The Chinese mitten crab dons furry gloves before winter even arrives

"When the north wind blows, crab legs itch; when chrysanthemums bloom, crab season comes."

Before we know it, the best season for savoring Chinese mitten crabs has arrived. The Chinese mitten crab (Eriocheir sinensis) is probably the most popular and best-known crab right now. As cold air arrives and the air grows increasingly dry, Chinese mitten crabs in peak season are becoming the darling of gourmands.

I really am not deliberately tempting you in the middle of the night! Image: Oriental IC

Within a crab carapace less than two inches wide lies fresh, sweet, plump, tender crab meat and fragrant, glutinous roe and paste. Savoring it slowly is enough to make one reminisce about it for the whole golden autumn. While enjoying this delicacy, let us also learn the story of the Chinese mitten crab.

Chinese mitten crab on the dining table. Image: Dennis Wong

A crab wearing "gloves"

The Chinese mitten crab, commonly known as the mitten crab, river crab, or hairy crab, is the largest of the four crab species in the genus Eriocheir, family Varunidae, phylum Arthropoda. Its carapace can reach 8 cm in width, and its body weight can exceed 500 grams.

Standard portrait of a Chinese mitten crab. Image: Sytske Dijksen / wiki commons

The Chinese mitten crab has a rounded body, a dark green back, and a grayish-white belly. Its most obvious feature is none other than that pair of crab claws covered inside and out with dense brown fuzz -- hence the name "mitten crab." In addition, because the claws look rather like mittens, it is called the "Chinese mitten crab" in English.

The furry claws of the Chinese mitten crab. Image: Natural History Museum

Among the four species in the same genus, the one most similar to the Chinese mitten crab is the Japanese mitten crab (E. japonica), whose distribution range also overlaps considerably with that of the Chinese mitten crab. The Japanese mitten crab is relatively smaller in size, but to reliably tell the two apart, one must observe the frontal teeth between their eyes: the frontal teeth of the Japanese mitten crab are wavy, while those of the Chinese mitten crab have 4 distinct small points.

A specimen of the Japanese mitten crab; note the shape of the frontal teeth. Image: Daderot / wiki commons

The frontal teeth of the Chinese mitten crab are clearly sharper. Image: Peter van der Sluijs / wiki commons

The other two mitten crabs -- E. leptognathus and E. rectus -- are much smaller in size. These two mitten crabs have frontal teeth divided into only 2 lobes, and only one side of their claws bears fuzz. E. leptognathus has fuzz only on the inner side of its claws, while E. rectus has fuzz only on the outer edge of its claws.

E. leptognathus, with fuzz distributed only on the inner side of the claws. Image: Dolphin in the Waves

The sea, where life begins

The Chinese mitten crab is a catadromous migratory crab -- simply put, it is born in seawater and grows in freshwater. It spends most of its life in freshwater, and "when the turn from autumn to winter comes, the crab follows the river downstream back to the sea." Around this time each year, Chinese mitten crabs reach sexual maturity and begin migrating toward the sea -- in other words, this is when their roe and paste are at their plumpest and most delicious.

The best time for Chinese mitten crabs lasts only about two or three months each year. Image: museilyvm

Nearly mature Chinese mitten crabs follow the rivers toward seawater, migrating to the nearshore waters of estuaries. When spring comes the following year, stimulated by salinity and temperature, they pair up and mate. After the female releases eggs, the fertilized eggs adhere to the setae on her abdomen. Under natural conditions, the zoea stage takes three to four months to hatch, and after hatching, the zoea begins a drifting life in seawater. The Chinese mitten crab has only one reproductive cycle in its lifetime; after completing the great task of passing on its lineage, it dies.

The mating posture of the Chinese mitten crab. Image: seerecht.org

The hatched zoea must go through 5 molts before transforming into a megalopa. With each molt, the body grows larger, and each molt is also a life-or-death hurdle. During the megalopa stage, the Chinese mitten crab begins to move toward freshwater. After one more molt, the Chinese mitten crab becomes a juvenile crab, its abdomen curled under the cephalothorax, losing its swimming ability and beginning a benthic life back in freshwater. Over the next year or so, after nearly 20 molts, the crab finally reaches sexual maturity and prepares for the next cycle.

The life history of the Chinese mitten crab. In the figure, 2 is the zoea, and 3 is the megalopa. Image: Xu Xingchuan (1994)

The flourishing Chinese mitten crab farming industry

The natural distribution range of the Chinese mitten crab in China is quite extensive, spanning the coastal provinces and cities of the Bohai Sea, the Yellow Sea, and the East China Sea. Because it can move freely between seawater and freshwater, it has left its footprints in rivers and lakes connected to the sea as well, including lakes more than 1,000 km from the ocean.

Initially, Chinese mitten crab farming in China relied basically on seedlings captured from the wild. However, the mixing of seedlings of three mitten crab species, along with environmental pollution and the alteration of nearshore areas and rivers, kept the output of Chinese mitten crabs in a state of chronic shortage. This situation only began to slowly turn around in the 1970s, when artificial breeding of Chinese mitten crabs succeeded.

A Chinese mitten crab farming base at Yangcheng Lake. Image: Zhang Jinqiao / Oriental IC

Now, across the vast areas suitable for Chinese mitten crabs, the farming industry is extremely prosperous, especially in the East China region. China's annual output of Chinese mitten crabs has exceeded 800,000 tons, with an output value of more than 50 billion yuan.

The crab that is "wanted"

On one side, people cannot get enough of Chinese mitten crabs; on the other side, they are the last thing people want to see.

When Europeans see this giant Chinese mitten crab in Kunshan, their hearts must be filled with incomprehension. Image: Jin Xintao / Oriental IC

Outside China, the natural distribution of the Chinese mitten crab is limited to the Yellow Sea coastal region of the Korean Peninsula, where it had lived in peace. However, with the coming and going of cargo ships crossing the sea, this tranquil scene changed at the beginning of the last century.

To sail stably on the ocean, cargo ships often take on a certain amount of water before setting off and discharge it as needed upon reaching their destination. But what is pumped in from the port is not only water -- Chinese mitten crabs also arrived unnoticed in European rivers along with the ballast water.

In 1912, the Chinese mitten crab was first discovered in a river in Germany. Because the environment was suitable, with no natural enemies to interfere and no human harvesting, the Chinese mitten crab began to reproduce rapidly. In just a few decades, they had invaded the Thames, the Rhine, and other major European river systems, running rampant everywhere, digging burrows and damaging water conservancy facilities and crowding out native species, becoming a major headache for ecological managers in European countries.

Map of the Chinese mitten crab invasion in Europe; red indicates established populations, blue indicates populations not yet confirmed as established. Image: S. Gollasch / europe-aliens.org

In 1992, the Chinese mitten crab even crossed the Pacific and was discovered in San Francisco Bay in the United States, where it successfully "settled down." Later, in 1998, the catadromous reproductive migration of Chinese mitten crabs caused a fish migration channel in Tracy, California, to become blocked, leading to mass deaths of fish from lack of oxygen. Now the Chinese mitten crab has become a "wanted" invasive species in the United States.

What did you say -- send Chinese foodies to wipe them out? You are still thinking too simply. China's invasive species have not yet been wiped out by foodies. Don't believe it? Go take a look at today's second article!

This article is the 276th article in Year 4 of the Species Calendar, from Species Calendar author @Dolphin in the Waves.

All right, one last question:

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Family Dorippidae, Japanese phantom crab