Chelicorophium curvispinum - Caspian mud shrimp
Originally, the Caspian mud shrimp was only found in the rivers flowing into the Caspian and the Black Sea [2].
First observation in Belgium
The Caspian mud shrimp is a species of shrimp that was first reported in Belgium in late 1981 in the Meuse, near Huy, under the name Corophium sp.
Spreading in Belgium
By 1990, the Caspian mud shrimp had already conquered the entire Meuse basin. During the 1990s, the species migrated via the canal network into the waters of Limburg, Flemish Brabant and Antwerp. It is mainly found in the canals in the east of Flanders (including Antwerp). In Belgium, this mud shrimp can mainly be found in fresh and slightly brackish water, with salt levels between 0.03 and 0.8 PSU [6]. By way of comparison, the seawater of the North Sea has a salinity of about 35 PSU.
Spreading in neighbouring countries
The Caspian mud shrimp spread from the Volga, Dnieper, Dniester and Danube rivers (all of which flow into the Black or Caspian Seas), through inland canals, to most of Europe [2] (Figure 1). The northern route consists of the Volga River in Russia to the Baltic Sea. The central route connects the Dnieper River in Ukraine, via rivers in Poland, to the German Rhine and the Baltic Sea. Its southern spread was made possible by the connection between the Danube – which flows into the Romanian Black Sea – and the Rhine in Germany [2].
The first sighting outside its natural area dates from 1912 at Müggelsee in Berlin, where it was described as a new species, namely Chelicorophium devium [7]. The Caspian mud shrimp was introduced into the Müggelsee from the Dnieper river in Ukraine via the canal system with transport boats [2]. In 1926, the Caspian mud shrimp was spotted in the brackish waters around the Baltic Sea. Via the Danube, the species is thought to have already reached Lake Balaton in Hungary before 1929 [8].
Figure 1: Distribution of the Caspian mud shrimp from its area of origin to Europe. © VLIZ, adapted from Bij de Vaate et al. (2002) [2].
In 1931, Chelicorophium curvispinum was also reported in the Polish Oder and Vistula rivers, which connect the Dnieper with the German waterways and the Baltic Sea. Presumably, it spread as a stowaway in ballast water from North German ports to England, where it was first observed in 1935 [9]. The observations in England are limited to two sightings, in 1935 and 1962
However, the observations in the Belgian Meuse took place before those in the Rhine, so the introduction in Belgium could not have taken place directly via the Rhine-Meuse Canal [5]. From Belgium, the Caspian mud shrimp has migrated upstream via the Meuse to France since 1981, where it was first reported in 1986 [3]. In France, this species migrates upstream at an average speed of 15 km per year [12].
In 1987, the Caspian mud shrimp was observed for the first time in the Netherlands [13]. The highest concentration of Caspian mud shrimp was found in the Rhine area, near the Dutch-German border, which led to the conclusion that the species had entered via the German Rhine and not via the Belgian Meuse. From 1991 onwards, the Dutch part of the Meuse was colonised. In that same year, the Dutch population increased enormously, and a record concentration of more than 750,000 Caspian mud shrimp per square metre was found [13]. In the following years, the species colonised all major waterways in the Netherlands [14].
The introduction of Ponto-Caspian species (i.e. species from the Caspian and Black Sea region) into European waters is due to the construction of canals connecting different rivers (Figure 1). Through this canal system, the Caspian mud shrimp could have been introduced – both actively and passively. It can spread in three different ways: of its own accord, by attachment to ship hulls or by transport in ballast water [2]. It is not known in which of these ways the Caspian mud shrimp reached the Belgian Meuse river.
The opening of the Main-Danube Canal in 1992 (the southern route) brought a new influx of Ponto-Caspian migrants to the German Rhine, the Dutch Lower Rhine and the Dutch and Belgian Meuse, respectively [2]. From here, some species, such as the bloody-red shrimp Hemimysis anomala, could enter the Belgian inland waters [15]. Once a river or canal has been colonised, the further spread can simply occur by passively drifting with the currents. This is how the Rhine and its basin would have been colonised after an introduction from the Danube and Main-Danube Canal [16].
