals appeared, which exist even now ^[1] .

During the existence of corals, the climate has changed, the level of the oceans has risen and decreased. The last strong decline in ocean level occurred 25-16 thousand years ago. About 16 thousand years ago, the melting of glaciers led to an increase in ocean level, which reached modern about 6 thousand years ago ^[1] .

For the emergence of a coral biocenosis, a combination of a number of conditions related to temperature, salinity, light exposure and a number of other abiotic factors is necessary. Germatypic corals are characterized by high stenobiontism (inability to tolerate significant deviations from optimal conditions) ^[24] . The optimal depth for the growth of coral reefs is 10-20 meters. The depth limit is not due to pressure, but to a decrease in illumination.

• Temperature

All germatypic corals are thermophilic. The bulk of coral reefs is located in an area where the temperature of the coldest month of the year does not fall below +18 ° C. However, sexual reproduction at this temperature is impossible, and vegetative slows down ^[24] . Typically, a drop in temperature below +18 ° C causes the death of reef-forming corals . The emergence of new colonies is limited to those areas where the temperature does not fall below +20.5 ° C, apparently this is the lower temperature limit for ovogenesis and spermatogenesis in hermatotype corals. The upper limit of existence exceeds +30 ° C. During day tides in shallow lagoons of the equatorial regions, where the greatest variety of forms and density of coral growth are observed, the water temperature can reach +35 ° C. The temperature within the reef-forming organisms remains stable all year round, annual fluctuations at the equator are 1-2 ° C, and in the tropics do not exceed 6 ° C ^[25] .

• Salinity of water

The average salinity on the surface of the oceans in the tropical zone is about 35.18 ‰. The lower limit of salinity at which coral reef formation is possible is 30–31 ‰ ^[26] . This explains the absence of madrepore corals in the estuaries of large rivers. The absence of corals along the Atlantic coast of South America is explained precisely by the desalination of sea water due to the Amazon. In addition to the mainland runoff, precipitation also affects the salinity of surface waters. Sometimes long rains that lower the salinity of water can cause mass death of polyps ^[27] . The salinity spectrum suitable for coral reef life is quite wide: various corals are widespread both in small inland seas with low salinity (30–31 ‰), washing the Sunda and Philippine archipelagos ( Celebess , Yavan , Banda , Bali , Flores , Sulu ) and The South China Sea and the Red Sea , where salinity reaches 40 ‰ ^[26] .

• sunlight

Most reef-forming organisms need sunlight to live. The physiological and biochemical processes during which lime is extracted from seawater and the formation of the skeleton of hermatotype corals are associated with photosynthesis and are more successful in the light. In their tissues there are unicellular algae, symbionts, symbioniums, which perform the functions of photosynthetic organs. In the area of ​​coral reef, the length of the day during the year does not change significantly: the day is almost equal to night, twilight is short. Near the equator, most of the year it is clear, in the tropics the number of cloudy days is not more than 70. The total solar radiation here is at least 140 kilocalories per 1 cm² per year. Probably, corals need direct sunlight: in the shaded areas of the reef their settlements are sparse. Colonies are not arranged vertically one above the other, but are distributed horizontally. Some types of corals that are not involved in the process of photosynthesis, like bright red tubastrae and purple hydrocoral distichopores , are not the basis of the reef. As depth increases, illumination quickly drops. The highest density of coral settlements is observed in the range of 15–25 m ^[28] .

• Turbulence

In the tropics, the solubility of gases in water is 1.5-2 times lower than in the polar region. The oxygen content in seawater in the equator region is 4.5-5 ml / l, and within the tropical zone no more than 6 ml / l. Phytoplankton is poorly developed here, so water on the surface is not enriched with oxygen due to photosynthesis. Oxygen starvation is especially acute in the lagoons. On the surface of corals, coralline algae grow in abundance, forming a strong calcareous crust. Algae are most abundant in places where the wave action is the strongest, and the water is saturated with dissolved gases as much as possible. Coral and algae grow best on the edges of the reef and on its outer slopes. The most powerful overgrowth of hermatotype corals is observed at the upper edge of the surf side. Fresh clean water, saturated with oxygen and rich in plankton provides respiration and nutrition of polyps ^[2] .

