Hemigrapsus takanoi Asakura and Watanabe , 2005 ( Crustacea : Decapoda : Brachyura : Grapsoidea ) : first records of the brush-clawed shore crab from Great Britain

The brush-clawed shore crab is reported from the River Medway, Kent and the River Colne, Essex, England. These represent the first records of Hemigrapsus takanoi Asakura and Watanabe, 2005 from Great Britain. If H. takanoi becomes established in GB, it may pose a threat to populations of the native shore crab Carcinus maenas.


Introduction
The varunine crab Hemigrapsus takanoi Asakura and Watanabe, 2005 is native to Northwest Pacific coastlines, occurring intertidally.Hemigrapsus takanoi was named and formally distinguished from the Northwest Pacific species H. penicillatus (de Haan, 1835) only in 2005 and, in Europe, it was initially identified as H. penicillatus (Gollasch 1996(Gollasch , 1999;;Noёl et al. 1997;d'Udekem d'Acoz 1998, 1999).Around this time, Masatsugu Takano recognised two sympatric forms of H. penicillatus in Japan, with statistically different-sized setal patches on the male chelae: form I possessed smaller setal patches than form II (Takano et al. 1997(Takano et al. , 1999)).Seeking clarification of the identity of European populations, Cédric d'Udekem d' Acoz (d'Udekem d'Acoz and Faasse 2002) sent Bay of Biscay Hemigrapsus material to Masatsugu Takano, who identified these as form II (Takano et al. 1997).Further examination by Cédric d'Udekem d 'Acoz (2004 pers. comm. to Asakura andWatanabe 2005: 287) of abundant material collected from France and the Netherlands revealed that all specimens lacked dark spots on the abdominal somites, further confirming European H. penicillatus as form II (Takano et al. 1997(Takano et al. , 1999)).Asakura and Watanabe (2005)    While visiting the Natural History Museum, London, Asakura examined and redetermined a male and female collected in 1996 by P.Y.Noёl from the French coast as H. takanoi.Yamasaki et al. (2011) were able to distinguish between H. penicillatus and H. takanoi based on sequence differences within the 16S rRNA region of mitochondrial DNA; their analysis of the sequence from one specimen collected in France as H. penicillatus (GenBank accession number AJ278835) re-identified it as H. takanoi.Also Markert et al. (2014) have recently confirmed, by molecular analysis, that the Asian brush-clawed crabs in the Wadden Sea are H. takanoi.
During a 4-year period (1992-1995), the ballast water, tank sediments, and hull fouling of 131 ships recently berthed at German ports were sampled and examined as part of a study on German shipping (Gollasch 1996;1999).On 14 August 1993, six specimens (2 male, 4 female) of H. takanoi (as H. penicillatus) were collected in hull samples from the car-carrier SPICA berthed in Bremerhaven.After the journey from Japan into European waters, this vessel had docked for a routine inspection and coating with antifouling paint (Gollasch 1996: 117;1999).As Gollasch (1999) stated at the time, this was the first direct observation of long-range transmission and survival of 'H.penicillatus' on a ship hull of a fast ocean-going vessel (SPICA was capable of an average cruising speed of 17 knots).These specimens were subsequently re-identified as H. takanoi (S.Gollasch, pers. comm. 2014).Noёl et al. (1997) record collecting a single specimen of H. takanoi (as H. penicillatus) from the estuary of Charente Maritime near La Rochelle on the Atlantic coast of France in March 1994.They reason that the crab could have been included with Asian oysters introduced near La Rochelle or in the Bay of Arcachon, or by shipping lines operating from Bordeaux and La Rochelle.Moreover, Noёl et al. (1997) could estimate the date of this introduction, stating that the intertidal fauna of La Rochelle had been systematically surveyed for years and this species had not been sighted before 1994.Noёl (1997) thus believed that the introduction of the species may have taken place in 1993.Gollasch (1999) later postulated that in 1993 the car-carrier SPICA passed the French coast on its passage to Bremerhaven, unintentionally transporting perhaps "several hundreds of specimens of H. penicillatus" and assumed these crabs either dropped off or were displaced from the hull in sufficient numbers to generate a new population.Whether this was the case is uncertain, but Noёl et al. (1997) reported that H. takanoi (as H. penicillatus) had spread rapidly and by December 1996 was present from Laredo, Spain (43°25′N, 003°23′W) north to Fromentine, France (46°53′N, 002°09′W), a distance of ca.700 km, in sheltered areas of the mid-littoral zone (Noёl et al. 1997, Figure 2).Hemigrapsus takanoi continued to spread along the NW coast of Europe with Dauvin et al. (2009, Figure 5) mapping its distribution with data up until July 2008 as far as the eastern Wadden Sea, Germany.Most recently however, Landschoff et al. (2013, Figure 1) record the brush-clawed shore crab as far north as Rømø Island, Denmark, but without details.Confirmation was obtained by email (dated 9 September 2014 to PFC from Jonas Geburzi, Zoologisches Museum, Universität zu Kiel), stating that on 15 March 2012, Karsten Reise (AWI Wadden Sea Station in List on Sylt) found numerous specimens in Havneby harbour, Rømø Island but that the species was not recorded at this locality in 2011.In 18 years, H. takanoi has therefore dispersed around the NW coastline of continental Europe a distance of ca.2200km.
The purpose of this current paper was to note the first GB records of H. takanoi from the River Medway, Kent, England, and from the River Colne, Essex, England, see Figure 1.2b, c) divided into three unequal parts (vs.a continuous finely striated ridge in H. sanguineus) and setal patches (Figure 2d) at the base of the joint between the propodus and dactylus on the male cheliped (vs. a fleshy vesicle at that site).2014) considered which invasive alien species were most likely to impact on native biodiversity but were not yet established in the wild in Great Britain.Hemigrapsus sanguineus and H. takanoi were both included in the top ten species as posing a 'high risk' (Roy et al. 2014, Table 2) to arrive in the next ten years.

