Shulou(Shulou.com)11/24 Report--

This article comes from the official account of Wechat: back to Park (ID:fanpu2019), author: Wulianhua

Sharks are not common economical fish. In some coastal areas of China, there is a tradition of eating small sharks, which can be made into fish balls or sour bamboo shoot soup. Can large sharks also eat? In July this year, an online celebrity staged a scene of eating a large shark and was arrested as a result of turning over.

However, people only know that she was arrested for eating protected animals, but they do not know that the happy expression of feasting and delight is the biggest lie.

Who believes who is stupid.

In a video circulated on the Internet last year, a Sichuan online celebrity blogger boiled and barbecued a shark about two meters long and praised the shark's delicacy. Due to the unique shape of the shark caudal fin in the video (not a general crooked shape but a fork), netizens strongly questioned. Finally, identified by the South China Sea Fisheries Institute of the Academy of Sciences by DNA, it is confirmed to be a national second-class protected animal-great white shark. Subsequently, the people's Procuratorate of Dongshan County in Fujian Province decided to approve the arrest of two people for selling the shark on suspicion of "endangering precious and endangered wild animals." the case is awaiting further sentencing.

We lament the demise of rare animals, and the harm caused by the video is much more than that. The exaggerated and false performance of the online celebrity blogger may lead people to mistakenly think that sharks taste similar to or more delicious than common meat, which is a complete departure from the common sense of life and the laws of science. Great white sharks are not fit to eat, and the scene shown in the video runs counter to the actual situation.

The scale-covered great white shark (Carcharodon carcharias) is an ancient fish and a ferocious top predator that not only hunts fish but also feeds on seabirds and marine mammals. Unlike our common fish on the market, great white sharks are cartilaginous fish. Except for the hard teeth, the bones of the whole body are made up of cartilage. It is scientifically classified as Lamnidae (Chondrichthidae).

The ancients had a long understanding of sharks. The six books of the Southern Song Dynasty said: "the sea is named for its skin such as sand, and the fetus is born, and the boat is full of the great ones." This may be a record of whale sharks (Rhincodon typus) or basking sharks (Cetorhinus maximus). Whale sharks are the largest living fish and often occur offshore. Gentle temperament, big mouth, filter feed on small animals and plants to make a living. Basking sharks are second only to whale sharks in size and also filter food with huge mouths. Like man-eating sharks, whale sharks and basking sharks are second-class protected animals in the country.

Figure 1. Three sources of second-class national protection of sharks: wikipedia, there is also a slight mistake in the records of the ancients that sharks are not scaleless. The scales of sharks, called placoid scale, are endemic to cartilaginous fishes and are the most primitive scales of existing fish. The shield scales are closely arranged on the shark skin and are very small. The length of a single scale is about 100 μ m and 200 μ m. Without the aid of a microscope, it is difficult to see that the ancients described sharks as "skin as sand" should be the conclusion drawn by reverse stroking of shark skin. Sharks are probably named because of their sand-like feel and taste.

Figure 2. Electron microscopic shark scales source: wikipedia shark shield scales and teeth are evolutionarily homologous and have similar tissue structure. The shield scale consists of a substrate buried in the skin and a scale spine exposed to the body surface. the outer part of the scale spine is covered with enamel produced by the epidermis, the dentin produced by the dermis inside, and the medullary cavity in the center. There is a hole in the center of the substrate, and there are blood vessels and nerves passing into the medullary cavity.

Figure 3. The structure and shape of shield scales [1] there are a large number of shield scales, which are closely embedded in the shark skin. Because the single scale is very small, the shark skin looks as if there are no fish scales. Stroking along the tail towards the head will produce a "sand" feel.

The morphology of shield scales varies with the species and distribution of sharks. The shield scale has a ribbed surface structure, and the width between the ribs is 50 μ m and 100 μ m. This rib structure optimizes the fluid structure of the fluid boundary layer on the shark's surface, suppresses and delays the occurrence of turbulence (also known as turbulence), and then reduces the resistance of the water body, thus reducing the energy consumption. this makes sharks one of the fastest swimming creatures in the ocean. This idea inspired humans to invent a swimsuit called "sharkskin", but it has been banned by FINA.

Figure 4. The scales of different parts of the small cat shark [2] the shield scales of the sharks also have excellent protective functions, and the tightly arranged shield scales enable the sharks to effectively prevent the attachment of marine fouling organisms. Barnacles, which are common in whales, are difficult to parasitize on the surface of sharks.

It is precisely because these scales are small, diverse and difficult to handle that the descaling process is particularly important for coastal residents when preparing small sharks, usually by scalding with hot water or peeling off the shark skin as a whole. On the other hand, the online celebrity videos cook shark skin without handling it at all, and it should taste like chewing sand, not as delicious as shown in the video.

So, won't it taste better to peel off the shark skin completely?

Special osmotic pressure regulation mechanism most sharks live in the sea. In order to adapt to the high osmotic environment of seawater, cartilaginous fish mainly rely on urea to regulate osmotic pressure. The urea content in shark blood is about 2% Mel 2.5%. When the urea content in the blood is high, the water permeates into the body, the blood concentration decreases, the micturition increases, and the urea loss increases. When the urea content in the blood decreases, the amount of water entering the body decreases, the urine volume decreases, and the urea concentration in the blood increases again, so that the osmotic pressure balance is maintained. In this process, the excess salt entering the body is excreted through the rectal gland (rectal gland). Thus it can be seen that urea is the main regulating factor of water dynamic balance of cartilaginous fish. Urea in sharks is broken down by urease secreted by bacteria after death to produce ammonia. Fermented shark meat, an Icelandic specialty, smells like urine dirt because urea is also found in human waste, and the smell of ammonia in toilets is not pleasant.

