The pressure in the deep sea is enormous. A submarine will be crushed if it exceeds the depth. So why can deep-sea fish move freely?

If a submarine with steel and iron frames dives deep, it will be crushed by the high pressure. Why can deep-sea fish live so freely under high pressure? This is a scientific question and it takes a long time to explain.

Let's talk about the concept of pressure and pressure first

Pressure refers to the force between the contact surfaces of two objects, or the vertical force of gas on the surface of liquid and solid, or the vertical force of liquid relative to the surface of solid. The force of pressure on an object is expressed in terms of pressure. For example, the sea level is 1 atmosphere, which is 101325Pa (Pascal), or 101.325kPa (kilopascal), 1013.25mPa (millibar or hectopascal).

The force exerted on a certain unit area is called pressure. The pressure of 1 atmosphere is 1kg/cm^2, which means that 1 kilogram of pressure is required on 1 square centimeter. If relative to 1m^2 (square meter), the pressure of 1 atmosphere is 10,000 kilograms.

Humans and all animals on the earth have evolved under this kind of pressure and pressure environment for a long time, so these animals have pressure corresponding to this pressure in their bodies. Of course, the pressure that animals in different regions bear is not very precise and uniform. The atmospheric pressure will change according to the altitude. The higher the altitude, the lower the pressure, and the lower the altitude, the greater the pressure.

Therefore, animals that survive on plateaus will not be able to adapt to living in plains or basins; conversely, animals from basins and plains will not be able to adapt to living in plateaus.

There is also a range for humans' ability to withstand pressure. For example, the atmospheric pressure on the Qinghai-Tibet Plateau is only 0.5 to 0.6 of that at sea level. People who live there for a long time can tolerate it, but people from lower altitudes will have altitude sickness when they go there. Similarly, people who were born and raised on the Qinghai-Tibet Plateau will also feel dizzy and intoxicated by oxygen when they come to the plains.

But human beings have a certain tolerance. For example, when diving to a depth of 10 meters, the pressure reaches 2 atmospheres (1 atmosphere of air pressure at sea level, and 1 atmosphere of pressure increases for every 10 meters of water depth), that is, the pressure reaches 2 kg/cm^2, which humans can also withstand. Trained people can dive to a depth of more than 100 meters, which is more than 10 atmospheres of pressure.

But it can't bear it if it goes deeper. The empty space such as the chest and abdomen will be crushed, and even the bones will be crushed and die. So people make submarines to protect themselves. Ordinary submarines can dive to a depth of several hundred meters, and military submarines can dive to a depth of 1,250 meters. But it can't bear it if it goes deeper. The steel plates will also be crushed and collapse.

Deep-sea submersibles can reach the deepest point of the Mariana Trench, 11,000 meters deep, where the atmospheric pressure reaches more than 1,100. This not only requires the submarine to have thick enough steel plate armor, but also to have a very high level of manufacturing technology to withstand such a high pressure.

So, deep-sea fish are mortal beings, so why can they withstand such great pressure?

The reason why deep-sea fish can resist deep-sea pressure is the result of evolution

The essence of species evolution is natural selection and survival of the fittest. Specifically, it means that only by adapting to the environment and the needs of competition can one survive and reproduce in the cruel environment and competition. Deep-sea fish are bred and evolved in the deep sea environment, so they can adapt to this environment.

Deep-sea fish have evolved to the point where they are fundamentally different from shallow-sea and shallow-water fish in terms of appearance, concrete macroscopic structures such as bones, and microscopic structures down to the cellular level.

First, let's talk about the microscopic structural characteristics of deep-sea fish.

Deep-sea fish have been living in a high-pressure dark environment from primitive cells to multicellular organisms, so their body structure is designed for this environment. Studies have found that deep-sea fish have more unsaturated fatty acids on their cell membranes, which enables the cell membranes to maintain high levels of fluidity in high-pressure environments and improves the efficiency of transporting nutrients to maintain sufficient life systems.

Intuitively speaking, the so-called unsaturated fatty acids are the kind of fatty acids that are easy to solidify and clump. For example, animal fats and oils become solid at room temperature because they contain too many unsaturated fatty acids. Vegetable oils contain more unsaturated fatty acids, so they are not easy to solidify and can remain fluid even in winter.

