Eriocheir sinensis - Chinese mitten crab

The Chinese mitten crab is native to the rivers and estuaries of East Asia, from the Taiwan Strait across China to North Korea ^{[2, 3]}.

First observation in Belgium

In 1993, the species was reported for the first time at Kruisschans, to the north of Antwerp ^{[4, 5]}.

Spreading in Belgium

Chinese mitten crabs can be found almost everywhere along the coast of Flanders, in the Scheldt and its tributaries and in various other (freshwater) channels in Flanders ^[5]. The species can move on land, which is why they are widespread in Flanders ^[6].

Spreading in neighbouring countries

The Chinese mitten crab was probably introduced to Europe from the Far East with ballast water. The first European sighting dates back to 1912 in the Aller, a tributary of the German Weser ^[7]. From there, the Chinese mitten crab spread to both the northern and southern parts of Europe.

Not long after its introduction into the Aller, the Chinese mitten crab was found in the Elbe. Between 1927 and 1954, this crab rapidly spread westwards from Germany through the North Sea and along the coasts of the English Channel ^{[8, 9]}. In 1954, the species reached Saint-Malo in Normandy and the crab also appeared in the Gironde and Loire estuaries (Bay of Biscay). Four years later, in 1958, the species was also reported at Hendaye, on the Franco-Spanish border ^[9]. Recently, the species has also been found as far south as Spain, in the Guadalquivir River ^[10], and the Tagus River in Portugal ^[11]. The species is also expanding its habitat to the Mediterranean area via the French Canal du Midi, although it does not seem to be able to establish itself there ^{[12, 13]}.

The Chinese mitten crab also migrated rapidly eastwards from Germany, reaching the Gulf of Finland in 1933 and the Gulf of Bothnia (Sweden) in 1934. The species spread along the European coasts at an average speed of 100-200 km per year (with peak speeds up to 500 km per year). The invasion of the British Isles began in 1973 from the River Thames and reached as far as the River Tyne, Yorkshire and the Irish coast in 2001 ^[9].

This crab has also been reported inland in Austria, Switzerland, the Czech Republic and Hungary ^[8], and even far to the east in the Caspian Sea (Iran) ^[14] and Persian Gulf (Iraq) ^[15]. Intermediate areas such as the White Sea ^[16], Volga River and the Sea of Azov (on the Black Sea) ^{[17, 18]} belong to its current range.

Recently, the Chinese mitten crab has also been found in rivers on the east and west coasts of the United States and Canada ^[19]. Genetic research showed that the crabs in San Francisco Bay (California) were not introduced from the Far East, but came from Europe ^[3].

The species probably arrived in Europe with ballast water from the Far East ^[11]. Other possibilities are that the species attached itself to ships’ hulls (biofouling) or that the species was deliberately introduced for human consumption ^[20].

In the estuaries and rivers that flow into the North Sea, there were originally no native migrating crustaceans (Brachyura) present, so there was little competition for the Chinese mitten crab. Its long planktonic phase, broad tolerance and great migratory capacity contribute to the success of the species in our latitudes.

Due to the favourable conditions, little competition and large food supply, the Chines mitten crab population in German waters grew very strongly in the 1930s-40s. In the late 1940s, however, their numbers declined sharply due to increased river pollution that eliminated the crabs' prey. However, the Chinese mitten crab itself can tolerate heavily polluted waters. The populations in the Netherlands and The United Kingdom showed similar evolutions, but to a lesser extent than in Germany. Recent improvements in river water quality have restored the abundance of crabs in European rivers.

The free-living (pelagic) larvae can drift with the currents, which has enabled this crab to quickly colonise the coasts of Europe ^[21]. Canals facilitate further distribution to inland waterways ^[9]. The Chinese mitten crab can also migrate over land and colonise other river basins in this way ^[22]. Shipping may have enhanced its further spread, as the Chinese mitten crab can hide among the fouling on ships’ hulls (e.g. in empty barnacle ‘shells’) ^[23].

The Chinese mitten crab is included in the list of Invasive Alien Species of Union concern (the Union list) (Regulation (EU) No 1143/2014) which means that the trade, possession and introduction of the species are prohibited. The EU Member States must detect and remove the species or, in the case of established populations, take measures to prevent further spread.

Since 1912, the financial impact of this crab species in Germany is estimated to be 80 million euros ^[23]. Chinese mitten crabs cause damage to fisheries by stealing bait, eating fish present in the nets and damaging the fishing nets. The crab also poses a threat to farmed fish and their food in aquaculture facilities ^{[7, 22-24]}. Furthermore, large numbers of Chinese mitten crabs can damage verges and dykes by digging burrows ^[25] and block the water supply of irrigation and water supply systems ^[23].

