The world's oceans cover more than 70% of the Earth's surface. It contains about 1.35 billion cubic kilometers of water, which is about 97% of all the water on the planet. The ocean supports all life on the planet and also makes it blue when viewed from space. Earth is the only planet in our solar system, which is known to contain liquid water.

Although the ocean is one continuous body of water, oceanographers have divided it into four main regions: Pacific, Atlantic, Indian and Arctic. Atlantic, Indian and Pacific Oceans merge into icy waters around Antarctica. Some experts identify this area as the fifth ocean, most often called the Southern Ocean.

To understand ocean life, you must first know its definition. The phrase "marine life" covers all organisms living in salt water, which includes a wide variety of plants, animals and microorganisms such as bacteria and.

There is a huge variety of marine species that range from tiny single-celled organisms to giant blue whales. As scientists discover new species, learn more about the genetic makeup of organisms, and study fossil specimens, they decide how to group ocean flora and fauna. The following is a list of the major types or taxonomic groups of living organisms in the oceans:

  • (Annelida);
  • (Arthropoda);
  • (Chordata);
  • (Cnidaria);
  • Ctenophores ( Ctenophora);
  • (Echinodermata);
  • (Mollusca)
  • (Porifera).

There are also several types of marine plants. The most common ones include Chlorophyta, or green algae, and Rhodophyta, or red algae.

Marine Life Adaptations

From the perspective of a land animal like us, the ocean can be a harsh environment. However, marine life is adapted to life in the ocean. Characteristics that help organisms thrive in marine environment, include the ability to regulate salt intake, organs for obtaining oxygen (for example, fish gills), resist high blood pressure water, adaptation to lack of light. Animals and plants that live in the intertidal zone deal with extreme temperatures, sunlight, wind and waves.

There are hundreds of thousands of species sea ​​life, from tiny zooplankton to giant whales. The classification of marine organisms is very variable. Each is adapted to its specific habitat. All oceanic organisms are forced to interact with several factors that do not pose problems for life on land:

  • Regulating salt intake;
  • Obtaining oxygen;
  • Adaptation to water pressure;
  • Waves and changes in water temperature;
  • Getting enough light.

Below we look at some ways to survive marine flora and fauna in this environment, which is very different from ours.

Salt regulation

Fish can drink salt water and remove excess salt through the gills. Seabirds also drink seawater, and excess salt is removed through "salt glands" into the nasal cavity and then shaken out by the bird. Whales do not drink salt water, but receive the necessary moisture from their bodies, which they feed on.

Oxygen

Fish and other organisms that live underwater can obtain oxygen from the water either through their gills or through their skin.

Marine mammals must come to the surface to breathe, so whales have breathing holes on the top of their heads, allowing them to inhale air from the atmosphere while keeping most of their body submerged.

Whales are able to remain underwater without breathing for an hour or more, as they use their lungs very efficiently, filling up to 90% of their lung capacity with each breath, and also store unusually a large number of oxygen in the blood and muscles during diving.

Temperature

Many ocean animals are cold-blooded (ectothermic), and their internal body temperature is the same as their environment. The exception is warm-blooded (endothermic) marine mammals, which must maintain a constant body temperature regardless of water temperature. They have a subcutaneous insulating layer consisting of fat and connective tissue. This layer subcutaneous fat allows them to maintain their internal body temperature approximately the same as that of their terrestrial relatives, even in the cold ocean. The bowhead whale's insulating layer can be more than 50 cm thick.

Water pressure

In the oceans, water pressure increases by 15 pounds per square inch every 10 meters. While some sea ​​creatures rarely change water depth, long-swimming animals such as whales, sea turtles and seals travel from shallow waters to great depths in a few days. How do they cope with pressure?

It is believed that the sperm whale is capable of diving more than 2.5 km below the ocean surface. One adaptation is that the lungs and chest shrink when diving to great depths.

The leatherback sea turtle can dive to more than 900 meters. Folding lungs and a flexible shell help them withstand high water pressure.

Wind and waves

Intertidal animals do not need to adapt to high water pressure, but must withstand strong wind and wave pressure. Many invertebrates and plants in this region have the ability to cling to rocks or other substrates and also have hard protective shells.

While large pelagic species such as whales and sharks are not affected by storms, their prey may be displaced. For example, whales hunt copepods, which can be scattered across different remote areas during strong wind and waves.

sunlight

Organisms that require light, such as tropical coral reefs and their associated algae, are found in shallow, clear waters easily transmitting sunlight.

