Gunfire, bullets and armor: the enchanting "assassin" of Nanki Shirahama, Japan

At 11 a.m. on July 8, 2022, at the Shirahama Aquarium on the coast of Shirahama Town, Japan, a peacock mantis shrimp smashed the aquarium twice; 30 minutes later, 120 kilometers away, next to Yamato-Saidaiji Station in Nara, a former Japanese politician was shot twice and fell to the ground.

For science communicators, there are theoretically plenty of topics available for assassination: you can use your knowledge of firearms to introduce shotguns, pistols, and homemade firearms. You can also combine ballistics and forensics to talk about the process of bullets entering and injuring the human body. You can also use scientific and technological historical materials to describe the origin and development of bulletproof vests. You can also use the principles of materials science and mechanics to popularize the "connection, transformation, and development" mechanism of bulletproof fabrics.

I chose to give the article an ambiguous title full of hints and a pretentious preface imitating the butterfly effect. This is because the protagonist of this article, the peacock mantis shrimp, perfectly combines all the scientific elements that can be strongly associated with the assassination: a Japanese assassin, a homemade pistol, a bullet that kills people, and a bulletproof vest that saves lives.

Written by | BiPuXianren

Produced by: Science Popularization China-Starry Sky Project

The MAX version of the mantis shrimp

Mantis shrimps are very popular among the public. They are eaten with relish by countless diners, are also used as mounts in emojis, and have even been paid tribute to by art masters. Many viewers learned from the movie "The God of Cookery" that mantis shrimps are also called sabre shrimps, and can be mixed with beef to make "bursting sabre beef balls". The name sabre shrimps comes from the fact that water will come out of their abdomens when they are caught.

Because mantis shrimp have mantis-like sickle-like forelimbs, English users call them "mantis shrimp", which literally means "mantis shrimp".

The peacock mantis shrimp and the mantis shrimp are close relatives, both belonging to the phylum Arthropoda, subphylum Crustacea, class Malacarides, subclass Prawns, order Stomatopoda (not strictly shrimps), but the former belongs to the family Odontodactylidae under the superfamily Gonodactyloidea, with the scientific name "Odontodactylus scyllarus"; the latter is a member of the family Squillidae under the superfamily Squilloidea, with the scientific name "Oratosquilla oratoria".

If you are not so particular, you can actually call the peacock mantis shrimp the peacock mantis shrimp.

Those standard shrimps, such as prawns, red shrimps, lobsters, etc., are distant relatives of mantis shrimps and belong to the class of soft armor - subclass of true soft armor - order of Decapoda.

Figure 1. Sketch of the external morphology of shrimp. Top: Common shrimp; Bottom: Mantis shrimp. [1]

When you see the peacock mantis shrimp for the first time, you will be moved by its sincerity, because its body, which is more than ten centimeters long, is as fancy as its name, like a miniature peacock spreading its tail, or like elongated rainbow amber.

A turquoise-like back, bright red and crimson legs and lower abdomen, red, orange, yellow, green, blue, indigo and white tentacles, dark blue eyestalks, and industrial punk-style big eyes.

Figure 2. Eye stalks are common in decapods, stomatopods, and other animals. They are rod-shaped and grow on the sides of the head, similar to antennae. The eyes mounted in the eye stalks can rotate freely. [2]

Photographers who encounter them will never miss the opportunity to freeze-frame their art, obsessive viewers will pull out their palettes to compare with them, and naive evolutionists speculate that their splendor is similar to that of peacocks, which attract the opposite sex and are monogamous.[3] But frankly, we are not sure whether they can appreciate their own charm in the same way that we do.

Seeing beauty but not judging color

The visual system of the peacock mantis shrimp can be said to be more industrial than punk.

Figure 3. The photoreceptor system of the mantis shrimp. [4]

Most mammals, including humans, have only three types of photoreceptors in their retinas, sensitive to red, green, and blue light. Peacock mantis shrimps have 12 different types of receptors in their eyes (some species have 16), eight of which cover the spectrum visible to humans and four in the ultraviolet. They can also sense polarized light.