Figure 5. Source: www.du.edu.eg author Sinicization is just like uremia when humans are unable to expel urea, high concentrations of urea in cartilaginous fish can also lead to protein denaturation, but sharks have obviously solved this problem. The concentration of trimethylamine oxide (TMAO) in sharks is significantly higher than that in other fish. Trimethylamine oxide can antagonize urea and maintain the normal morphology of protein. After shark death, trimethylamine oxide reductase produced by bacteria reduces trimethylamine oxide to trimethylamine (TMA), which is the main source of fishy smell. Trimethylamine is a volatile malodorous substance, and bacteria continue to convert trimethylamine oxide to trimethylamine after shark death, so shark meat will smell more and more strongly with the increase of time of death. The great white shark shown in the video has been transported from Fujian to Sichuan, and its smell must be bad, not as pleasant as the video shows.

Trimethylamine oxide not only makes sharks smell bad after death, but also makes eating fresh shark meat dangerous.

Figure 6. Urea stretches the structure of the protein, and after adding TMAO, the protein will return to the folded state [3] for sharks, trimethylamine oxide (TMAO) can keep the protein folded and help maintain osmotic pressure and fight the cold environment, but for the human body, trimethylamine oxide and its metabolite trimethylamine are toxic to a certain extent, and excessive consumption can lead to symptoms similar to drunkenness and, in severe cases, death.

Chart metabolism of 7TMAO in the human body [4] after intake of food rich in trimethylamine oxide (TMAO), TMAO is converted to trimethylamine (TMA) by bacterial fermentation in the intestine, and then TMA is re-oxidized by Flavin monooxygenase 3 (FMO3) in the liver to tasteless trimethylamine oxide (TMAO).

Some studies suggest that once eating too much fresh shark meat (containing trimethylamine oxide), trimethylamine in the intestines will not have time to deal with, accumulate in the body, mild intestinal problems, serious cases resemble drunkenness, more serious loss of consciousness or life. This may be because after some processes, trimethylamine produces quaternary ammonium salts, neurotoxins similar to poisonous snails.

Therefore, Greenland locals will use "shark-sick" to describe drunken people, and Icelanders can only ferment the meat of fresh Greenland sleeping sharks for a long time to avoid food poisoning. Even so, the strong smell of ammonia makes Icelandic fermented shark meat (H á karl) the most disgusting food in the world-similar to a long-unwashed urinal.

Fig. 8 Iceland's fermented shark meat generally needs wine to dilute its odor. [6] some large sharks (such as bull sharks, great white sharks) are top predators in the sea. Great white sharks are not afraid of anyone but a few species of whales.

Fig. 9 A summary of the marine food web? source: Sally Palmer@utmsi.utexas.edu great white shark is in the position of top predator (top predator) and has a very long life. If the intake of food contains heavy metals that cannot be metabolized, the content of heavy metals in shark meat will be superimposed layer by layer, reaching a very high level. Studies have found that sharks in the Bahamas have high levels of mercury (Hg), arsenic (As) and chromium (Cr). The content of mercury in Icelandic fermented products is also very high, about 1.62.7 ppm. In addition, polychlorinated biphenyls, DDT and microplastics also accumulate in sharks, and when eating, consider whether to eat them together.

To sum up, the rough skin and chemicals in the body of a large shark make it very poor in eatability. The only common edible large shark product is Icelandic fermented shark meat, and it is widely selected as the most frightening fermented food.

If you want to try the taste of sharks, offshore non-dangerous small sharks are a good choice, their scales are easy to deal with, the levels of urea and trimethylamine oxide are not high, and a small amount of food is a good choice. Most importantly, small offshore sharks are much more fecund than large sharks. Female great white sharks do not mature sexually until they are in their 30s. Due to the way of oviparity, the number of offspring is only a few to a dozen at a time.

Let the great white shark go.

reference

[1] Xie Congxin. Ichthyology [M]. Beijing: China Agricultural Press, 2010.

[2] Feld, K.A., Kolborg, A.N., Nyborg, C. M., Salewski, M., Steffensen, J. F., & Berg-S ø rensen, K. (2019). Dermal denticles of three slowly swimming shark species: microscopy and flow visualization. Biomimetics, 4 (2), 38.

[3] Ganguly, P.P., Pol á k, J., van der Vegt, N. F., Heyda, J., & Shea, J. E. (2020). Protein stability in TMAO and mixed urea-TMAO solutions. The Journal of Physical Chemistry B, 12429, 6181-6197.

[4] Schmidt, A. C., & Leroux, J. C. (2020). Treatments of trimethylaminuria: where we are and where we might be heading. Drug discovery today, 25 (9), 1710-1717.

[5] Anthoni, U.N., Christophersen, C., Gram, L.A., Nielsen, N. H., & Nielsen, P. (1991). Poisonings from flesh of the Greenland shark Somniosus microcephalus may be due to trimethylamine. Toxicon: official journal of the International Society on Toxinology, 29 (10), 1205-1212.

[6] Sk å ra, T.A., Axelsson, L.S., Stef á nsson, G., Ekstrand, B., & Hagen, H. (2015). Fermented and ripened fish products in the northern European countries. Journal of Ethnic Foods, 2 (1), 18-24.

[7] Shipley, O.N., Lee, C. S.S., Fisher, N. S.A., Sternlicht, J. K.Kattan, S.A., Staaterman, E. R., Hammerschlag, N.A. & Gallagher, A.J. (2021). Metal concentrations in coastal sharks from The Bahamas with a focus on the Caribbean Reef shark. Scientific reports, 11 (1).

Welcome to subscribe "Shulou Technology Information " to get latest news, interesting things and hot topics in the IT industry, and controls the hottest and latest Internet news, technology news and IT industry trends.