Because underwater visibility and light levels can change, whales do not rely on vision to find food. Instead, they find prey using echolocation and hearing.

In the depths of the ocean abyss, some fish have lost their eyes or pigmentation because they simply are not needed. Other organisms are bioluminescent, using light-producing organs or their own light-producing organs to attract prey.

Distribution of life in the seas and oceans

From the coastline to the deepest seabed, the ocean is teeming with life. Hundreds of thousands of marine species range from microscopic algae to the blue whale that has ever lived on Earth.

The ocean has five main zones of life, each with unique adaptations of organisms to its particular marine environment.

Euphotic zone

The euphotic zone is sunlit top layer ocean, up to approximately 200 meters in depth. The euphotic zone is also known as the photic zone and can be present in both lakes with seas and the ocean.

Sunlight in the photic zone allows the process of photosynthesis to occur. is the process by which some organisms convert solar energy and carbon dioxide from the atmosphere into nutrients (proteins, fats, carbohydrates, etc.) and oxygen. In the ocean, photosynthesis is carried out by plants and algae. Seaweeds are similar to land plants: they have roots, stems and leaves.

Phytoplankton, microscopic organisms that include plants, algae and bacteria, also live in the euphotic zone. Billions of microorganisms form huge green or blue patches in the ocean, which are the foundation of oceans and seas. Through photosynthesis, phytoplankton are responsible for producing almost half of the oxygen released into the Earth's atmosphere. Small animals such as krill (a type of shrimp), fish and microorganisms called zooplankton all feed on phytoplankton. In turn, these animals are eaten by whales, large fish, seabirds and humans.

Mesopelagic zone

The next zone, extending to a depth of about 1000 meters, is called the mesopelagic zone. This zone is also known as the twilight zone because the light within it is very dim. The lack of sunlight means that there are virtually no plants in the mesopelagic zone, but large fish and whales dive there to hunt. The fish in this area are small and luminous.

Bathypelagic zone

Sometimes animals from the mesopelagic zone (such as sperm whales and squid) dive into the bathypelagic zone, which reaches depths of about 4,000 meters. The bathypelagic zone is also known as the midnight zone because light does not reach it.

Animals that live in the bathypelagic zone are small, but they often have huge mouths, sharp teeth and expanding stomachs that allow them to eat any food that falls into their mouths. Much of this food comes from the remains of plants and animals descending from the upper pelagic zones. Many bathypelagic animals do not have eyes because they are not needed in the dark. Because the pressure is so high, it is difficult to find nutrients. Fish in the bathypelagic zone move slowly and have strong gills to extract oxygen from the water.

Abyssopelagic zone

The water at the bottom of the ocean, in the abyssopelagic zone, is very salty and cold (2 degrees Celsius or 35 degrees Fahrenheit). At depths of up to 6,000 meters, the pressure is very strong - 11,000 pounds per square inch. This makes life impossible for most animals. The fauna of this zone, in order to cope with the harsh conditions of the ecosystem, has developed bizarre adaptive features.

Many animals in this zone, including squid and fish, are bioluminescent, meaning they produce light through chemical reactions in their bodies. For example, the anglerfish has a bright appendage located in front of its huge, toothy mouth. When the light attracts small fish, the anglerfish simply snaps its jaws to eat its prey.

Ultra Abyssal

The deepest zone of the ocean, found in faults and canyons, is called the ultra-abyssal. Few organisms live here, such as isopods, a type of crustacean related to crabs and shrimp.

Such as sponges and sea cucumbers thrive in the abyssopelagic and ultra-abyssal zones. Like many starfish and jellyfish, these animals depend almost entirely on the settling remains of dead plants and animals called marine detritus.

However, not all bottom dwellers depend on marine detritus. In 1977, oceanographers discovered a community of creatures on the ocean floor feeding on bacteria around openings called hydrothermal vents. These vents lead hot water, enriched with minerals from the depths of the Earth. The minerals feed unique bacteria, which in turn feed animals such as crabs, clams and tube worms.

Threats to marine life

Despite relatively little understanding of the ocean and its inhabitants, human activity has caused enormous harm to this fragile ecosystem. We constantly see on television and in newspapers that yet another marine species has become endangered. The problem may seem depressing, but there is hope and many things each of us can do to save the ocean.

