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Big yellow croaker is a seafood that everyone loves to eat. Fish belly made of big yellow croaker's swim bladder, shark fin, sea cucumber and abalone are listed as one of the four dry goods in China.

However, you may not know what the big yellow croaker experienced before it died to have such a yellow color. Today, let's talk about the mystery of "yellow croaker."

Large yellow croaker dishes (Photo source: macaucloser.com) Large yellow croaker (Larimichthys crocea) is the most popular marine fish in China. The annual yield of large yellow croaker in China exceeds that of any other artificially cultured marine fish, and it is also the highest in the world.

According to the Food and Agriculture Organization of the United Nations (FAO), China produces half of the global production of large yellow croaker. According to the China Fishery Statistics Yearbook published in 2019, large yellow croaker production was the highest among mariculture fish, reaching 198,000 tons, and it also increased by 11.2% over the previous year.

In fact, from the 1980s, China began to artificially cultivate large yellow croaker. In order to expand the population of wild large yellow croaker, since 1987, China has also released artificially bred large yellow croaker into nature in the sea area near Ningde.

It is also because we love eating big yellow croaker too much, and in 2014, Chinese researchers sequenced its entire genome. Because of this, in January 2015, the yellow croaker became the 200th species in the world to have its genome fully sequenced.

Although large yellow croaker has so many fans in our country, fans may not know that large yellow croaker is not always yellow, they only "yellow" at a certain time.

Large yellow croaker (Photo: FAO) Large yellow croaker has completely different skin by day and night: it is silvery white by day, but turns yellow at night.

Yellow is the symbol of large yellow croaker, yellow croaker and yellow croaker prices can be 20 times different. Therefore, in order to preserve the yellow color of large yellow croaker, fishermen and farmers usually catch it at night and kill it in the absence of light. Zhejiang Taizhou large yellow croaker breeding Yu Chun introduced, if the yellow croaker died to see light, they are still silver-white.

Yu Chun, a large yellow croaker farmer, introduced that strong light should not be used when fishing large yellow croaker. After fishing, cotton quilt should be used to cover it to make it die without light, otherwise the yellow color of large yellow croaker will disappear (picture source: watermark). As for why large yellow croaker has "night mode," and what principle this "night mode" is, in fact, we don't know why we love to eat large yellow croaker.

Therefore, in the field of large yellow croaker farming, a lot of research has been put into the problem of how to make large yellow croaker yellow. In a paper published in Aquaculture in 2014, researchers at Ocean University of China said the problem of skin color in large yellow croakers had not been solved.

However, regarding the big yellow croaker love "yellow" this matter, now can confirm the following facts.

First of all, this yellow color comes from the skin glands under each scale of the yellow croaker belly, and this yellow color is not made by the yellow croaker itself, but eaten.

The researchers found that the yellow color of Pseudosciaena crocea mainly comes from carotenoids, the most abundant of which is lutein, in addition to some pigments such as carotene and astaxanthin.

Carotenoids make flowers yellow (image: wikimedia) researchers have long known that all fish fail to synthesize carotenoids, so the color of yellow fish depends entirely on the carotenoids they ingest. To turn yellow, farmers add carotenoids and other substances to their ingredients.

However, the speed of "yellow" through "food supplement" is too slow, and the process of "coloring" takes several months. If we can understand the principle of yellow croaker, farmers can further reduce the breeding cost and more people can eat this seafood.

In order for croakers to yellow, farmers feed them carotenoids (photo credit: pixnio). So the question is, why do croakers yellow only at night?

In 2019, Chinese researchers made important progress. Chen Shixi, a researcher at the State Key Laboratory of Offshore Marine Environmental Science at Xiamen University, and his colleagues found for the first time that the yellow color of Pseudosciaena crocea depends on the distribution density of yellow-like pigment cells.

Specifically, yellow-like pigment cells of Pseudosciaena crocea congregate during the day and separate at night. When the xanthoid cells disperse, the skin turns yellow.

Changes in pigment distribution in the back (first row) and belly skin (second row) of large yellow croaker during the day (first column), night (second column) and after MSH application (third column). (Photo source: (doi) 10.1016 / j.aquaculture.2019.03.074) Because there are only xanthoid cells on the belly of the big yellow croaker, there are no melanin-carrying cells that store melanin, so the yellow color of their belly is more obvious than other parts of the body.

So how did the big yellow croaker know to switch skin at night?

Changes in body color of bony fish depend on the nervous and endocrine systems. Among them, melanocyte stimulating hormone (MSH) secreted by pituitary gland plays an important role. In order to verify whether the large yellow croaker is "yellow" by MSH, these researchers also injected MSH into the large yellow croaker, and the large yellow croaker turned yellow as expected.

After using MSH, the big yellow croaker turned yellow (Photo source: (doi) 10.1016 / j.aquaculture.2019.03.074) They also wanted to know how light caused the big yellow croaker to change color. In general, light stimulates fish eyes to produce MSH, which then discolors the fish.

To see if the yellow croaker is also colored by the eyes, they did another interesting experiment. They wrap the croaker's body in opaque material, but not its head. After 30 minutes, the yellow croaker's body will turn yellow, but its head will still be silver-white.

Large yellow croaker dish (Photo credit: foodgem.sg) This is strange. Logically speaking, if the big yellow croaker is sensitive to light by the eyes and secretes MSH after being stimulated by the pituitary gland, then the whole body should be discolored together. It makes no sense that only the shaded part will be discolored. That is to say, the yellow croaker does not rely solely on the light received by the eye, but their skin may also be involved, but the mechanism behind this is unclear.

Finally, the purpose of the big yellow croaker's discoloration is also a mystery. If it was to protect themselves at night from predators below, why didn't the night give them black bellies? After all, the "background wall" above is black.

It seems that despite raising so many large yellow croakers, eating so many large yellow croakers, and paying so many researchers 'efforts, we still know too little about it.

Big Yellow Croaker: I'll die in the light? It doesn't exist.

Source: Wikipedia

References:

https://docs.qq.com/doc/DVFVRYXBEVnVZVHpu

This article comes from Weixin Official Accounts: Bring Science Home (ID: steamforkids), Author: Everything

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