A high proportion of unsaturated fatty acids allows deep-sea fish to still have soft cell membranes even in the high-pressure environment of cold deep-sea water, so the cells are not easily damaged. At the same time, the protein of deep-sea fish also has pressure-resistant characteristics. For example, the α-actin in its body undergoes amino acid substitutions at multiple sites, including the binding sites for calcium ions and ATP, which can ensure that the actin works normally under high-pressure environments.

In addition, the number and types of chemical bonds in the protein of deep-sea fish have also changed, resulting in changes in the tertiary structure of the protein, which strengthens the rigidity of the structure and improves its adaptability to high-pressure environments; the trimethylamine oxide content in deep-sea fish is also much higher than that in shallow-sea fish. This is an important protein stabilizer that can help proteins restore their original structure and function, thereby ensuring cell activity.

However, trimethylamine is the main source of fishy smell. After the death of deep-sea fish, trimethylamine is oxidized and decomposed into trimethylamine. Therefore, deep-sea fish smell more fishy than shallow-sea fish after death.

Macroscopic structural characteristics of deep-sea fish

We know that gas is very easy to compress, while liquid or solid is more difficult to compress. Therefore, deep-sea fish first abandoned the swim bladder in their evolution. The so-called swim bladder is the fish bubble we see in the abdominal cavity when we slaughter diving fish. It is a "tool" used by shallow-water fish to adjust their ascent and descent. There is gas in the bubble. If you want to rise, you fill it with more gas to make the fish bubble bigger, thereby gaining greater buoyancy; if you want to sink, you discharge the gas to make the fish bubble smaller and increase the body density.

If this fish bubble is in the deep sea, it will be crushed. After abandoning this fish bubble organ, deep-sea fish mainly use lipids in their bodies to adjust their buoyancy. Let's use an analogy. An empty glass bottle is easily crushed by the pressure of the deep sea, while a glass bottle filled with water will not break even if it sinks into the deep sea. This is because a relatively balanced pressure is formed inside and outside the bottle.

Deep-sea fish also have less bone and muscle content than shallow-sea fish, but relatively more lipids and gelatin, and the proportion of cartilage in their bones is much higher than that of shallow-sea fish. As the saying goes, too much rigidity will easily break, and deep-sea fish use cartilage and lipid gelatin to resist the pressure of the deep sea, so they can move freely under high pressure.

Therefore, deep-sea fish have the ability to resist high pressure, that is, the internal pressure and environmental pressure form a balance, so they can resist the high pressure in the deep sea. But real deep-sea fish can only live in the deep sea. If they swim to shallow water or are caught, they will suddenly lose the high pressure balance, and the cell substances of deep-sea fish will seep out from inside. Some will even burst due to the high pressure in their bodies and die soon.

Therefore, the deep-sea fish sold on the market are unlikely to be alive. Usually, ice storage is prepared on the fishing boat, and the fish are frozen and preserved immediately after being caught.

Several representative fish species in the sea at different depths

There are many kinds of deep-sea fish, but strictly speaking, some common fish on the market cannot be called deep-sea fish, such as tiger grouper, blue grouper, pink grouper, garfish, maca fish, red fish, etc. These fish generally live in the sea at a depth of tens of meters, and there are not many that are more than 100 meters. Therefore, some fish can be kept in the aquarium of the restaurant. These fish can only be called sea fish more accurately. Real deep-sea fish cannot be kept in an aquarium.

The deeper the water, the darker it is and the greater the pressure. Therefore, the fish that live in deeper waters look weirder and more terrifying, like alien species. Generally speaking, deep-sea fish have big eyes, big mouths, and sharp teeth. Some parts of the body of many deep-sea fish glow, which can seduce the same species to mate in the dark environment, and can also be used to lure prey.

Generally, at a depth of 200 meters, the weak light can no longer carry out photosynthesis, so there are no green plants at this depth, and there are not many fish. There is a fish called oarfish, commonly known as the sea dragon king; in the sea below 200 meters, there are some crabs, blue whales and emperor penguins, which can dive to a depth of up to 500 meters.

At 600 meters, it is basically a dark world with extremely weak light. There is a kind of post-atmospheric steel fish active here with a transparent head and eyes like balls hanging on the left and right halves of the head. The eyeballs only look up because there is no need to look down and they can't see anything at all. Below 700 meters live some deep-sea fish such as European hairtail and eel, and at a depth of 900 meters there are giant squids.

