Callinectes sapidus - Blue crab

The Blue crab is a species of crab native to the Atlantic coast of America, from Nova Scotia in the north to Uruguay in the south ^{[2, 3]}. The adult Blue crab lives in the shallow waters of estuaries and river mouths and can be found down to a depth of 35 metres. After mating, female crabs migrate to the sea to lay their eggs, after which the juveniles migrate from the ocean back to the estuaries ^[3].

First observation in Belgium

The first report of the Blue crab in the study area dates back to 1950 when two cooked individuals were found in the Dutch Western Scheldt near Vlissingen. As these were probably crabs thrown overboard from a ship, this is not considered a sighting ^[4]. On the 4^th of September 1973, a live male individual was observed near Terneuzen (also in the Western Scheldt) ^[5].

In Belgium, one dead individual was found in 1981 in the cooling water system of the Bayer factory in Antwerp ^[5]. In October 1993, the first living (male) crab was found in the Scheldt River in the cooling water system of the Doel nuclear power plant ^[6]. This is considered the first official observation in Belgium.

Spreading in Belgium

Off the Belgian coast, the species was reported on both the West (Oostduinkerke) and East coast (Knokke-Heist). The species is regularly observed by coastal fishermen. In 2004, they brought several individuals to the Ostend aquarium, as well as a living female with eggs to the Sea Life Centre in Blankenberge ^[7]. In 2006, some female crabs with eggs were also caught by shrimp fishermen ^[8]. The many reports of this species off our coast make some scientists suspect that the Blue crab is a permanent resident in some harbours and/or river mouths ^{[9, 10]}.

After 1993, the species was reported several times in the Sea Scheldt, which was also the case in 2011 ^[11]. Therefore, it is suspected that a population has settled in the Sea Scheldt (near Antwerp) ^[12]. This population is possibly linked to the established population in the Dutch Western Scheldt ^[13].

Spreading in neighbouring countries

In recent decades, the Blue crab has been successfully introduced to various places in Europe and Asia, sometimes accidentally and sometimes deliberately.

The first European report dates back to 1900 ^[14] when a male individual was found in the harbour of Rochefort (southwest France). The following sighting in France was delayed by 60 years, when a plaice fisherman retrieved the species from the Gironde estuary near Bordeaux on the 16^th of September 1960 ^[15]. More recently, the Blue crab has been observed mainly in and around the Seine River (northern France). The low numbers along the French coasts suggest that there is no established population here ^[13].

In the Netherlands, the species was observed for the first time in 1932. In the period between 1932 and 1989, there were 22 reports of this crab, mainly in the Western Scheldt and the Wadden Sea ^[5]. In the ports of Amsterdam and Rotterdam, individuals have been found every year since 1995. The first sighting from the Eastern Scheldt dates from 2002 ^[13].

The Blue crab has also been found in Denmark (the Sound near Copenhagen), Germany (Elbe estuary), Spain (Guadalquivir estuary) and England (Thames)  ^{[5, 13]}.

This species of crab has also established itself permanently in the Mediterranean, particularly in the eastern part. In Greece, the Blue crab was able to establish itself during the 1930s, after which it quickly became a popular species on the local market. During the 1960s, overfishing and pollution led to the collapse of the population. As of 2011, a decimated population still exists along the eastern coasts of Greece ^[13] and in the Adriatic Sea ^[16].

The annual migration pattern in Western European waters is similar to American waters. During summer, the species remains in brackish water, whereas in winter, they migrate towards the sea to reproduce in the deeper and saltier water.

It is unknown how the Blue crab got introduced to Europe. It is assumed that several independent releases have occurred, whether or not in the same way. The most likely vector of transport for the larvae is the ballast water of ships ^[13].

Blue crabs can withstand fluctuating temperatures and can survive in a wide range of salinity levels. The species has been observed at temperatures ranging from 3 to 35 °C and in virtually fresh to extremely saline water (up to 48 PSU) ^[3]. The Blue crab can moult and reproduce in water with a temperature between 15 and 30 °C. A female can – under favourable conditions – produce up to two million eggs per brood.