Figure 4. The human eye’s photoreceptor channels are poorly matched compared to those of mantis shrimp. [4]

However, the luxurious retinal configuration, 12 photosensitive channels, does not give them excellent color resolution. Scientists have found that humans can distinguish the difference between colors with a wavelength difference of 5 nanometers, while mantis shrimps struggle in the range of 15-25 nanometers.

Figure 5. Humans can tell the difference between the left and middle columns with 50% accuracy, while mantis shrimp can only tell the difference between the right and middle columns with equal accuracy. [5]

The reason for this contrast is that their visual system focuses on "technical recognition" rather than "artistic appreciation."

The cones in the human eye are closely interconnected, forming a complex mechanism; differences in the responses of these cells to light, combined with the processing of the response signals by neural computing networks, allow us to perceive subtle differences in color.

In contrast, the cones of mantis shrimp "work independently of each other, without the need for complex neural calculations." When light-sensitive information leaves the retina and reaches the central nervous system, there will be "a large number of parallel data streams," allowing the visual headquarters to process several pieces of data at the same time. This type of visual system often has difficulty accurately distinguishing different colors, but can quickly identify the presence of colors, which helps mantis shrimps keenly detect natural enemies or prey [4, 6].

Mantis Gun

For an assassin, sharpness alone is not enough. A homemade pistol is the key to killing someone in a crowd of people.

The peacock mantis shrimp has two pistols, namely its two hammer-like raptorial appendages/legs.

The Mantis Gun works in much the same way as a bow and arrow.

The archer's hand moves in the opposite direction of the arrow, causing the bow string to accumulate elastic potential energy, and then releases it, converting the potential energy of the string into the kinetic energy of the arrow. The mantis gun uses a more complex mechanism to achieve the power of a compound bow.

Figure 6. Schematic diagram of the structure of the predatory forelimbs as an analogy to a mechanical structure. [7]

We can observe the operation of this group of mechanisms through the figure above. Since there are a lot of obscure arthropod-specific terms in the figure, for the sake of simplicity and popularization, we will temporarily put aside all biological concepts and only treat the predatory forelimbs as mechanical structures.

It is not difficult to see that the essence of the Mantis Gun lies in "one bullet, two hearts":

The "saddle" refers to the spring, which is responsible for providing power to the mechanism. The peacock mantis shrimp will use the strong contractile muscles of the predatory forelimbs (not shown in the picture) to greatly squeeze the spring, thereby storing up potential energy. At this time, the extensor muscles (shown in red in the picture) are also in a contracted state.

"Two centers" refers to two pivot points, the first pivot point and the second pivot point, which are the bottom end point (black dot) of "merus-v" and the connection point (white dot) of "carpus" and "merus-v" in the figure. If the tight spring is released, the potential energy will be released instantly, the extensor muscle will stretch, the merus-v will rotate counterclockwise around the first pivot point, and the carpus will rotate clockwise around the second pivot point, and the "sickle" fixed on the carpus, namely "propodus+dactyl", will swing rapidly, with the dactyl giving direct damage to the target.

Figure 7. When the saddle spring is released, the potential energy is immediately converted into kinetic energy, and the extensor muscles that were originally in a contracted state will stretch. [7]

Gather strength and attack. [8] [Please go to the "Fanpu" public account to watch the video]

How fast does the sickle swing? The data is the most intuitive [7, 9, 10]:

The acceleration after startup can reach 10400 times the acceleration of gravity, which is more than 105m/s2, comparable to a .22 caliber bullet and the piston in an F1 engine.

A complete swing takes about 2.7 milliseconds, with an average speed of 83 kilometers per hour; it takes humans 100-400 milliseconds to blink, but the peacock mantis shrimp can complete hundreds of kills in the blink of an eye.

The striking force can reach 1500 Newtons, thousands of times its own body weight, enough to easily penetrate the shell of prey and even break the aquarium glass; the striking force of a professional boxer is about 5000 Newtons.

It must be pointed out that the above data were all achieved underwater...

