Hemigrapsus takanoi - Brush-clawed shore crab

The Brush-clawed shore crab - also known as the 'Asian brush crab' - was originally found along the East Asian coast, from Russian Sakhalin to Taiwan. ^[5]. Here, this crab is common in the intertidal zone (i.e. the zone between the high and low water line) along rocky shores, under rocks and in mud ^[3].

First observation in Belgium

The first sighting of the Brush-clawed shore crab in the study area was on the 9^th of December 2001 near Rhittem, a village on the eastern shore of the Dutch Western Scheldt ^[6]. The first observation in Belgium took place on the 19^th of August 2003, in the subtidal zone between mussel clumps on the pontoons in the Montgomery dock of the Ostend marina ^[7].

Spreading in Belgium

The first sighting in Ostend immediately led to the start of an inventory campaign, which proved successful: along the bank of the Ostend channel (south of the eastern pier), large numbers of Brush-clawed shore crabs were found, hiding underneath the mussels. Furthermore, the species was also present in Zeebrugge: on the 21^st of August 2003, Brush-clawed shore crabs were found in ‘het Verbindingsdok’ (literal translation in English: ‘the Connection dock’), on a rock covered with mussels ^[7].

This biological inventory revealed that by the end of 2003 the Brush-clawed shore crab was already a common sight in Belgian seaports and marinas: in Ostend, Zeebrugge, Blankenberge and Nieuwpoort ^[8] the exotic species was found, as well as on groynes in Duinbergen and in Zwin Nature Park ^[7]. This non-indigenous crab can also be found between the mussel beds on the groynes in Knokke-Heist ^[9]. Today, the Brush-clawed shore crab can also be found in Antwerp, near Doel ^[10].

In general, it can be said that this exotic crab has become abundant along the Belgian coast ^[11]. In a 2010 study on various sites in the Flemish harbours, the species was mainly found in the vicinity of artificial substrates ^[12]. The species also appears to be common among the oyster beds of the non-native Japanese oyster Crassostrea gigas ^[7], which is particularly abundant on hard substrates in the Flemish ports ^[11].

Spreading in neighbouring countries

The first sighting in Europe dates from the 14^th of August 1993, in Bremerhaven, Germany. On the hull of a transport ship, which was carrying cars from Japan, six Brush-clawed shore crabs were found alive between the biofouling layers. It was estimated that the ship must have carried several hundred individuals with it ^[5]. However, the first report of an established population did not come from Bremerhaven harbour, but from the Bay of Biscay near La Rochelle (France; March 1994). Subsequently, Hemigrapsus takanoi rapidly expanded its range and, by the end of 1996, occurred from Laredo in Spain to Fromentine in France. The species soon became very common here and up to 20 crabs per m² were found in some places ^[13]. In France, the species further expanded its range and in 1999 this exotic species was also found in Le Havre. A few years later (2006), a very dense population – more than 60 specimens per m² – was discovered near Dunkirk. In Brittany, the Brush-clawed shore crab remains largely absent ^[14].

The first Dutch specimen was collected on the 21^st of April 2000 near Goese Sas in the Eastern Scheldt ^[3]. Soon the species appeared everywhere in the Eastern Scheldt. From the end of 2001, the Brush-clawed shore crab was observed in the Western Scheldt ^[6] and it was predicted that this invader would become one of the most common crab species in the Netherlands ^[15]. Since 2006, individuals have been found in the Dutch Wadden Sea ^[16]. The species is also found in Lake Grevelingen (the Netherlands). In Zeeland, it has also been observed in the subtidal zone, down to a depth of at least 20 metres, but always near to the coast ^[17].

On the 2^nd of December 2007, the first Brush-clawed shore crabs – apart from those found on the hull of a ship in 1993 – were discovered in Germany (Lower Saxony), near Norddeich ^[18]. In 2013, the species was found in Southern England ^[19]. Established populations now spread from Lower Saxony in Germany to the Cotentin Peninsula in France (54°N-49°N) ^[20].

The Brush-clawed shore crab is thought to have arrived in our region because of the shipping industry. The Asian brush crab can hide underneath and between mussels, oysters and seaweed on ships' hulls ^[5] and thus continued its spread from the ports. On the other hand, it has also been suggested that Brush-clawed shore crabs in ballast water may give rise to new populations in Europe ^[14]. Both planktonic larvae – typical of crustaceans – and adult individuals can be transported by this route ^[21]. Another possibility is that this non-indigenous species was introduced through the import of Asian oysters, such as the Japanese oyster Crassostrea gigas ^[21], to La Rochelle or the nearby Arcachon bay ^[13].

Secondary (local) dispersal occurs by larval transport via the prevailing sea currents, but can also be facilitated by recreational boating ^[7].