In addition, they are opportunistic omnivores, so they can survive in different habitat types. They feed mainly on shellfish, fish, amphipods, other crabs, worms, echinoderms, organic waste, algae, vascular plants and even insects. In its early development, this species has many planktonic larval stages. These can easily spread over large distances by hitching a ride on the sea currents or as stowaways in the ballast water of ships. Most likely the low winter temperatures of our regions prevent an explosive increase in this species ^[3].

The assumption that the Blue crab has established itself permanently in our study area remains a topic of discussion. On the one hand, it is suggested that only sufficiently high temperature industrial cooling water can maintain a stable population, as is the case in Doel. However, whether the seawater of the North Sea is warm enough for the development of young larvae remains to be seen ^[3].

It is noteworthy that no sightings of juveniles have been reported so far ^[3]. This is possibly due to the reduced salinity in the Sea Scheldt. Larvae of this species would only survive at salinity levels above 22 PSU ^[16]. For comparison, the salt content of seawater in the North Sea is approximately 35 PSU. Right before the hatching of the eggs, the females move to saltier water ^[16].

The growth of crabs is highly dependent on the water temperature and the food availability and quality. The Blue crab is probably most affected in our regions by the low water temperature during the winter months, which makes permanent settlement in the southern North Sea difficult. Temperatures below 10 °C hinder the development of the sexual organs (gonads) and drastically reduce the growth rate of the Blue crab. Therefore, the Blue crab can only grow at temperatures above 15 °C ^[3]. This species could benefit from the warming climate in our regions, which raises concerns about the ecological and economic impact it could have in the future ^[13]. In addition to low water temperatures, the permanent settlement of the Blue crab in Northern Europe is hindered by environmental pollution and unsuitable habitat.

The Blue crab is a highly mobile organism. The ‘paddle-like’ structures on the last pair of legs make it a good, active swimmer. This species of crab can cover a distance of 140 metres in one hour, which means that individuals can spread into a new area very quickly. In addition, the planktonic larvae can be transported over long distances by floating passively on the sea currents or in ships’ ballast water tanks ^{[3, 17]}.

Crabs can control the number of other species in their habitat, especially young shellfish, mussel and oyster populations. An explosive increase in the crab stock can lead to shifts in the ecosystem. These shifts can have a negative impact on species important to fisheries. So far, such an ecosystem effect has not been observed in our waters, probably due to the relatively limited numbers of Blue crabs in our region.

Even in other regions where the Blue crab is an exotic species (e.g. Mediterranean), no negative effects on the ecosystem have been observed. This is possibly related to a lack of research on the subject ^[13].

The fact that the Blue crab can be consumed means that it can be commercially exploited in the event of large populations. This measure proved successful in Greece, where commercial fishing decimated the population ^[13]. However, the crab does cause economic damage to the fisheries. The Blue crab damages the fishing nets and the fish caught in them ^[16]. In addition, the crab is a potential carrier of the Vibrio cholerae bacterium that is responsible for cholera epidemics, thus posing a risk to public health ^[13].

The crab is easily recognisable by the typical blue colour of its legs, the orange-blue colour of its claws and the large lateral spines. The carapace in males can measure up to 22.3 cm in diameter, for females this is limited to 17.5 cm ^[3].

[1]         World Register of Marine Species (WoRMS) (2020). Callinectes sapidus Rathbun, 1896. [http://www.marinespecies.org/aphia.php?p=taxdetails&id=107379] (2020-11-17).

[2]         Christiansen, M.E. (1969). Crustacea Decapoda Brachyura. Marine invertebrates of Scandinavia, 2. Universitetsforlaget: Oslo. 143 pp. [www.vliz.be/en/imis?module=ref&refid=121060]

[3]         Adema, J.P.H.M. (1984). De Blauwe zwemkrab, Callinectes sapidus Rathbun, 1896, (1965-1990). Vita Marina Zeebiol. Doc. Geleedpotigen. Vita Marina Zeebiologische Dokumentatie: zeebiologie, zeeaquariologie, malacologie. Stichting Biologia Maritima: Den Haag: pp. 67-70. [www.vliz.be/en/imis?module=ref&refid=75675]

[4]         Den Hartog, C.; Holthuis, L.B. (1951). De Noord-Amerikaanse "Blue Crab" in Nederland. Levende Nat. 54: 121-125. [http://www.vliz.be/en/imis?module=ref&refid=207307]

[5]         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]

