Rhithropanopeus harrisii - Zuiderzee crab
SCIENTIFIC NAME
Rhithropanopeus harrisii (Gould, 1841)
1981 - 1990
NW Atlantic
Unknown
The estuarine mud crab originates from the east coast of North America: from Nova Scotia in southeast Canada to Mexico. This crab lives at depths of 0-8 meters on soils covered with a thin layer of clay or mud, waste of wood, plant material and the remains of shell, which it uses as a hiding place. Sometimes, it even digs holes in the clay [2].
First observation in Belgium
In 1985, a dead individual of this crab – and a little later that year a pincer – was found for the first time near Doel, in the brackish river basin of the Western Scheldt [3]. Two years later, in 1987, another pincer was found at the same location [2]. However, it wasn’t until 1991 that the first living individuals were observed in the cooling water of the Doel Nuclear Power Station [4].
Spreading in Belgium
After 1991, several reports were made of the estuarine mud crab near Doel and Lillo [5-7]. In addition, there were reports from the more temperate saline part of the Scheldt estuary (1996, 1997) [8]. This crab was reported again in the Sea Scheldt near Lilo (2004) [9] and Doel (2007) [10]. More recently, the crab was found in the Sea Scheldt as far as Steendorp, and in a hyperbenthic sample taken near the ‘Notealer’ (Hingene) [11].
In the Ghent-Terneuzen Canal, a permanently established population has been present since 1999, stretching from the centre of Ghent to the Dutch border [12].
In 2009, the estuarine mud crab was found around the Port of Nieuwpoort [13] and in the Yser near the ‘Ganzepoot’ (Dutch for ‘goose foot’) [14] in 2009. At this last location, the individuals live between rocks covered with the Bay barnacle Amphibalanus improvisus and between reed beds [15].
Spreading in neighbouring countries
In the second half of the 19th century (1874), the estuarine mud crab was discovered in the Netherlands as a new immigrant. This crab was originally described as Pilumnus tridentatus. Only in 1949, it was recognized as Rhithropanopeus harrisii from North America [2].
In 1874, this crab was already common in the brackish southern part of the Zuiderzee, hence its Dutch name ‘Zuiderzeekrabbetje’ (in English: ‘Zuiderzee crab’). At that time, the Zuiderzee was not yet separated from the North Sea. This connection with the North Sea caused the northern part to be saltier than the southern part, making it more difficult to survive here for R. harrisii. The estuarine mud crab was also found in other rivers and lakes in North and South Holland and Groningen [16].
In 1932, the Zuiderzee was separated from the North Sea by a dam and its name changed to the Ijsselmeer. The salinity decreased, enabling the estuarine mud crab to spread further to the north; until this species reached a peak in 1936. Since then, the conditions of the Ijsselmeer resembled those of fresh water and the number of R. harrisii populations reduced [16]. In the rest of the Netherlands, the species remained, and since 1960, this crab has been present in the North Sea Canal (near Amsterdam), Zeeland, the south of South Holland and Nijmegen [2].
Until 1936, this crab was unknown in Europe outside the Netherlands. However, after 1936, the species spread rapidly. This is probably because the population in the Ijsselmeer reached its peak at that time and could, therefore, easily expand its range [2].
The first observation in northern Germany dates from 1936 [17]. In 1939, the species was observed in the south of Russia. According to fishermen, the species was already present in this area since 1936 [16]. These sightings were followed with introductions to the Sea of Azov, the Black Sea and the Caspian Sea. In the Baltic Sea, first Poland (in 1951) [2], then Denmark [18], Lithuania (in 2000), Finland (in 2009), and most recently Estonia (in 2011) were colonised by this species [19-21].
In France, the estuarine mud crab was first reported from the Canal of Caen à la Mer (in 1953) and the Canal de Tancarville (in 1956), both situated in Normandy [22]. This species was subsequently also found in the brackish part of the Gironde (in 1957) [23] and at the Loire Estuary (in 1968), both in the west of France [24].
In Great Britain, this crab has only recently been observed (since 1996), and only within the Cardiff Docks, in the southwest of England [25].
Currently, the estuarine mud crab occurs widespread in Europe from the Baltic Sea, along the European Atlantic coast (including Great Britain) to the Mediterranean Sea and the Black Sea [26].
It is unknown how the estuarine mud crab came to Europe from North America. Some suggest it happened via transport with ballast water of ships or by attachment to ship hulls [27].
However, both options remain uncertain and need to be further investigated [22, 28]. Introduction via ballast water is unlikely, as the first European reports date back to 1874 when ballast water was barely used. However, before the beginning of the 20th century, stones, sand and mud were used as ballast, in which different species (e.g. snails) might have been transported [28]. Possibly this species was spread in this way.
The omnivorous estuarine mud crab preys upon dead organisms, freshwater hydroids, worms, shellfish, copepods, marine isopods, opossum shrimps (Mysida), mosquito larvae and algae [2]. Although the female crabs produce only one brood per year, they can carry 1,280-4,800 eggs [29], which isn’t particularly high for a crab [15].
R. harrisii can tolerate a wide gradient of salinities. The estuarine mud crab is usually found in waters of about 0.1 PSU (fresh) to 15 PSU (brackish) [2]. Exceptionally, this species can be found in an environment with a salinity of 25 PSU (salt brackish) [30]. For comparison: the seawater of the North Sea is on average 35 PSU.
Temperature is a more important environmental factor determining the presence or absence of the estuarine mud crab. To be able to lay and develop eggs, the temperature should be above 20°C. If necessary, it can migrate vertically in the water column, allowing it to find the most appropriate temperature. The estuarine mud crab can also bury itself in the soil or hide between shells to hibernate, allowing it to survive colder temperatures [21].
Secondary spreading within Europe was facilitated by shipping. The larvae of the estuarine mud crab are free-swimming and remain in this stage for about 23 days [29]. Therefore, these crabs can spread via the ballast water of ships both as an adult or larvae. Furthermore, the species can spread between estuaries, rivers and lakes attached to ship hulls [25].
Economically, the presence of many estuarine mud crab individuals can obstruct the flow of water in industrial pipes [31].
In the Baltic Sea, this invasive species had a negative impact on the local food web. Here, it caused trophic cascades, which resulted in a decrease in species richness and biodiversity. The abundance of gastropods and crustaceans decreased by 99% and 75%, respectively. Chironomids (Chironomidae) even disappeared completely. The community, which was dominated by herbivores and periphyton-grazing gastropods and crustaceans, shifted to a community dominated by mussels with little herbivores. This allowed for more epiphytic algae to grow [32]. It is clear that the crab has had a major impact on this ecosystem and it may well cause a similar trophic cascade in other regions where it is not part of the native fauna [19].
The body (carapace) and the legs of the estuarine mud crab have a dark greyish-green colour, while the outside of the pincers is white. However, due to the growth of single-celled algae, they can look rather dark-brown to black. Therefore, fishermen often talk about the ‘black crab’ instead of the estuarine mud crab. The claws of the pincers remain white since due to their abundant use the algae do not get the chance to grow there. Females are usually smaller than males and can reach a width of up to 2 cm. Males grow up to 2.6 cm wide. Their body is taller-than-wide, has a rounded shape and is smooth. The legs are hairy around the edges [2].
Males and females are not solely distinguishable based on their size. Males have large pincers and a small triangular abdomen (folded towards the belly of the crab) with seven segments of which three are merged. In females, this abdomen is broad and oval, and there are no merged segments. The abdomen of both sexes is hairy along the edges [2].
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