Video of a bullet coming out of a gun and a mantis shrimp punching [8] [Please go to the "Fanpu" public account to watch the video]

Soul-breaking Cavitation Bomb

At this point, we have to admire the mantis gun's terrifying firepower. However, since it is called a gun, it should be equipped with bullets.

The bullets of the mantis gun are called "cavitation bullets". They are not blank bullets or hollow bullets. They are cavitation bullets whose English name is "cavitation bubble".

Cavitation is a fluid mechanics phenomenon: when a liquid encounters a rapid change in pressure, a vapor cavity is formed inside it. The liquid then fills the cavity very quickly, so fast that the cavitation bubble collapses, or implodes, generating a violent shock wave.

Figure 8. The process of a cavitation bubble collapsing and impacting the plane below it [11]

Because the predatory forelimbs strike so fast, the water in the impact area evaporates instantly, forming a large number of cavitation bubbles, which then quickly implode collectively, completing the cavitation bullet killing.

Theoretically, the collapse of a cavitation bubble with a diameter of 2.7 mm can produce an impact with a strength exceeding 9 MPa (900 N/cm2). The cavitation bullet produced by the mantis gun is equivalent to a 500 N impact[12].

For the prey, even if they manage to withstand the fierce blow, they cannot escape the secondary blow of the cavitation bullet.

The fierce killing power helps the peacock mantis shrimp stand at the top of the marine food chain. Only killer whales, sharks, two- to three-meter-long bluefin tuna, one- to two-meter-long barracudas and other absolutely crushing creatures can impose sanctions on the peacock mantis shrimp of the size of mantis shrimp. In many cases, they will hunt objects larger than themselves, such as various crabs south of the Kii Peninsula in Japan, various large shrimps around Guam in the United States, and certain fish in the waters of Taiwan, China.

It should be pointed out here that the peacock mantis shrimp, which possesses the style of Japanese Bushido, not only lives near Japan, but is also widely distributed in the coastal waters of the Indian Ocean and the Pacific Ocean.

Figure 9. Global distribution of peacock mantis shrimp. The yellow area indicates the sea areas where they live. Nanki Shirahama is one of the main distribution areas of peacock mantis shrimp in Japan. [13]

Armor protects the gun from being hurt

At this point, the elements of "assassin", "shooting" and "bullet" in the title of the article have all been mentioned, and the only thing left is "armor".

Assassins also need to wear bulletproof vests, but the armor of peacock mantis shrimps is not worn on their bodies, nor is it used to defend against enemies, but is installed on the mantis gun. Without this pair of bulletproof kits, their assassination will be like the Seven Wounds Fist, killing the enemy and injuring themselves.

As mentioned above, the fighting style of the peacock mantis shrimp can be described as fierce and swift, and the premise of fierceness is that the striking part of the predatory forelimbs - that is, the "dactyl" that comes into direct contact with the target - is hard and heavy enough. Whether it is hard and heavy depends on the internal material structure. (The dactyl and propodus have been introduced in the previous article through pictures. You can think of the dactyl as the blade of a sickle or the hammer head of a hammer, and the propodus as the handle.)

Figure 10. Schematic diagram of the killing part of the predatory forelimb, which is the main body of the mantis gun. This part is composed of the dactyl and propodus. [6]

Scientists have discovered that the front end of the dactyl, the red "impact front" in the image above, contains a high concentration of minerals, mainly hydroxyapatite, and has a high degree of crystallinity. It is the hardest, most rigid and least deformable part of the entire structure, with an elastic modulus of up to 65-70GPa.

The elastic modulus is a physical quantity that describes the elasticity of a material. Its value reflects the ease with which a material deforms under the action of an external force. The larger the elastic modulus, the stiffer the material and the less likely it is to deform. Of course, there are many ways in which external forces can act, such as stretching outward, squeezing inward, or shearing laterally; the elastic modulus naturally also includes many types.

The elastic modulus here reflects the ease with which a material deforms under compression. The experimenters used the nanoindentation method to measure the compressive elastic modulus of the impact front.