The threats presented below are not in any particular order, as they are more pressing in some regions than others, and some ocean creatures face multiple threats:

  • Ocean acidification- If you've ever owned an aquarium, you know that the correct pH of the water is an important part of keeping your fish healthy.
  • Changing of the climate- we constantly hear about global warming, and for good reason - it negatively affects both marine and terrestrial life.
  • Overfishing is a worldwide problem that has depleted many important commercial species fish.
  • Poaching and illegal trade- despite laws passed to protect sea ​​creatures, illegal fishing still flourishes today.
  • Networks - marine species from small invertebrates to large whales may become entangled and die in abandoned fishing nets.
  • Garbage and pollution- various animals can become entangled in debris, as well as in nets, and oil spills cause enormous damage to most marine life.
  • Habitat loss- As the world's population grows, human pressure on coastlines, wetlands, kelp forests, mangroves, beaches, rocky shores and coral reefs, which are home to thousands of species, increases.
  • Invasive species - species introduced into a new ecosystem can cause serious harm to their native inhabitants, since due to the lack of natural predators they may experience a population explosion.
  • Seagoing vessels - ships can cause fatal damage to large marine mammals, and also create a lot of noise, carry invasive species, destroy coral reefs with anchors, and lead to the release of chemical substances into the ocean and atmosphere.
  • Ocean noise - there is a lot of natural noise in the ocean that is an integral part of this ecosystem, but artificial noise can disrupt the rhythm of life of many marine inhabitants.

Charles

Why do the oceans have "low productivity" in terms of photosynthesis?

80% of the world's photosynthesis occurs in the ocean. Despite this, the oceans also have low productivity- they cover 75% earth's surface, but of the annual 170 billion tons of dry weight recorded as a result of photosynthesis, they provide only 55 billion tons. Aren't these two facts that I encountered separately contradictory? If the oceans fix 80% of the total C O X 2 " role="presentation" style="position: relative;"> C O X C O X 2 " role="presentation" style="position: relative;"> C O X 2 " role="presentation" style="position: relative;"> 2 C O X 2 " role="presentation" style="position: relative;"> C O X 2 " role="presentation" style="position: relative;">C C O X 2 " role="presentation" style="position: relative;">O C O X 2 " role="presentation" style="position: relative;">X C O X 2 " role="presentation" style="position: relative;">2 fixed by photosynthesis on the ground and releases 80% of total number O X 2 " role="presentation" style="position: relative;"> O X O X 2 " role="presentation" style="position: relative;"> O X 2 " role="presentation" style="position: relative;"> 2 O X 2 " role="presentation" style="position: relative;"> O X 2 " role="presentation" style="position: relative;">O O X 2 " role="presentation" style="position: relative;">X O X 2 " role="presentation" style="position: relative;">2 Released by photosynthesis on Earth, they must also have accounted for 80% of the dry weight. Is there a way to reconcile these facts? In any case, if 80% of photosynthesis occurs in the oceans, it hardly seems low productivity - then why are the oceans said to have low primary productivity (many reasons are also given for this - that light is not available at all depths in the oceans, etc.)? More photosynthesis must mean more productivity!

C_Z_

It would be helpful if you could point out where you found these two statistics (80% of the world's productivity comes from the ocean, and the oceans produce 55/170 million tons of dry weight)

Answers

chocoly

First, we must know what are the most important criteria for photosynthesis; these are: light, CO 2, water, nutrients. docenti.unicam.it/tmp/2619.ppt Secondly, the productivity you are talking about should be called "primary productivity" and is calculated by dividing the amount of carbon converted per unit area (m2) by time. www2.unime.it/snchimambiente/PrPriFattMag.doc

Thus, due to the fact that oceans cover a large area of ​​the world, marine microorganisms can convert large amounts of inorganic carbon into organic carbon (the principle of photosynthesis). The big problem in the oceans is the availability nutrients; they tend to deposit or react with water or other chemicals, even though marine photosynthetic organisms are mostly found on the surface, where light is of course present. This consequently reduces the potential for photosynthetic productivity of the oceans.

WYSIWYG♦

MTGradwell

If the oceans fix 80% of the total CO2CO2 fixed by photosynthesis on earth, and release 80% of the total O2O2 fixed by photosynthesis on earth, they must also account for 80% of the resulting dry weight.

Firstly, what is meant by "O 2 released"? Does this mean that "O 2 is released from the oceans into the atmosphere, where it contributes to excess growth"? This cannot be the case since the amount of O2 in the atmosphere is fairly constant and there is evidence that it is significantly lower than in Jurassic times. In general, global O2 sinks should balance O2 sources or, if anything, slightly exceed them, causing current atmospheric CO2 levels to gradually increase at the expense of O2 levels.