The European climate is similar to that in the natural habitat (Japan and China) of the Brush-clawed shore crab. This exotic species prefers to settle underneath stones between the high and low water line, a habitat that is abundant along the Belgian coast, with its many harbours, dykes and groynes ^[5]. Specific abiotic factors such as pollution and physical disturbance are characteristic of port areas ^[21]. These characteristics seem to favour the occurrence of Hemigrapsus takanoi at these sites ^[21]. The crabs are said to be "prepared" for such polluted habitats because of the unfavourable conditions during their transportation in the ship's ballast water ^[22]. This gives them a great advantage over the native species that cannot cope with these conditions. They also feel more comfortable in sheltered places where hydrodynamics are low, which is typical of port areas ^[21]. Although, these crabs seem to thrive in even more sheltered regions, such as the Dutch Eastern Scheldt ^[9].

In recent years, the surfaces of hard substrates have increased. This is partly due to the emergence of Japanese oyster beds but is also the result of the construction of artificial hard substrates in the context of, for example, offshore wind farms. These processes may have facilitated the spread of the Brush-clawed shore crab ^[7]. Furthermore, the frequent co-occurrence of the Japanese oyster Crassostrea gigas and the Brush-clawed shore crab may be the result of an overlap in habitat preference. Another explanation could be that both species of crab feed on juvenile oysters ^[7].

The reproductive period in Hemigrapsus takanoi is very long (about six months) compared to other crabs of the Hemigrapsus genus, which are sometimes only reproductively active for six days ^[21]. Sometimes these animals produce more than five broods per year ^{[7, 21]}. This, together with a short egg development period – sometimes as little as 12 days – and rapid development of the larvae into adults – up to several months – enables the species to colonise areas quickly ^[7].

Ultimately, this species has a strong tolerance to variable salinities, and it can be found in environments with a salinity between 15 and 35 PSU ^[23]. By way of comparison, the seawater in the North Sea has an average salinity of 35 PSU

The Brush-clawed shore crab needs some type of shelter. It finds these in the form of shellfish banks, groynes and harbour constructions. Where these are not available – e.g. on the beach – the species will find it difficult to survive. The fact that the Brush-clawed shore crab is found in the Scheldt estuary ^[10] shows that it is resistant to lower salt levels ^{[5, 7]}.

The Brush-clawed shore crab can occur in high abundances: up to 80 individuals per m² ^[24]. These high numbers were measured on the French Opal Coast ^{[14, 21]} and Lake Veerse in the Dutch delta ^[24]. There is no doubt that a non-indigenous species, which occurs in such large numbers after only a few years, influences the local (eco)systems.

Brush-clawed shore crabs are often found in the company of juvenile common shore crabs Carcinus maenas. Both species are opportunists in terms of habitat and food choice. It is therefore questionable whether the common shore crab (or several native crab species) can be outcompeted over time as a result of competition for space and food resources with the Brush-clawed shore crab ^{[7, 25]}. A Dutch study from 2010, showed that Hemigrapsus takanoi now competes with juvenile common shore crabs by chasing them out of their hiding places. This caused a sharp decline in the number of common shore crabs in Dutch waters. However, this does not appear to be the case in soft sediments, where the shore crab can survive and reproduce because Hemigrapsus takanoi is hardly present there ^[24]. Meanwhile, at many locations along the Dutch Eastern Scheldt, an increase in the number of common shore crabs can be observed, while the Brush-clawed shore crab populations are decreasing. It is regularly observed that large specimens of common shore crabs feed on Brush-clawed shore crab. ^[26].

However, scientists warn of highly fluctuating trends in the numbers of Hemigrapsus takanoi ^[21]. A population decline one year does not indicate a possible trend the following year. Frequent monitoring is therefore strongly recommended, as this species may eventually pose a threat to biodiversity and the functioning of coastal communities ^[21].

On the other hand, measures to halt the advance of this exotic species are not obvious and have not been worked out for the time being. As a result, ecological shifts in the native fauna and flora cannot be ruled out in the long run. ^[2]

The Brush-clawed shore crab is a fairly small species of crab. The form of its carapace is approximately like a square and can be up to 28 mm wide. Both sides of the carapace have three spines ^[2]. Males have larger claws than females, with a tuft of yellow bristles. The carapace is brown, green or grey and the underside of the body is whitish. Young individuals sometimes ^{[2, 10]}.

Little is known about the life cycle of Hemigrapsus takanoi, although it is probably very similar to the life cycle of Hemigrapsus penicillatus. Before 2005, these species were considered to be the same. Female Brush-clawed shore crabs can bear several broods per year during the summer months. The development time of the hatchlings and larvae strongly depends upon the water temperature, although maturity is usually reached after a few months ^[24].

In the future, the colonisation success of the Brush-clawed shore crab may further increase, allowing the expansion of the area occupied by this species to locations that are currently considered unsuitable based on excessively cold temperatures ^[27]. Common shore crabs Carcinus maenas usually seek deeper water in the colder winter months, away from the shore. Brush-clawed shore crabs, on the other hand, do not migrate. The cold, however, makes them sluggish, so that in winter they lead a more reclusive life ^[7]. The species can be found both in intertidal areas and at greater depths, up to 20 metres ^[17].

The Brush-clawed shore crab may be confused with the closely related Japanese shore crab Hemigrapsus sanguineus, a more recent non-indigenous species introduced to the European coasts. This species has a more spotted carapace and a leather-like vesicle instead of the pluck of hair in between the claws. The Brush-clawed shore crab is slightly smaller than the Japanese shore crab and has a more orange to brown colour. The pereiopods have, in contrast to the Japanese shore crab, no horizontal stripes and rather short and robust setae ^[18].