• Substrate

Most reefs form on a fixed basis. Coral does not develop on separate stones and calcareous blocks. Corals that live on ridges with high turbulence cannot tolerate siltation. Whereas on the fringing reefs in the zone between the ridge and the shore there are areas with a muddy bottom where their own coral fauna develops. Large mushroom-shaped corals grow on a loose substrate, the wide base of which does not allow them to sink into the silt. A number of branched corals ( Acropolis Kuelcha , Psammocore , blackish porite ) settling in silted lagoons are rooted with outgrowths. Coral does not form settlements on sandy soils, as the sands are mobile ^[29] .

According to the modern relation to sea level, reefs are divided into:

1) level, reaching the peak surface of the tidal zone or mature, having reached the maximum possible height for the existence of reef-builders (germatypes) at a given sea level ^[30] ;

2) elevated - located above, in its structure are well-defined hermatotype corals above the upper limit of their existence ^[31] ;

3) submerged - either dead, due to tectonic lowering, plunged to a depth where reef-building organisms cannot exist, or living, located below the water's edge, with a peak that does not dry out at low tide ^{[32] [33]} .

In relation to the coastline, reefs are divided into:

• fringing or coastal reefs
• barrier reefs
• atolls
• intra-lagoon reefs - patch reefs, pinnack reefs and coral hills. Isolated buildings that rise above the bottom in the form of hills and ridges. They are formed by fast-growing coral colonies Acropora , Stylophora , Pontes , etc. Intralagoon branching colonies have thinner and more easily broken branches compared to similar corals that live outside the lagoon. Between dead branches, mollusks , echinoderms , polychaetes quickly settle, the surface is covered with crusts of calcareous algae. Clefts and niches serve as a refuge for fish ^[34] .

The coral reef ecosystem is divided into zones that represent different types of habitat. Usually there are several zones: lagoon, reef-flat, inner slope and outer reef (reef rock) ^[35] . All zones are ecologically interconnected. Life on the reef and ocean processes create opportunities for constant mixing of water, sediment, nutrients and organisms.

The outer slope faces the open sea, is composed of coral limestone, covered with live corals and algae. Usually consists of an inclined platform in the lower part and the upper zone of spurs and hollows ^[36] or spurs and channels ^[37] . The outer slope is crowned with a ridge rising above sea level, and a relatively flat calcareous plain - reef-flat - stretches behind it. The crest is the site of the most active coral growth. Reef-flat is divided into external, internal and zone of block accumulation or rampart (solid shaft of cemented blocks with ravines). The inner slope of the reef passes to the bottom of the lagoon, where coral and halimed sand and silt accumulate and intra-lagoon reefs are formed ^[36] .

Living corals are colonies of polyps with a calcareous skeleton. Usually these are tiny organisms, however some species reach 30 cm across. A coral colony consists of numerous polyps connected to the colony's common body with lower ends. Colonial polyps have no sole ^[38] .

Reef-forming polyps live exclusively in the euphotic zone at a depth of up to 50 m. The polyps themselves are not capable of photosynthesis, but they live in symbiosis with algae symbiodiniums . These algae live in the tissues of the polyp and produce organic nutrients. Thanks to symbiosis, corals grow much faster in clear water, where more light penetrates. Without algae, growth would be too slow for large coral reefs to form. Corals receive up to 90% of their nutrition through symbiosis ^[39] . In addition, it is believed that the oxygen contained in the waters washing the Great Barrier Reef is not enough for polyps to breathe, therefore, without algae producing oxygen, most corals would die from a lack of oxygen ^[40] . The production of photosynthesis on coral reefs reaches 5–20 g / cm² per day, which is almost 2 times higher than the volume of primary phytoplankton production in the surrounding waters ^[35] .

Reefs grow due to the deposition of calcareous skeletons of polyps. Waves and animals that feed on polyps ( sponges , parrot fish , sea ​​urchins ) destroy the calcareous structure of the reef, which is deposited around the reef and on the bottom of the lagoon in the form of sand. Many other organisms of the reef biocenosis contribute to the deposition of calcium carbonate in the same way ^[41] . Coralline algae strengthens corals, forming a calcareous crust on the surface.