So by "released" we mean "released by the process of photosynthesis at the moment of its action."

The oceans fix 80% of the total CO 2 fixed through photosynthesis, yes, but they also break it down at the same rate. For every algae cell that is photosynthetic, there is one that is dead or dying and is consumed by bacteria (which consume O2), or it itself consumes oxygen to maintain its metabolic processes at night. Thus, the net amount of O 2 released by the oceans is close to zero.

We must now ask what we mean by "performance" in this context. If a CO2 molecule becomes fixed due to algae activity, but then almost immediately becomes unfixed again, is that considered "productivity"? But blink and you'll miss it! Even if you don't blink, it's unlikely to be measurable. The dry weight of algae at the end of the process is the same as at the beginning. therefore, if we define "productivity" as "increase in algae dry mass", then the productivity would be zero.

For algae photosynthesis to have a sustainable effect on global CO 2 or O 2 levels, the fixed CO 2 must be incorporated into something less rapid than algae. Something like cod or hake, which can be collected and placed on tables as a bonus. "Productivity" usually refers to the ability of the oceans to replenish these things after harvest, and this is really small compared to the ability of the earth to produce repeat harvests.

It would be a different story if we viewed algae as potentially suitable for mass harvesting, so that its ability to grow like wildfire in the presence of fertilizer runoff from the land was seen as "productivity" rather than a profound nuisance. But that's not true.

In other words, we tend to define "productivity" in terms of what's good for us as a species, and algae tends to be not.

Charles

Why do the oceans have "low productivity" in terms of photosynthesis?

80% of the world's photosynthesis occurs in the ocean. Despite this, the oceans also have low productivity - they cover 75% of the earth's surface, but of the annual 170 billion tons of dry weight recorded through photosynthesis, they provide only 55 billion tons. Aren't these two facts that I encountered separately contradictory? If the oceans fix 80% of the total C O X 2 " role="presentation" style="position: relative;"> C O X C O X 2 " role="presentation" style="position: relative;"> C O X 2 " role="presentation" style="position: relative;"> 2 C O X 2 " role="presentation" style="position: relative;"> C O X 2 " role="presentation" style="position: relative;">C C O X 2 " role="presentation" style="position: relative;">O C O X 2 " role="presentation" style="position: relative;">X C O X 2 " role="presentation" style="position: relative;">2 fixed by photosynthesis on earth and releases 80% of the total O X 2 " role="presentation" style="position: relative;"> O X O X 2 " role="presentation" style="position: relative;"> O X 2 " role="presentation" style="position: relative;"> 2 O X 2 " role="presentation" style="position: relative;"> O X 2 " role="presentation" style="position: relative;">O O X 2 " role="presentation" style="position: relative;">X O X 2 " role="presentation" style="position: relative;">2 Released by photosynthesis on Earth, they must also have accounted for 80% of the dry weight. Is there a way to reconcile these facts? In any case, if 80% of photosynthesis occurs in the oceans, it hardly seems low productivity - then why are the oceans said to have low primary productivity (many reasons are also given for this - that light is not available at all depths in the oceans, etc.)? More photosynthesis must mean more productivity!

C_Z_

It would be helpful if you could point out where you found these two statistics (80% of the world's productivity comes from the ocean, and the oceans produce 55/170 million tons of dry weight)

Answers

chocoly

First, we must know what are the most important criteria for photosynthesis; these are: light, CO 2, water, nutrients. docenti.unicam.it/tmp/2619.ppt Secondly, the productivity you are talking about should be called "primary productivity" and is calculated by dividing the amount of carbon converted per unit area (m2) by time. www2.unime.it/snchimambiente/PrPriFattMag.doc

Thus, due to the fact that oceans cover a large area of ​​the world, marine microorganisms can convert large amounts of inorganic carbon into organic carbon (the principle of photosynthesis). A big problem in the oceans is nutrient availability; they tend to deposit or react with water or other chemicals, even though marine photosynthetic organisms are mostly found on the surface, where light is of course present. This consequently reduces the potential for photosynthetic productivity of the oceans.

WYSIWYG♦

MTGradwell

If the oceans fix 80% of the total CO2CO2 fixed by photosynthesis on earth, and release 80% of the total O2O2 fixed by photosynthesis on earth, they must also account for 80% of the resulting dry weight.