At 1,000 meters, it is a completely dark world, and there are even fewer fish at this depth. However, at a depth of 1,300 meters, there is a sea turtle called the leatherback turtle living here, and the European sharpnose shark also lives at this depth. As the depth increases, there are fewer and fewer fish, but the South China Sea elephant leopard can dive to 1,700 meters.

At 2,000 meters, there is a strange fish called the black dragon fish, which looks like an alien species, with two long whiskers underneath that glow and are used to attract prey; and the huge sperm whale can dive to a depth of 3,000 meters. Their main food is giant squid. According to statistics, sperm whales eat 5 million giant squids every year around the world.

At four thousand meters, there is a deep-sea fish called viperfish. It has long teeth and big eyes. It is not easy to catch prey in the dark deep sea, so they have evolved sharp long teeth. Once the prey is bitten, there is no way to escape.

Human diving depth

It is very rare to see fish in the deep sea below 4,000 meters, but in April 2017, a Chinese deep-sea exploration team saw a white fish at a depth of 8,150 meters; coincidentally, in August of the same year, a Japanese exploration team also found the same fish in the same place, and some people think it may be the same fish. In such deep waters, fish are already very rare, and people named this fish Mariana snailfish.

The deepest creature is a giant single-celled organism that lives at 10,660 meters below the sea. This single-celled organism is 20 centimeters long.

Humans have been trying to explore deeper waters, including diving without equipment and with equipment, as well as unmanned submarines and manned submarines. So far, the deepest depth of human unequipped diving is 113 meters, created by a French explorer named Jean Nery in 2006; the world's deepest depth of equipped diving is 332 meters, created by a 41-year-old Egyptian man, Ahmed Gabr, in 2014.

Human submersibles have been able to dive to the deepest part of the Mariana Trench. The person who has set the deepest record for manned submarines is the famous American director James Cameron. This filmmaker, who is known for directing blockbusters such as "Titanic", "Terminator" and "Avatar", has a strong childlike innocence and desire for adventure. In March 2012, he took the "Deepsea Challenge" submarine and dived to the bottom of the Mariana Trench at a depth of 10,929 meters, setting a world record for manned deep diving, which has not been broken so far.

China ranks second in manned submersible deep diving. On June 24, 2012, China's Jiaolong submersible conducted a deep diving experiment in the Mariana Trench, reaching a maximum depth of 7,062 meters; on November 10, 2020, the Fendouzhe successfully landed on the bottom of the Mariana Trench, reaching a depth of 10,909 meters.

The world record for unmanned deep-sea submersible is held by Japan's Kaitou, which reached a depth of 10,970 meters in 1995. China's developed submersible Kaidou dived to a depth of 10,767 meters on July 28, 2016, ranking second in the world.

This is a contest between humans and deep-sea animals and the special natural environment of deep-sea high pressure. With the further advancement of science and technology, human exploration and development of the ocean will continue to improve.

Why are deep-sea fish more popular?

Scarcity makes things valuable. Deep-sea fish are rare and difficult to catch. According to the law of value, they are naturally very popular with humans. In addition to the law of value, deep-sea fish have many advantages over shallow-sea fish.

First of all, deep-sea fish are cleaner and healthier seafood and are the main source of delicious dishes such as sashimi and fish sashimi because of the high salinity and pressure of the sea water, very few microorganisms such as parasites and bacteria, and industrial pollution has less impact on the deep sea.

Deep-sea fish contains more and higher nutrients that are beneficial to the human body. It is not only rich in protein, but also contains many special amino acids, fatty acids, DHA, etc. that the human body needs, such as unsaturated fatty acids, mineral trace elements such as calcium, iron, zinc, vitamins A, B, C, etc.

Therefore, eating more deep-sea fish can help promote balanced nutrition, regulate the balance of nutrients in the body, promote development, improve brain health, lower cholesterol, make blood vessels healthier, reduce the occurrence and risk of cardiovascular diseases, effectively control coronary heart disease, etc.

Some scientific institutions and enterprises extract various nutritional health products such as deep-sea fish oil from deep-sea fish to meet human health needs, which are very popular.

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