These crabs are generalist omnivores and eat algae, fish eggs, waste and various species of invertebrates, including worms and shellfish ^{[7, 25]}. They can pose a threat to local flora and fauna due to their aggressive voracity ^[22] and compete with native and introduced crayfish for the same niche ^{[22, 23]}. This places it among the top 100 worst invasive species in the world. It can also be the host of the crayfish plague, a disease that also belongs to this top because it is lethal for crayfish. This plague occurs naturally in North America, where the native crayfish have been able to adapt over time. However, when this disease strikes elsewhere, e.g. in Europe, it can cause the entire local crayfish population to go extinct ^[26].

In its area of origin (Asia), Chinese mitten crabs are carriers of the parasitic flatworm ‘oriental lung fluke’ Paragonimus westermani ^[11]. If the crabs are not adequately cooked when consumed, there is a risk that these parasites infect humans. However, the likelihood of this parasite causing problems (such as lung or neurological diseases) in Northern Europe is small, as the necessary intermediate host in its cycle (a species of snail) does not exist here ^[23].

Until recently, attempts in the past decades to catch as many juvenile crabs as possible while they are migrating upstream have been insufficiently efficient ^[20]. In February 2018, a crab trap was installed in the Kleine Nete (Small Nete) river in Grobbendonk by the Flemish Environment Agency (VMM) in cooperation with Ghent University, in which, according to press reports, more than 1 million crabs were caught during the first year and a half.

Another solution would be to use the crab as a food source for humans. Hunting the crabs could significantly reduce their numbers. In China, the Chinese mitten crab is considered a delicacy and the species has been sold there for some time. The female gonads are particularly prized.

Recently, Europeans have discovered the gastronomic delights of this species. Until a few years ago, the Chinese mitten crab had no commercial importance but was rather seen as a pest. Nowadays, this species of crab appears to be a welcome extra source of income for Dutch eel fishermen, especially because eel stocks in our region are in sharp decline. The majority of Chinese mitten crabs caught are exported to Italy and the Czech Republic. Many fishermen also supply directly to Chinese restaurants in the Netherlands. Especially Chinese and Thai people living in the Netherlands have increased their demand for Chinese mitten crab. In 2003, the yield in Den Oever was 11 tonnes with a value of 36,000 euros ^[27]. One year later, the yield increased already to 22 tonnes with a value of 83,600 euros. In Belgium, there is no such official circuit (yet) ^[28].

Because Chinese people eat so many of these crabs, there is a concern about the overfishing of the population in the Yangtze River. In China, the Chinese mitten crab is now cultivated on a massive scale for local consumption (500,000 tonnes in 2005; 1.14 billion dollars per year). ^[9]. Furthermore, the crabs are used as bait for eel fishing, in the production of fishmeal, for fertiliser in agriculture and in cosmetic products ^[23].

The Chinese mitten crab is not a true marine species. It is a freshwater species that migrates from streams, rivers and canals to estuaries in late summer to breed ^[7]. Sometimes they come out of the water to continue their journey on land. This way, they can move between different river basins ^[22].

After mating, egg-bearing females migrate further out to sea and remain in deep water at the bottom of estuaries and coastal waters throughout the winter. A female can carry 250,000 to 1 million eggs. Only in spring do they return to the brackish estuaries, where the eggs can hatch into larvae ^[7]. The planktonic larvae remain in the brackish water until they settle and metamorphose into small, juvenile crabs. These juveniles gradually migrate up the river to freshwater, where they complete their life cycle. Not all individuals migrate upriver, some continue to inhabit brackish water ^[25]. After two to three years they are sexually mature ^[2].

Chinese mitten crabs can travel enormous distances. They have been found in Prague about 750 km upstream in the Elbe River. In China, specimens have been reported in the Yangtze River, approximately 1,300 km from the sea ^[22].

The Chinese mitten crab can be easily recognised by its brown ‘woolly gloves on its claws, especially in larger males. The claws have a white end. The carapace is round (3 to 10 cm wide) and contains four lateral spines (the fourth pair is very small). They have a small protrusion between the eyes. The legs of an adult crab are generally more than twice as long as the carapace.