Firstly, what is meant by "O 2 released"? Does this mean that "O 2 is released from the oceans into the atmosphere, where it contributes to excess growth"? This cannot be the case since the amount of O2 in the atmosphere is fairly constant and there is evidence that it is significantly lower than in Jurassic times. In general, global O2 sinks should balance O2 sources or, if anything, slightly exceed them, causing current atmospheric CO2 levels to gradually increase at the expense of O2 levels.

So by "released" we mean "released by the process of photosynthesis at the moment of its action."

The oceans fix 80% of the total CO 2 fixed through photosynthesis, yes, but they also break it down at the same rate. For every algae cell that is photosynthetic, there is one that is dead or dying and is consumed by bacteria (which consume O2), or it itself consumes oxygen to maintain its metabolic processes at night. Thus, the net amount of O 2 released by the oceans is close to zero.

We must now ask what we mean by "performance" in this context. If a CO2 molecule becomes fixed due to algae activity, but then almost immediately becomes unfixed again, is that considered "productivity"? But blink and you'll miss it! Even if you don't blink, it's unlikely to be measurable. The dry weight of algae at the end of the process is the same as at the beginning. therefore, if we define "productivity" as "increase in algae dry mass", then the productivity would be zero.

For algae photosynthesis to have a sustainable effect on global CO 2 or O 2 levels, the fixed CO 2 must be incorporated into something less rapid than algae. Something like cod or hake, which can be collected and placed on tables as a bonus. "Productivity" usually refers to the ability of the oceans to replenish these things after harvest, and this is really small compared to the ability of the earth to produce repeat harvests.

It would be a different story if we viewed algae as potentially suitable for mass harvesting, so that its ability to grow like wildfire in the presence of fertilizer runoff from the land was seen as "productivity" rather than a profound nuisance. But that's not true.

In other words, we tend to define "productivity" in terms of what's good for us as a species, and algae tends to be not.

Oceans and seas occupy 71% (more than 360 million km2) of the Earth's surface. They contain about 1370 million km3 of water. Five huge oceans - Pacific, Atlantic, Indian, Arctic and Southern - are connected to each other through the open sea. In some parts of the Arctic and Southern Oceans, a permanently frozen continental shelf has formed, extending from the coast (shelf ice). In slightly warmer areas, the sea freezes only in winter, forming pack ice (large floating ice fields up to 2 m thick). Some marine animals use the wind to travel across the sea. Physalia ("Portuguese man-of-war") has a gas-filled bladder that helps catch the wind. Yantina releases air bubbles that serve as her float raft.

The average depth of water in the oceans is 4000 m, but in some ocean depressions it can reach 11 thousand m. Under the influence of wind, waves, tides and currents, ocean water is in constant motion. Waves raised by the wind do not affect deep water masses. This is done by the tides, which move water at intervals corresponding to the phases of the moon. Currents carry water between oceans. Surface currents, moving, slowly rotate clockwise in the Northern Hemisphere and counterclockwise in the Southern Hemisphere.

Ocean bottom:

Most of the ocean floor is flat, but in some places mountains rise thousands of meters above it. Sometimes they rise above the surface of the water in the form of islands. Many of these islands are active or extinct volcanoes. Through central part At the bottom of a number of oceans there are mountain ranges. They are constantly growing due to the outpouring of volcanic lava. Each new flow that carries rock to the surface of underwater ridges forms the topography of the ocean floor.

The ocean floor is mostly covered with sand or silt - they are brought by rivers. In some places there are hot springs, from which sulfur and other minerals are deposited. The remains of microscopic plants and animals sink from the surface of the ocean to the bottom, forming a layer of tiny particles (organic sediment). Under pressure from overlying water and new sediment layers, the loose sediment slowly turns into rock.

Oceanic zones:

In depth, the ocean can be divided into three zones. In the sunny surface waters above - the so-called photosynthetic zone - most ocean fish swim, as well as plankton (a community of billions of microscopic creatures that live in the water column). Beneath the photosynthesis zone lie the dimly lit twilight zone and the deep, cold waters of the gloom zone. Fewer life forms are found in the lower zones - mainly carnivorous (predatory) fish live there.

In most of the ocean water the temperature is approximately the same - about 4 °C. As a person dives deeper, the pressure of water on him from above constantly increases, making it difficult to move quickly. On great depths, in addition, the temperature drops to 2 °C. The light becomes less and less until finally, at a depth of 1000 m, complete darkness reigns.