Species of Coral

In general, hard corals that form a reef can be divided into branchy brittle (madrepor) and massive, rocky (brain and mendrine corals). Branched corals are usually found on a shallow and flat bottom. They are painted in blue, pale lilac, purple, red, pink, light green and yellow. Sometimes the tops have a contrasting color, for example, green branches with lilac tops.

Brain corals can reach more than 4 meters in diameter. They live at a greater depth compared to branchy. The surface of the brain coral is covered with meandering crevices. Brown predominates in color, sometimes in combination with green. Dense porites form a kind of bowl, the base of which consists of dead corals, and living ones are located at the edges. The edges grow, increasingly increasing the diameter of the bowl, which can reach 8 m. Live porite colonies are painted in pale purple, the tentacles of the polyps are greenish-gray.

At the bottom of the bays, individual mushroom-shaped corals sometimes come across. Their lower flat part fits snugly to the bottom, and the upper consists of vertical plates converging in the center of the circle. Mushroom coral, unlike branched and massive hard corals, which are colonies, is an independent living organism. In each such coral, only one polyp lives, whose tentacles reach a length of 7.5 cm. Mushroom-shaped corals are painted in greenish and brownish colors. The coloration persists even when the polyp draws in the tentacles ^[40] .

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  Brain Corals

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  Acropora cervicornis

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  Porite coral

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  Mushroom coral

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  Meandrina meandrites

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  Black coral

The basis of the biocenosis of the vast majority of coral reefs is composed of six-rayed madreporic corals ( Acroporidae , Poritidae , Pocilloporidae , Faviidae , etc.), eight-rayed corals ( Tubiporidae , Helioporidae ), hydrocoral Milliporidae, and Coralitolitholithidae, ³⁶ , algae , Lithium littora . Reefs dominated by algae, sponges, and soft corals are under development ^[42] .

Coral reefs form one of the most important ecosystems of the oceans . They are characterized by extremely high productivity. Autotrophic photosynthetic production of the reef ecosystem ranges from 50-300 g of raw biomass per 1 m ² per day. Unlike other highly productive ocean communities in the reef ecosystem, it remains unchanged year-round. The three-dimensional spatial structure of reefs provides high primary production, which in turn supplies energy to dense communities of heterotrophs ( benthos , zooplankton and fish). In the coral reef zone, up to 9% of the world's total fish stocks are concentrated. In addition, it serves as a place for spawning and maturation of juveniles of many pelagic fish ^[2] .

Fringing reefs below the low tide benefit from interactions with mangroves above low tide and thickets of sea grass in the intermediate zone. Reefs protect mangroves and algae from strong tidal currents and waves that can erode the soil in which they are rooted, and the latter in turn prevent siltation, abundant fresh gutter and pollution. Many coral reef inhabitants feed on algae and find shelter and breeding conditions on the reef ^[43] .

A variety of fish, seabirds , sponges , gnawing intestines , worms , crustaceans , mollusks , echinoderms , ascidia , sea ​​turtles and snakes live on the reef. With the exception of humans and dolphins, mammals rarely visit reefs ^[5] . Reef biomass is directly dependent on biodiversity ^[44] .

The night and day inhabitants of the reef are very different from each other: the same shelters can be regularly settled by different species at different times of the day.

Some coral reef dwellers provide people with valuable medicines. So, an extract from ascidia is widely used in the fight against viral infections, and a drug for the treatment of skin cancer is made from a substance that protects polyps from the sun.

The coral reefs of the Red Sea, the Caribbean and the northeast coast of Australia have long become a tourist mecca and a source of income for the local population.

• Coral island
• Reef fish

• Naumov D.V. , Propp M.V. , Rybakov S.N. Corral World. - L .: Gidrometeoizdat , 1985 .-- 360 p. - 100,000 copies.

• Biologists have compiled a list of coral reef suicide bombers
• Coral reefs and islands // Brockhaus and Efron Encyclopedic Dictionary : 86 tons (82 tons and 4 extra). - SPb. , 1890-1907.