Since 2003, another exotic crab with fuzzy claws has appeared in our region: the Asian brush crab Hemigrapsus takanoi. This species is smaller than the Chinese mitten crab and has a square carapace without spines between the eyes ^[29].

[1]          World Register of Marine Species (WoRMS) (2020). Eriocheir sinensis H. Milne Edwards, 1853. [http://www.marinespecies.org/aphia.php?p=taxdetails&id=107451] (2020-11-17).

[2]          Adema, J.P.H.M. (1991). De krabben van Nederland en Belgie (Crustacea, Decapoda, Brachyura). Nationaal Natuurhistorisch Museum: Leiden. ISBN 90-73239-02-8. 244 pp. [http://www.vliz.be/en/imis?module=ref&refid=5876]

[3]          Hänfling, B.; Carvalho, G.R.; Brandl, R. (2002). mt-DNA sequences and possible invasion pathways of the Chinese mitten crab. Mar. Ecol. Prog. Ser. 238: 307-310. [http://www.vliz.be/en/imis?module=ref&refid=125094]

[4]          Lestage, J.A. (1935). La présence en Belgique du Crabe chinois (Eriocheir sinensis H. Milne-Edwards). Ann. Soc. R. Zool. Bel. 66: 113-118. [http://www.vliz.be/en/imis?module=ref&refid=113652]

[5]          Wouters, K. (2002). On the distribution of alien non-marine and estuarine macro-crustaceans in Belgium. Bull. Kon. Belg. Inst. Natuurwet. Biologie 72: 119-129. [http://www.vliz.be/en/imis?module=ref&refid=35968]

[6]          Waarnemingen afkomstig van Waarnemingen.be: een initiatief van Natuurpunt Studie vzw en de Stichting Natuurinformatie (2019). Chinese Wolhandkrab - Eriocheir sinensis H. Milne Edwards, 1853. [https://waarnemingen.be/species/16663/maps/?start_date=2019-06-08&interval=2592000&end_date=2019-07-08&map_type=grid10k] (2019-07-09).

[7]          Panning, A. (1939). The Chinese Mitten crab. Smithsonian Ann. Rep. 1938: 361-375. [http://www.vliz.be/en/imis?module=ref&refid=125070]

[8]          Herborg, L.M.; Rushton, S.P.; Clare, A.S.; Bentley, M.G. (2003). Spread of the Chinese mitten crab (Eriocheir sinensis H. Milne Edwards) in Continental Europe: analysis of a historical data set. Hydrobiologia 503: 21-28. [http://www.vliz.be/en/imis?module=ref&refid=58402]

[9]          Herborg, L.M.; Rushton, S.P.; Clare, A.S.; Bentley, M.G. (2005). The invasion of the Chinese mitten crab (Eriocheir sinensis) in the United Kingdom and its comparison to continental Europe. Biological Invasions 7(6): 959-968. [http://www.vliz.be/en/imis?module=ref&refid=121273]

[10]        Cuesta, J.A.; González-Ortegón, E.; Rodriguez, A.; Baldó, F.; Vilas, C.; Drake, P. (2006). The decapod crustacean community of the Guadalquivir Estuary (SW Spain): Seasonal and inter-year changes in community structure. Hydrobiologia 557(1): 85-95. [http://www.vliz.be/en/imis?module=ref&refid=117021]

[11]        Cabral, H.N.; Costa, M.J. (1999). On the occurrence of the Chinese mitten crab, Eriocheir sinsensis, in Portugal (Decapoda, Brachyura). Crustaceana 72(1): 55-58. [http://www.vliz.be/en/imis?module=ref&refid=125007]

[12]        Petit, G. (1960). Le crabe chinois est parvenu en Méditerranée. Vie Milieu 11: 133-136. [http://www.vliz.be/en/imis?module=ref&refid=125046]

[13]        Galil, B.S.; Froglia, C.; Noël, P. (2002). CIESM atlas of exotic species in the Mediterranean: 2. Crustaceans: decapods and stomatopods. CIESM Publishers: Monaco. ISBN 92-990003-2-8. 192 pp. [http://www.vliz.be/en/imis?module=ref&refid=59207]

[14]        Robbins, R.S.; Sakari, M.; Baluchi, S.N.; Clark, P.F. (2006). The occurrence of Eriocheir sinensis H. Milne Edwards, 1853 (Crustacea: Brachyura: Varunidae) from the Caspian Sea region, Iran. Aquat. Invasions 1(1): 32-34. [http://www.vliz.be/en/imis?module=ref&refid=97419]