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[4]          Asakura, A.; Watanabe, S. (2005). Hemigrapsus takanoi, new species, a sibling species of the common Japanese intertidal crab H. penicillatus (Decapoda: Brachyura: Grapsoidea). J. Crust. Biol. 25(2): 279-292. [http://www.vliz.be/en/imis?module=ref&refid=75377]

[5]          Gollasch, S. (1999). The Asian decapod Hemigrapsus penicillatus (de Haan), 1835) (Grapsidae, Decapoda) introduced in European waters: status quo and future perspective. Helgol. Meeresunters. 52(3-4): 359-366. [http://www.vliz.be/imis?module=ref&refid=120952]

[6]          Faasse, M.A.; Nijland, R.; d'Udekem d'Acoz, C. (2002). Opmars van de Penseelkrab Hemigrapsis penicillatus De Haan, 1935 in Nederland. Het Zeepaard 62(2): 41-44. [www.vliz.be/en/imis?module=ref&refid=24046]

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[12]        Hebbelinck, L. (2010). Monitoring van exotische macro-invertebraten in de Vlaamse havens. MSc Thesis. Universiteit Gent, Faculteit Bio-Ingenieurswetenschappen: Gent. 87 pp. [http://www.vliz.be/en/imis?module=ref&refid=197447]

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[14]        Dauvin, J.-C.; Tous Rius, A.; Ruellet, T. (2009). Recent expansion of two invasive crabs species Hemigrapsus sanguineus (de Haan, 1835) and H. takanoi Asakura and Watanabe 2005 along the Opal Coast, France. Aquat. Invasions 4(3): 120-465. [http://www.vliz.be/en/imis?module=ref&refid=206505]

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[17]        d'Udekem d'Acoz, C. (2020). Persoonlijke mededeling

[18]        Obert, B.; Herlyn, M.; Grotjahn, M. (2007). First records of two crabs from the North West Pacific Hemigrapsus sanguineus and H. takanoi at the coast of Lower Saxony, Germany. WSN 33(1): 21-22. [www.vliz.be/en/imis?module=ref&refid=206451]

[19]        Ashelby, C.W.; Sewell, J.; Rostron, J.; Shrubsole, R.; Child, T.; Clark, P.F. (2017). Evidence for the invasion and successful establishment of Hemigrapsus takanoi Asakura & Watanabe, 2005 (Decapoda, Varunidae) in Great Britain. Crustaceana 90(6): 695-708. [http://www.vliz.be/nl/catalogus?module=ref&refid=312322]

[20]        Epifanio, C.E. (2013). Invasion Biology of the Asian Shore Crab Hemigrapsus Sanguineus: A Review. J. Exp. Mar. Biol. Ecol. 441: 33-49. [http://www.vliz.be/en/imis?module=ref&refid=287904]

[21]        Gothland, M.; Dauvin, J.C.; Denis, L.; Dufossé, F.; Jobert, S.; Ovaert, J.; Spilmont, N. (2014). Biological traits explain the distribution and colonisation ability of the invasive shore crab Hemigrapsus takano. Est., Coast. and Shelf Sci. 142: 41-49. [http://www.vliz.be/nl/catalogus?module=ref&refid=297362]

[22]        Crooks, J.A.; Chang, A.L.; Ruiz, G.M. (2011). Aquatic pollution increases the relative success of invasive species. Biol. Invasions 13(1): 165-176. [http://www.vliz.be/nl/catalogus?module=ref&refid=297365]

[23]        Mingkid, W.M.; Yokota, M.; Watanabe, S. (2006). Salinity tolerance of larvae in the penicillate crab Hemigrapsus takanoi (Decapoda: Brachyura: Grapsidae). La Mer. 43: 17-21. [http://www.vliz.be/nl/catalogus?module=ref&refid=297369]

[24]        Van den Brink, A.M.; Wijnhoven, S.; McLay, C.L. (2012). Competition and niche segregation following the arrival of Hemigrapsus takanoi in the formerly Carcinus maenas dominated Dutch delta. J. Sea Res. 73: 126-136. [http://www.vliz.be/nl/catalogus?module=ref&refid=230998]

[25]        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]

[26]        Faasse, M. (2019). Persoonlijke mededeling

[27]        Van den Brink, A.; Godschalk, M.; Smaal, A.; Lindeboom, H.; McLay, C.L. (2013). Some like it hot: the effect of temperature on brood development in the invasive crab Hemigrapsus takanoi (Decapoda: Brachyura: Varunidae). J. Mar. Biol. Ass. U.K. 93(1): 189-196. [http://www.vliz.be/en/imis?module=ref&refid=234291]

VLIZ Alien Species Consortium (2020). Hemigrapsus takanoi – Brush-clawed shore crab (Asia brush crab). Non-native species of the Belgian part of the North Sea and bordering estuaries anno 2020. Flemish Institute for the Sea (VLIZ). 8 pp.