[6]         Van Damme, P.; Maes, J. (1993). De Blauwe Zwemkrab Callinectes sapidus Rathbun, 1896 in de Westerschelde (België). De Strandvlo 13(4): 120-121. [www.vliz.be/en/imis?module=ref&refid=18820]

[7]         Kerckhof, F.; Haelters, J. (2005). Enkele opmerkelijke waarnemingen en strandingen in 2004 en 2005. De Strandvlo 15(3-4): 101-105. [http://www.vliz.be/en/imis?module=ref&refid=78684]

[8]         Kerckhof, F. (2007). National report Belgium, 2006, in: ICES Advisory Committee on the Marine Environment. Report of the Working Group on Introductions and Transfers of Marine Organisms (WGITMO) 21-23 March 2007 Dubrovnik, Croatia. CM Documents - ICES. CM 2007(ACME:05). ICES: Copenhagen: pp. 44-48. [http://www.vliz.be/en/imis?module=ref&refid=208968]

[9]         Kerckhof, F.; Haelters, J.; Gollasch, S. (2007). Alien species in the marine and brackish ecosystem: the situation in Belgian waters. Aquat. Invasions 2(3): 243-257. [http://www.vliz.be/en/imis?module=ref&refid=114365]

[10]       Rappé, G. (1985). Vestigt de Blauwe zwemkrab, Callinectes sapidus zich blijvend in de Zuidelijke Noordzee? De Strandvlo 5(1): 8-11. [www.vliz.be/en/imis?module=ref&refid=18185]

[11]       Waarnemingen afkomstig van Waarnemingen.be: een initiatief van Natuurpunt Studie vzw en de Stichting Natuurinformatie (2018). Blauwe Zwemkrab - Callinectes sapidus Rathbun, 1896 [https://waarnemingen.be/soort/view/26995?from=1900-11-10&to=2011-11-10&prov=0&maand=0&os=0] (2018-08-30).

[12]       Kerckhof, F. (2001). National report for Belgium, 2000, in: ICES Advisory Committee on the Marine Environment. Report of the working group on introductions and transfers of marine organisms, Barcelona, Spain, 21–23 March 2001. CM Documents - ICES. CM 2001(ACME: 08). ICES: Copenhagen: pp. 24-26. [http://www.vliz.be/en/imis?module=ref&refid=208978]

[13]       Nehring, S. (2011). Invasion history and success of the American Blue crab Callinectes sapidus in European and adjacent waters, in: Galil, B.S. et al. In the wrong place - alien marine crustaceans: Distribution, biology and impacts. , 6. Invading Nature - Springer Series in Invasion Ecology: pp. 607-624. [http://www.vliz.be/en/imis?module=ref&refid=205538]

[14]       Bouvier, E.L. (1901). Sur un Callinectes sapidus M. Rathbun trouvé à Rochefort. Bulletin du Muséum d'histoire naturelle 7: 16-17. [http://www.vliz.be/en/imis?module=ref&refid=121061]

[15]       Holthuis, L.B. (1969). Enkele interessante Nederlandse Crustacea. Bijdragen tot de faunistiek van Nederland I. Zoologische Bijdragen 11: 34-48, pl. I. [http://www.vliz.be/en/imis?module=ref&refid=207599]

[16]       Dulcic, J.; Tutman, P.; Matic-Skoko, S.; Glamuzina, B. (2011). Six years from first record to population establishment: The case of the Blue crab, Callinectes sapidus Rathbun, 1896 (Brachyura, Portunidae) in the Neretva river delta (South-eastern Adriatic Sea, Croatia). Crustaceana 84(10): 1211-1220. [www.vliz.be/en/imis?module=ref&refid=208815]

[17]       ICES Advisory Committee on the Marine Environment (2006). Report of the Working Group on Introductions and Transfers of Marine Organisms (WGITMO) 16-17 March 2006 Oostende, Belgium. CM Documents - ICES. CM 2006(ACME:05). ICES: Copenhagen. 330 pp. [http://www.vliz.be/en/imis?module=ref&refid=111237]

VLIZ Alien Species Consortium (2020). Callinectes sapidus – Blue crab. Non-native species of the Belgian part of the North Sea and bordering estuaries anno 2020. Flemish Institute for the Sea (VLIZ). 6 pp.