The habitat of the Meuse and Scheldt rivers is characterised by the absence of other species of mud tube-building amphipods
This non-native mud shrimp tolerates low levels of oxygen to a minimum of 0.3 mg O2 per litre [19], multiplies best in hard water (i.e. water with relatively high ion content), needs a minimum of sodium ions (Na+; >0.5 mM) [20] and tolerates temperatures between 7 and 32 °C [21]. Another requirement is the constant supply of organic material and silt. However, these conditions are present in almost all large waterways in Belgium and the Netherlands [13].
In nutrient-rich (eutrophic) waterways, an opportunistically feeding species, such as the Caspian mud shrimp, can produce many offspring. Combined with its short life cycle – an adult can produce up to three generations a year that can reach maturity in a matter of weeks – this allows the species to quickly become dominant in new suitable habitats [18]
The highest number of Caspian mud shrimps are observed in slow-flowing water on a hard substrate, about one metre below the water level. After all, the ideal site for a colony of mud shrimps is a location with a balance between a sufficiently strong current – to be able to filter enough food from the water column – and a not too strong current, so that the mud tube is not washed away. Tubes attached higher in the water column are more vulnerable to tidal effects and waves from passing ships
A mild form of pollution and a slightly increased salt concentration give this species a competitive advantage, as these mud shrimps show a certain degree of tolerance [22]. A salinity higher than 6 PSU prevents the occurrence of the Caspian mud shrimp [23].
The presence of a large number of Caspian mud shrimps – on stony habitats there are often more than 100,000 individuals per m² – caused drastic changes in the macrofauna of the Rhine [14]. Organisms that adhere to similar substrates and/or feed in similar ways were displaced. For example, the advance of the Caspian mud shrimp in Dutch waters has been accompanied by a sharp decline in the non-native zebra mussel Dreissena polymorpha, the exotic tiger scud Gammarus tigrinus and the caddisfly Hydropsyche contubernalis [18].
The zebra mussel in particular suffers from direct competition with the Caspian mud shrimp. This non-indigenous mud shrimp hides in homemade mud burrows [24]. In the Rhine, this thick layer of mud tubes prevents the zebra mussels from adhering to the underlying stones. Those that succeed in establishing themselves run the risk of suffocating under the mud
The negative relationship between the zebra mussel and the Caspian mud shrimp is an exception. Most relationships between species from the Ponto-Caspian region are neutral or even positive, facilitating the invasion of new species. Today, even the British Isles, which for the time being have been relatively spared from alien species from the Ponto-Caspian region, must now be on their guard for a so-called ‘invasional meltdown’. The islands are relatively easy to reach from Western Europe and an increasing number of non-native species are found here, which in turn promote the invasion of other species. This allows an increasing number of non-native species to establish themselves at the expense of indigenous biodiversity [26].
Nonetheless, some beneficial effects for biodiversity have been attributed to the Caspian mud shrimp. For example, large numbers of mud shrimps could filter out considerable amounts of organic material from the water column, making the water clearer. This promotes light penetration and subsequently stimulates the growth of plants and algae living on the bottom [25].
In 1995, the introduction of a new exotic species, namely the killer shrimp Dikerogammarus villosus, into the Rhine appeared to control the numbers of Caspian mud shrimp. These days the stony habitats of the Rhine are dominated by both the Caspian mud shrimp and the killer shrimp [24].
Together with many other adherent organisms, the Caspian mud shrimp is part of the so-called biofouling community. Biofouling can affect various substrates and even cause economic damage. Preventing fouling on ship hulls by cleaning and treating them with biofouling paint costs a lot of money [27]. Moreover, many of these paints cause damage to the ecosystem long after they have been withdrawn from circulation (e.g. tributyltin or TBT) [28]
The Caspian mud shrimp is a filter feeder that feeds on organic material filtered from the water column. A special behaviour of the Caspian mud shrimp is that it hides in homemade burrows made of mud
Caspian mud shrimps are generally darker than the native mud shrimp species, have a yellowish colour and are covered with brown stripes and dots
The species can also be distinguished from other similar species by a number of anatomical features [5]. However, these can only be observed with a binocular.
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