Life at the surface:

Plant and animal plankton in the photosynthesis zone is food for small animals, such as crustaceans, shrimp, and juveniles starfish, crabs and other marine life. Away from sheltered coastal waters animal world less diverse, but many fish live here and large mammals- for example, whales, dolphins, porpoises. Some of them (baleen whales, giant sharks) feed by filtering water and ingesting plankton contained in it. Others (white sharks, barracudas) prey on other fish.

Life in the depths of the sea:

In cold, dark waters ocean depths hunting animals are able to detect the silhouettes of their victims in the dimmest light, barely penetrating from above. Here, many fish have silvery scales on their sides: they reflect any light and camouflage the shape of their owners. Some fish, flat on the sides, have a very narrow silhouette, barely noticeable. Many fish have huge mouths and can eat prey that is larger than them. Howliods and hatchetfish swim with their large mouths open, grabbing whatever they can along the way.

From the surface to the very bottom, the ocean is alive with the life of a variety of animals and plants. Just like on land, almost all life here depends on plants. The main food is billions of microscopic plants called phytoplankton, which are carried by currents. Using sun rays, they create their food from sea, carbon dioxide and minerals. During this process, called photosynthesis, phytoplankton produce 70% of atmospheric oxygen. Phytoplankton consists mainly of small plants called diatoms. In a cup sea ​​water there can be up to 50 thousand. Phytoplankton can only live near the surface where there is enough light for photosynthesis. Another part of plankton - zooplankton - does not participate in photosynthesis and therefore can live deeper. Zooplankton are tiny animals. They feed on phytoplankton or eat each other. Zooplankton includes juveniles - larvae of crabs, shrimp, jellyfish and fish. Most of them do not look like adults at all. Both types of plankton serve as food for fish and other animals - from small jellyfish to huge whales and sharks. The amount of plankton varies from place to place and from season to season. Most plankton are found on the continental shelf and at the poles. Krill is a type of zooplankton. Most krill are found in the Southern Ocean. Plankton also lives in fresh waters. If you can, look at a drop of water from a pond or river or a drop of sea water under a microscope

Food chains and pyramids

Animals eat plants or other animals and themselves serve as food for other species. More than 90% of sea inhabitants end their lives in the stomachs of others. All life in the ocean is thus connected into a huge food chain, starting with phytoplankton. To feed one large animal, you need many small ones, so there are always fewer large animals than small ones. This can be depicted as a food pyramid. To increase its weight by 1 kg, tuna needs to eat 10 kg of mackerel. To obtain 10 kg of mackerel you need 100 kg of young herring. For 100 kg of young herring you need 1000 kg of zooplankton. To feed 1000 kg of zooplankton, you need 10,000 kg of phytoplankton.

ocean floors

The thickness of the ocean can be divided into layers, or zones, according to the amount of light and heat that penetrates from the surface (see also the article ““). The deeper the zone, the colder and darker it is. All plants and most animals are found in the top two zones. The sunny zone gives life to all plants and a wide variety of animals. IN twilight zone Only a little light from the surface penetrates. The largest inhabitants here are fish, squid and octopuses. In the dark zone it is about 4 degrees Celsius. Animals here feed mainly on the “rain” of dead plankton that falls from the surface. The abyssal zone is completely dark and icy cold. The few animals that live there live under constant high blood pressure. Animals are also found in ocean depressions, at depths of more than 6 km from the surface. They feed on what falls from above. About 60% deep sea fish have their own glow to find food, detect enemies and give signals to relatives.

Coral reefs


Coral reefs found in shallow waters in warm, clear tropical waters. They are made up of the skeletons of small animals called coral polyps. When old polyps die, new ones begin to grow on their skeletons. The oldest reefs began to grow many thousands of years ago. One type of coral reef is an atoll, which is shaped like a ring or a horseshoe. The formation of atolls is shown below. Coral reefs began to grow around volcanic island. After the volcano subsided, the island began to sink to the bottom. The reef continues to grow as the island sinks. A lagoon forms in the middle of the reef salt Lake). When the island sank completely, the coral reef formed an atoll - a ring reef with a lagoon in the middle. Coral reefs are more diverse in life than other parts of the ocean. A third of all ocean fish species are found there. The largest is the Great Barrier Reef on the east coast of Australia. It stretches for 2027 km and shelters 3000 species