The compressive elastic modulus of a copper block is about 130 GPa, or 1.3×1011 Pa, and the elastic modulus of a steel fiber is about 36 GPa. However, the elastic modulus of the impact front with a thickness of only 50-70 μm is as high as 70 GPa, which is rare. This level of material hardness certainly ensures that the material is both strong and powerful, and that every punch hits the flesh[6].

But we all know that the strongest things are easy to break, and being too strong can easily cause internal injuries and often cannot last long. While the mantis guns are fiercely attacking the enemy, they will also bear the reverse impact force. So how do they avoid being attacked by the reaction force?

The reason is that there are clearly layered buffer zones behind the impact front, which are the gun guards.

Using a variety of characterization methods, the researchers found that the material structure of the buffer zone is very different from that of the impact zone. The impact zone is mainly composed of apatite mineral, which is hard and brittle; the buffer zone is a combination of inorganic minerals and organic polymers, in which the inorganic material is mainly amorphous calcium phosphate and calcium bicarbonate, and the polymer is chitin, which is elastic and soft, dispersed in the inorganic material, responsible for absorbing impact energy and playing a shock-absorbing role[6].

Figure 11. The material structure of the transition zone is still mainly minerals, but the composition has changed compared to the impact front, and the elastic modulus has dropped to 35 GPa. The elastic moduli of the two buffer zones are only 5 GPa and 10 GPa. [6]

Figure 12. The chitin in the buffer zone forms a unique, elegant, and functional helical structure. [6]

To use an inappropriate analogy, minerals are like a large box of glass bottles. If the box falls to the ground, the glass will probably break. But if you fill the box with sponges, or chitin, the glass will not be easily damaged.

Thanks to its chitin spiral armor, the mantis shrimp is both hard and flexible, and has excellent damage resistance. It can withstand thousands of high-energy impacts and finally be refurbished during the mantis shrimp's periodic molting.[14]

The armor of the peacock mantis shrimp has also brought inspiration to the bulletproof field.

Researchers in the United States have designed a super-strong composite material made of carbon fiber layers, imitating the structure of this natural material. Test results show that the new material can absorb 15-20% more impact energy than commonly used materials in the aerospace industry[15]. In addition, they have also 3D printed a material structure inspired by the mantis gun[16].

David Kisailus, a biomimetic materials scientist and corresponding author of the study, has long been devoted to observing and copying peacock mantis shrimps, and has received a large grant from the US Department of Defense. "As we study the predatory forelimbs of mantis shrimps more and more, we realize that its structure can improve many things we use every day, such as stronger bulletproof vests, football helmets, airplanes and cars."

References

[1] https://aquariumbreeder.com/dwarf-shrimp-external-anatomy/#open

[2] https://www.thoughtco.com/mantis-shrimp-facts-4582442

[3] https://oceana.org/marine-life/peacock-mantis-shrimp/

[4] https://scholarblogs.emory.edu/artsbrain/2020/03/05/vision-in-mantis-shrimp/

[5] https://www.nationalgeographic.com/science/article/the-mantis-shrimp-sees-like-a-satellite

[6] https://www.science.org/doi/abs/10.1126/science.1245824

[7] https://www.nature.com/articles/428819a

[8] https://youtu.be/ti2Uoc1RXuQ

[9] https://journals.biologists.com/jeb/article/208/19/3655/15838/Extreme-impact-and-cavitation-forces-of-a

[10] https://ourmarinespecies.com/c-lobsters/mantis-shrimps/

[11] https://doi.org/10.1051/epjconf/201714302119

[12] https://arc.aiaa.org/doi/10.2514/3.8027

[13] https://kids.nationalgeographic.com/animals/invertebrates/facts/peacockmantisshrimp

[14] https://onlinelibrary.wiley.com/doi/10.1002/jmor.1051460104

[15] https://phys.org/news/2014-04-mantis-shrimp-stronger-airplanes.html

[16] https://onlinelibrary.wiley.com/doi/10.1002/adma.201600786

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