Many people ignore how poisonous dead hairy crabs are!

It's the season for eating hairy crabs again. The plump and sweet crab roe and tender and sweet crab meat are the biggest expectations of the golden autumn season every year. But it's really not easy to eat delicious hairy crabs.

First of all, the high prices always make people torn between satisfying their appetite or holding their wallets; and after finally making a decision, they need to distinguish the real and fake hairy crabs from the dazzling array of products online, and at the same time find options with good quality and cost-effectiveness; finally, after receiving the hairy crab gift box with great expectations, they have to overcome the last difficulty - accurately finding the dead hairy crabs from a pile of crabs that are trapped tightly and motionless.

Especially for hairy crabs purchased online, it is easy to die due to adverse environmental factors such as high temperature and lack of oxygen during the long journey. And it is particularly important to pick dead crabs, which is not only evidence for us to claim compensation from the merchants, but also can prevent us from accidentally eating ourselves to the hospital.

At this point, foodies who hate trouble always ask a soul-searching question: They look similar on the surface, so why can’t we eat dead hairy crabs?

Sub-title: Why can’t we eat dead hairy crabs?

The simplest reason is that dead hairy crabs don’t taste good.

The reason why we eat hairy crabs is because of their delicious taste, which comes from the extremely high free amino acid content in the hairy crabs. Once the hairy crabs die, these free amino acids will be quickly destroyed, and the hairy crab meat will become loose. At this time, the hairy crabs can be eaten, but they are really not delicious. If the hairy crabs have been dead for a long time, a large amount of volatile compounds will be produced during the decay process, and they smell full of "the smell of death", and ordinary people can't eat them at all.

If not tasting good isn’t enough to stop you, what about the possibility of being poisoned if eaten?

Sub-heading: There are a lot of microorganisms in dead hairy crabs

Eating dead hairy crabs can really cause food poisoning. In fact, river crabs, represented by hairy crabs, cannot be eaten as long as they are dead, and this is related to their living habits.

Let’s take the hairy crab as an example. Whether it is the artificially cultivated population or the so-called “wild population” after escape and spread, they all prefer to live in mud and silt environments such as riverbeds and ditch bottoms. The amount of microorganisms in this environment is very large, and hairy crabs carry a large number of microorganisms both inside and outside their bodies.

When hairy crabs are alive, their own immunity is enough to resist the invasion of these microorganisms, but once the hairy crabs die, the microorganisms will quickly invade the entire body of the hairy crab.

The hairy crab itself has a high water content, and contains extremely high levels of free amino acids and nitrogenous compounds, which are favorable conditions for the reproduction of microorganisms. Once the hairy crab dies, its immune system collapses, and microorganisms will quickly invade the hairy crab's body, turning it into a base for their own reproduction. As the hairy crab's death time increases, the total number of bacteria in the hairy crab's body will gradually exceed the threshold for safe consumption, and the hairy crab's appearance will gradually change from a shiny green shell and white meat to a dull black color, the crab paste will rot, and the meat will become watery, and the whole crab will exude a smell of death.

Moreover, the microorganisms attack the hairy crab carcass from both inside and outside. In addition to the microorganisms lurking on the hairy crab's surface in daily life, there are also a large number of microorganisms in the hairy crab's digestive system, especially in the stomach.

This is because the hairy crab is an omnivorous animal. It not only feeds on fresh small fish, shrimps, snails, etc., but also eats some dead or even slightly rotten aquatic bodies, which naturally allows the hairy crab's stomach to collect many microorganisms. As the hairy crab dies, its body system collapses, and its stomach can no longer hold these microorganisms. They will work together with the microorganisms on the surface of the body to quickly "eat" the hairy crab's body.

Sub-heading: Histamine in hairy crab carcasses is also dangerous

In addition to the risk of excessive microorganisms, the reproduction of microorganisms in the bodies of hairy crabs will also produce another more dangerous substance - histamine.

Microorganisms that multiply in large numbers use free amino acids as raw materials to produce large amounts of cadaverine, putrescine, histamine, tyramine, and small amounts of spermidine and spermine. The higher the temperature of the environment in which the hairy crab carcass is located, the faster these biogenic amines are produced. In a study specifically on the changes in the composition of hairy crabs after death, it was found that the content of biogenic amines in the crab roe and crab meat began to increase sharply 5 hours after the hairy crab died. After 24 hours, the content of biogenic amines in the abdominal meat and crab roe exceeded 200mg/100g. Although it has not yet reached a concentration that will cause poisoning if eaten, people with weaker immunity can easily be affected by the dual attack of excessive microorganisms and excessive biogenic amines.

Among biogenic amines, histamine is the most dangerous. Although histamine is a substance naturally present in animals, and our own skin, lungs, and intestinal mucosal cells contain a large amount of histamine, this does not prevent humans from being poisoned by excessive histamine intake.

Normally, a trace amount of histamine intake will not have any effect, but usually more than 100 mg of histamine may cause discomfort to the human body, and because each person's tolerance is different, the poisoning dose is not completely absolute. Once infected, flushing and heat sensation on the face, chest and skin of the whole body may occur within tens of minutes to hours, accompanied by a series of symptoms such as general discomfort, mustard congestion, headache, nausea, diarrhea, tachycardia, chest tightness, and decreased blood pressure. In severe cases, shock may occur on the spot. Many people may mistakenly think that it is an allergy, but it is actually histamine poisoning. Fortunately, in most cases histamine poisoning is not fatal, the symptoms are usually mild, and health can be restored within 1 to 2 days.

Although there are relatively few cases of histamine poisoning caused by hairy crabs in my country, it is important to know that a dead hairy crab contains not only excessive biogenic amines, but also a large number of unknown microorganisms. When these multiple risk factors are combined, people with weak immunity are very likely to be infected. Even if you don't end up in the hospital, it's not a good experience to have to "live in love" with the toilet for several days instead of enjoying the delicious food.

Refer：

1. Wang, Y., Zhu, Y., Shi, W., & Wang, X. (2020). Quality evaluation of living and postmortem Chinese mitten crabs (Eriocheir sinensis). Food Science & Nutrition, 8(5), 2327-2340.

2. Ghosh, A., Rathore, A., Gaba, S., Kamli, MR, Maigoro, AY, Kwon, HW, ... & Malik, A. (2025). The Chinese mitten crab (Eriocheir sinensis) and its microbiome: A review. Aquaculture, 595, 741518.

3. Jiang, S., Zhang, W., Qian,

Author: Hydra

Reviewer: Zhang Yu, researcher/PhD, Chinese Center for Disease Control and Prevention, national health science expert