[15]        Clark, P.F.; Rainbow, P.S.; Robbins, S.M.; Smith, B.; Yeomans, W.E.; Thomas, M.; Dobson, G. (1998). The alien Chinese mitten crab, Eriocheir sinensis (Crustacea: Decapoda: Brachyura), in the Thames catchment. J. Mar. Biol. Ass. U.K. 78(4): 1215-1221. [http://www.vliz.be/en/imis?module=ref&refid=125099]

[16]        Berger, V.J.; Naumov, A.D. (2002). Biological invasion in the White Sea, in: Leppäkoski, E. Invasive aquatic species of Europe: distribution, impacts and management. . Kluwer Academic: Dordrecht, The Netherlands: pp. 235-239. [http://www.vliz.be/en/imis?module=ref&refid=40604]

[17]        Murina, V.V.; A.G., A. (2001). Chinese Crab, Eriocheir sinsensis is an invader into the basin of the Sea of Azov. Ekol. Morya 55: 37-39. [http://www.vliz.be/en/imis?module=ref&refid=125031]

[18]        Gomoiu, M.-T.; Alexandrov, B.; Shadrin, N.; Zaitsev, Y. (2002). The Black Sea: a recipient, donor and transit area for alien species, in: Leppäkoski, E. et al. Invasive aquatic species of Europe: distribution, impacts and management. Kluwer Academic: Dordrecht, The Netherlands: pp. 341-350. [http://www.vliz.be/en/imis?module=ref&refid=40614]

[19]        Cohen, A.N.; Carlton, J.T. (1997). Transoceanic transport mechanisms: the indroduction of the Chinese mitten crab Eriocheir sinesis to California. Pac. Sci. 51(1): 1-11. [http://www.vliz.be/en/imis?module=ref&refid=125012]

[20]        DAISIE (Delivering Alien Invasive Species Inventories for Europe) (2009). Handbook of alien species in Europe. Invading Nature - Springer Series in Invasion Ecology, 3. Springer: Dordrecht, The Netherlands. ISBN 978-1-4020-8279-5. xxviii, 399 pp. [http://www.vliz.be/en/imis?module=ref&refid=134923]

[21]        Eno, N.C.; Clark, R.A.; Sanderson, W.G. (Ed.) (1997). Non-native marine species in British waters: a review and directory. Joint Nature Conservation Committee: Peterborough. ISBN 1-86107-442-5. 152 pp. [http://www.vliz.be/nl/imis?module=ref&refid=24400]

[22]        Rainbow, O.; Robbins, R.; Clark, P. (2003). Alien Invaders: Chinese mitten crabs in the Thames and spreading. Biologist 50(5): 227-230. [http://www.vliz.be/en/imis?module=ref&refid=125078]

[23]        Global Invasive Species Database (2005). Eriocheir sinensis. [http://www.issg.org/database/species/ecology.asp?si=38&fr=1&sts] (2018-09-07).

[24]        Rudnick, D.A.; Resh, V.H. (2002). A survey to examine the effects of the Chinese mitten crab on commercial fisheries in northern California. IEP Newsletter 15(1): 19-21. [http://www.vliz.be/en/imis?module=ref&refid=125138]

[25]        Rudnick, D.A.; Halat, K.H.; Resh, V.H. (2000). Distribution, Ecology and Potential Impacts of the Chinese Mitten Crab (Eriocheir sinensis) in San Francisco Bay. Technical Completion Reports, 206. University of California Water Resources Center: Riverside, USA. 74 pp. [http://www.vliz.be/en/imis?module=ref&refid=124525]

[26]        Schrimpf, A.; Schmidt, T.; Schultz, R. (2014). Invasive Chinese mitten crab (Eriocheir sinensis) transmits crayfish plague pathogen (Aphanomyces astaci). Aquat. Invasions 9(2): 203-209. [http://www.vliz.be/nl/catalogus?module=ref&refid=297313]

[27]        De Vleet. Ecomare (2007). Chinese wolhandkrab. [http://www.zeeinzicht.nl/vleet/index.php?use_template=vleet_template.html&item=vleet&pageid=chinese-wolhandkrab.htm ] (2007-10-15).

[28]        Breine, J. (2018). Persoonlijke mededeling

[29]        Soors, J. (2011). Persoonlijke mededeling

VLIZ Alien Species Consortium (2020). Eriocheir sinensis – Chinese mitten crab. Non-native species of the Belgian part of the North Sea and bordering estuaries anno 2020. Flemish Institute for the Sea (VLIZ). 7 pp.