Proasellus coxalis -

Proasellus coxalis was described in 1892 based on specimens from Lake Kinneret (Israel) ^[2]. However, it is thought to come from southern Italy and the Greek Aegean islands ^[2], although some consider the entire Mediterranean region to be the area of origin ^[3].

First observation in Belgium

In Belgium, Proasellus coxalis was observed for the first time in 1998. ^[4]. The species was found in some samples taken by the Flemish Environment Agency (VMM) at various locations in Flanders. Although these were mainly freshwater samples, Proasellus coxalis was found in brackish water in the study area ^[5].

Spreading in Belgium

Proasellus coxalis is observed in and around the Ghent-Terneuzen Canal, such as in the Avrijevaart and several creeks north of Ghent near the Dutch border. The species is also found in the nearby Leopold Canal, which flows into the sea at Zeebrugge ^[5]. Furthermore, this isopod was reported in the upper reaches of Raambeek in Heist-op-den-berg, near Mechelen ^[2], and the Antwerp to North-South connection ^[5]. Although it is a typical freshwater species, it also thrives in some Belgian brackish waters ^[6].

Spreading in neighbouring countries

The first observation outside its area of origin was in 1931, in the German Rhine ^[7]. Presumably, the species came from the estuary of the Rhône in southern France and ended up in the Rhine via the Rhône-Rhine Canal ^{[7, 8]}. Through the Dortmund-Ems Canal, the species reached in 1987 the Ems Estuary in Northern Germany ^[7]. Since the 1950s, this freshwater isopod has been found in most inland waters of northern Germany, such as the Ems, the Elbe, the Ruhr and the Weser. In the Upper Rhine in southern Germany, this species is quite rare ^[7].

In the Netherlands, Proasellus coxalis was sighted in 1978 ^[2], when the species was found in a few streams in the province of Limburg. Today, the species is quite common there ^{[9, 10]}. In the Central Netherlands, this freshwater isopod is present in the Waal (near Nijmegen), the Rhine (near Arnhem) ^[11] and the Meuse ^[12]. After re-examining previously collected material, it became clear that the species was already present in Dutch Limburg (at Vlodorp on the Ruhr river) since 1948, but at the time had been mistaken for the non-native freshwater isopod Proasellus meridianus ^[9]

Proasellus coxalis probably spread in the inland waters of Europe through inland shipping – as a stowaway in ship’s ballast water. The creation of connecting channels allowed for autonomous dispersal between previously separated river basins ^{[3, 8]}.

Nutrient-rich water usually contains a lot of organic waste, which serves as a food source for Proasellus coxalis. In nutrient-poor water, this isopod experiences competition with the indigenous water louse Asellus aquaticus ^[11]. In general, the concentrations of Proasellus coxalis can be considered to be limited ^[5].

This freshwater isopod prefers water with little current since it is a poor swimmer and the adults mainly live on the bottom. Therefore, dispersal occurs mainly via juveniles carried away with the currents. As the currents at the sites of occurrence are often slow, dispersal is limited ^[13].

Although Proasellus coxalis is a typical freshwater species, it can thrive in brackish water, such as in estuaries ^[14]. The species can be found in German and Belgian waterways at salinities up to 5 PSU ^{[3, 15]}. By way of comparison, the seawater of the North Sea has a salt concentration of approximately 35 PSU.

There are no known effects on the ecosystem linked to the occurrence of this isopod.

Freshwater isopods look similar to terrestrial isopods ^[9], although they are usually flattened and have longer antennae ^[16]. Males are between 7 and 10 mm long and females between 5 and 8 mm ^[2]. They have a grey-brown to beige colouration ^[2] and appear colourless in places without sunlight ^[3].

Proasellus coxalis lives on the bottom or between rocks and water plants in stagnant or slow-flowing waters ^{[9, 14]}. They feed on dead organic material. Like most freshwater isopods, this species carries fertilized eggs in a brood pouch until all larval stages are completed. Some plates connected to the first five pairs of legs slide like roof tiles over each other and form the brood pouch ^[9].

Proasellus coxalis can be distinguished from the water louse Asellus aquaticus and Proasellus meridianus by looking at the spots on their heads. A. aquaticus has two white spots, while P. coxalis and P. meridianus have only one. The latter can be distinguished by looking at the shape of the spot. P. meridianus has a regularly shaped spot (trapezoidal), while P. coxalis has an irregular one ^[9]. Furthermore, the species get identified by comparing their swimming legs (pleopods). The hairs on the branches (exopodites) of these pleopods differ among the species ^[9].

Two individuals of Proasellus coxalis can look very different. Scientists subdivided this species into 28 groups or subspecies ^[17]. Since some of these subspecies occur in Western Europe, it is assumed that there occurred several independent introductions of this species of isopod ^[13]

[1]          World Register of Marine Species (WoRMS) (2020). Proasellus coxalis (Dollfus, 1892). [http://www.marinespecies.org/aphia.php?p=taxdetails&id=233891] (2020-11-17).