Discussion
Considering its current NW European distribution, these UK records of H. takanoi are not a great surprise.However, it is only possible to speculate as to the introduction vector(s) of these new arrivals.The nearest established populations of H. takanoi are along the continental coast of the English Channel and North Sea from Boulognesur-mer, France to Knokke Heist, Belgium (Dauvin et al. 2009) on the coastline opposite to where the GB specimens were found in Kent and Essex.However according to Dauvin and Delhay (2010, Figure 1) larval dispersal is an unlikely vector as surface currents would tend to carry the larvae north-westwards towards the Netherlands.Transport by shipping, either as hull fouling or in ballast water is a distinct possibility, as is transfer with commercial oysters.Genetic profiling will be needed and hydrographic movements investigated to infer the source(s) of any GB populations.
The native habitat of H. takanoi ranges from cold-temperate to warm-temperate (Gollasch 1999).Its native habitats include muddy and rocky shores and it can be found in sheltered estuaries and port areas.The juvenile and adult stages are tolerant of salinities as low as 9 (Gittenberger et al. 2010;Soors et al. 2010), although the larvae require higher salinity conditions (Mingkid et al. 2006).The habitats where the crabs were found in GB match these conditions.Both are estuarine environments with soft sediments.At Gillingham shelter is provided by the marina basin, whereas at Brightlingsea the oyster reef serves this function.The escape of the Pacific oyster C. gigas from commercial oyster beds is leading to the establishment of wild reefs along parts of the southern GB coast (Herbert et al. 2012).This ecosystem engineering has created areas of hard substrate and more heterogeneous habitats, which can provide spatial refuges against predators for small crabs such as H. takanoi (van den Brink et al. ), and the first male pleopods differ between the species (Asakura and Watanabe 2005: Figure6A, B cf. Figure6C, D).

Figure 1 .
Figure 1.Showing locations of first Great Britain records of Hemigrapsus takanoi.

Recently
Seeley et al. (2015) reported two GB sightings of H. sanguineus from Glamorgan, south Wales, 2 May 2014 and Kent, southern England, 14 May 2014.Now H. takanoi has been reported from a Medway marina in North Kent, 5 August 2014, and from an estuary in Essex, 7 May 2013.Minchin et al. (2013) list 90 alien species in British brackish and marine waters; 35 originated from the North Pacific (particularly the North-West Pacific) of which 82% have become established in British waters.The two Hemigrapsus crab
2012).There appear to be no environmental conditions preventing H. takanoi from becoming established around the UK and Irish coastlines in the future.If H. takanoi becomes established in GB, it may pose a threat to populations of the native shore crab Carcinus maenas.According to Dauvin et al. (2009), d'Udekem d'Acoz reported a drastic reduction in the number of juvenile C. maenas on some Dutch shores with high densities of H. takanoi.Dauvin et al. (2009) also observed that H. takanoi dominated C. maenas in Dunkirk harbour.
May 2013, NHMUK reg.2015.2780-27781.These crabs were found in preserved samples taken from a wild Pacific oyster reef (Crassostrea gigas) growing on tidal mudflats in a lower estuarine environment, see Figure3.The specimens were confirmed as H. takanoi by Paul Clark on 27 November 2014.Of the 15 samples analysed one contained the two H. takanoi and a single Carcinus maenas.