[2]          Vercauteren, T.; Wouters, K.A. (2008). Proasellus coxalis sensu auctorum (Crustacea, Isopoda) in de bovenloop van de Raambeek te Heist-op-den-Berg: eerste vaststelling van deze zoetwaterpissebed in België. Antenne: tijdschrift van de Antwerpse Koepel voor Natuurstudie 2(4): 12-16. [http://www.vliz.be/en/imis?module=ref&refid=207789]

[3]          Nehring, S.; Leuchs, H. (1999). Neozoa (Makrozoobenthos) an der deutschen Nordseeküste: eine Übersicht. Bericht BfG, 1200. Bundesanstalt für Gewässerkunde = Federal Institute of Hydrology: Koblenz. 131 pp. [http://www.vliz.be/en/imis?module=ref&refid=120661]

[4]          Messiaen, M.; Lock, K.; Gabriels, W.; Vercauteren, T.H.; Wouters, K.; Boets, P.; Goethals, P.L.M. (2010). Alien macrocrustaceans in freshwater ecosystems in the eastern part of Flanders (Belgium). Belg. J. Zool. 140(1): 30-39. [http://www.vliz.be/en/imis?module=ref&refid=206040]

[5]          Boets, P. (2019). Persoonlijke mededeling

[6]          Boets, P.; Lock, K.; Goethals, P.L.M. (2011). Shifts in the gammarid (Amhipoda) fauna of brackish polder waters in Flanders (Belgium). J. Crust. Biol. 31(2): 270-277. [http://www.vliz.be/en/imis?module=ref&refid=211034]

[7]          Holdich, D.; Pölck, M. (2007). Invasive crustaceans in European inland waters, in: Gherardi, F. Biological invaders in inland waters: profiles, distribution, and threats. Invading Nature - Springer Series in Invasion Ecology: Dordrecht: pp. 29-75. [http://www.vliz.be/en/imis?module=ref&refid=207787]

[8]          Nehring, S. (2005). International shipping – a risk for aquatic biodiversity in Germany, in: Nentwig, W. et al. Biological invasions – From ecology to control, 6. NeoBiota: Berlin: pp. 125-143. [http://www.vliz.be/en/imis?module=ref&refid=207779]

[9]          Huwae, P.; Rappé, G. (2003). Waterpissebedden: een determineertabel voor de zoet-, brak- en zoutwaterpissebedden van Nederland en België. Wetenschappelijke Mededelingen van de Koninklijke Nederlandse Natuurhistorische Vereniging, 226. KNNV Uitgeverij: Utrecht. ISBN 90-5011-171-8. 55 pp. [http://www.vliz.be/en/imis?module=ref&refid=40665]

[10]        Tolkamp, H. (1983). Beken in Noord- en Midden-Limburg. Natura (Amst.) 81(1): 94-101. [http://www.vliz.be/imis/imis.php?module=ref&refid=208160]

[11]        Vermonden, K.; Leuven, R.S.E.W.; Van der Velde, G. (2010). Environmental factors determining invasibility of urban waters for exotic macroinvertebrates. Diversity Distrib. 16: 1009-1021. [www.vliz.be/en/imis?module=ref&refid=207790]

[12]        Josens, G.; Bij de Vaate, A.; Usseglio-Polatera, P.; Cammaerts, R.; Chérot, F.; Grisez, F.; Verboonen, P.; Vanden Bossche, J.P. (2005). Native and exotic Amphipoda and other Peracarida in the River Meuse: new assemblages emerge from a fast changing fauna. Hydrobiologia 542(1): 203-220. [http://www.vliz.be/en/imis?module=ref&refid=207710]

[13]        Ketmaier, V. (2002). Isolation by distance, gene flow and phylogeography in the Proasellus coxalis-group (Crustacea, Isopoda) in Central Italy: allozyme data. Aquat. Sci. 64: 66-75. [http://www.vliz.be/en/imis?module=ref&refid=207788]

[14]        van der Velde, G.; Rajagopal, S.; Kelleher, B.; Muskó, I.; Bij de Vaate, A. (2000). Ecological impact of crustacean invaders: general considerations and examples from the Rhine River, in: von Vaupel Klein, J.C. et al. The Biodiversity Crisis and Crustacea: Proceedings of the Fourth International Crustacean Congress, Amsterdam, Netherlands, 20-24 July 1998, volume 2. Crustacean Issues, 12. A.A. Balkema: Rotterdam, Brookfield: pp. 3-33. [http://www.vliz.be/en/imis?module=ref&refid=207016]

[15]        Boets, P.; Lock, K.; Goethals, P.L.M. (2011). Using long-term monitoring to investigate the changes in species composition in the harbour of Ghent (Belgium). Hydrobiologia 663(1): 155-166. [http://www.vliz.be/en/imis?module=ref&refid=201947]

[16]        Waterwereld (2018). zoetwaterpissebed of Assellus, waterezeltje, waterluis. [http://www.waterwereld.nu/zoetwaterpissebed.php] (2018-08-06).

[17]        Stoch, F.; Valentino, F.; Volpi, E. (1996). Taxonomic and biogeographic analysis of the Proasellus coxalis-group (Crustacea, Isopoda, Asellidae) in Sicily, with description of Proasellus montalentii n. sp. Hydrobiologia 317(3): 247-258. [http://www.vliz.be/nl/catalogus?module=ref&refid=208341]

VLIZ Alien Species Consortium (2020). Proasellus coxalis. Non-native species of the Belgian part of the North Sea and bordering estuaries anno 2020. Flemish Institute for the Sea (VLIZ). 5 pp.