Unio crassus


Taxonomy [top]

Kingdom Phylum Class Order Family

Scientific Name: Unio crassus
Species Authority: Philipsson, 1788
Common Name/s:
English Thick Shelled River Mussel
Taxonomic Notes: There are many unresolved questions about this species' taxonomy and the placement of geographic variants as either subspecies or distinct species. For example, Graf (2007), following the Biological Species Concept, discerns three geographic groups within Unio crassus, namely U. crassus species group, U. c. gontieri and U. c. mongolicus. Kantor et al. (2010), following the comparative method, divide Graf's Unio crassus species group into six distinct species.

It is therefore evident that molecular research on the Unio crassus complex is urgently needed. It has been suggested that the species concept of the Comparatory Model is too typological. The result is the description of numerous ‘species’ that occur sympatrically (instead of geographically isolated) without any solid evidence how a radiative evolution did take place in the past. We hence follow Graf’s Biological Species Concept for Unio crassus although, as this author states himself, this hypothesis also does not rest on a solid taxonomic footing. This was indeed corroborated by the fact that recently the Iberian populations of U. crassus have been lifted from the complex on the basis of molecular research and recognised as a distinct endemic species, Unio tumidiformis Reis & Araujo, 2009.

It is highly probable that the Siberian subspecies, U. crassus mongolicus will prove to be either to belong to the Unio douglasiae-complex (East Asia) as Zhadin (1952) already suggested, or is a distinct species (see, for example, Kantor et al. 2010). This may also be the case for Unio crassus gontieri that may be closer to Unio tigridis or Unio terminalis from the Tigris-Euphrate basin (Graf 2007) and by extension for Unio crassus bruguierianus of the same basin. We also agree with Van Helsdingen et al. (1996) that molecular investigation may show that the subspecies U. crassus carneus occurring in Greece and adjacent territories is also a separate species. Its main habitat, eutrofied lakes, is indeed not the same as that of the other European subspecies (lowland rivers with a low concentration of nitrates).

The main question is if these taxonomic contradictions and unsolved questions about the degree of genetic distinction between diverse ‘subspecies’ make it impossible to formulate a Red List assessment. The synonym list for this species is extensive and is given in Graf and Cummings (2010).

Assessment Information [top]

Red List Category & Criteria: Endangered A2ace ver 3.1
Year Published: 2011
Date Assessed: 2011-07-22
Assessor/s: Van Damme, D.
Reviewer/s: Böhm, M. & Collen, B.
Contributor/s: Dyer, E., Soulsby, A.-M., Whitton, F., McGuinness, S., De Silva, R., Milligan, H.T., Kasthala, G., Herdson, R., Thorley, J., McMillan, K. & Collins, A.
Unio crassus has been assessed as Endangered. Former assessments on this species have been based too much on references pre-dating the 1980s.

The recent evidence presented in this current assessment shows that a dramatic decline of the populations (90%) first started in the most industrialised regions in Europe with the highest population densities and a well developed agro-industry (i.e., in West European countries), leading to extirpation or Critically Endangered populations in the 1970s.

The second wave of population decline seems to have taken place during the period 1980–2000 in many countries of the formerly Communist Eastern Block and equally was intensified in western and southeastern Europe as the result of modernisation (steep increase of waste waters from domestic use) and intensifying agro-industrialisation.

In the northern part of its Asian range, it is unresolved whether the specimens belong to a single species or represent a number of distinct species belonging to Middendorfiana. The taxonomic status is also uncertain for the populations of the former Transcaucasian part of the USSR (Georgia, Armenia, Azerbaijan) and the adjoining Tigris-Euphrates basin in Turkey, Syria and Iraq (here, populations may belong to the Levantine Unio terminalis complex (Eolymnium), although it is known that these populations were already scarce in the 19th century. Similarly, in the Asian parts of its range the species (provided it is indeed Unio crassus) was already rare to very rare in the first half of the 20th century and there are no indications that conditions have improved; on the contrary, conditions appear to have declined.

Only in the Baltic countries and European Russia the situation appears to be still relatively good for the species. As a result of this evidence, the decline of U. crassus in Europe over the last three generations (ca. 45–60 years) should be estimated as being >50% and the threats are known and increasing. Hence, the species is assessed as Endangered (EN A2ace), regardless of the unresolved taxonomic problems. If the subspecies are assessed separately, then Unio crassus crassus and Unio crassus gontieri should most likely be listed as Endangered, and Unio crassus mongolicus as Critically Endangered.
1994 Endangered (Groombridge 1994)
1990 Vulnerable (IUCN 1990)

Geographic Range [top]

Range Description: This species is considered a widespread variable species incorporating numerous subspecies and synonyms. It is reported from the Atlantic to the Ural Mountains (European Russia) (Graf 2007), is widespread throughout Europe (Bank et al. 2006) and is found in the Tigris-Euphrates region (Middle East) and further east into the Amur Basin (eastern Russian) (Graf 2007). The species is absent in the U.K., Ireland, Iceland, Italy and the Iberian peninsula. Its distribution was formerly continuous but is now extremely patchy over most of its whole range (D. Van Damme pers. comm. 2011).

The subspecies U. c. crassus has a distribution throughout most of Middle Europe; U. c. gontieri is found in the Ukraine, the Caucasian-northern Pontic region and the transcaucasian Arax River basin; U. c. mongolicus is found in the Amur Basin and Primorye, north to Magadan (Siberia) (Graf 2007).
Albania; Andorra; Armenia (Armenia); Austria; Azerbaijan; Bahrain; Belarus; Belgium; Bosnia and Herzegovina; Bulgaria; Croatia; Czech Republic; Denmark; Estonia; Finland; France (France (mainland)); Georgia; Germany; Greece; Hungary; Iran, Islamic Republic of; Iraq; Israel; Italy; Jordan; Kuwait; Latvia; Lebanon; Liechtenstein; Lithuania; Luxembourg; Macedonia, the former Yugoslav Republic of; Moldova; Monaco; Montenegro; Netherlands; Norway; Oman; Poland; Portugal; Qatar; Romania; Russian Federation (Altay, Amur, Buryatiya, Central European Russia); Saudi Arabia; Serbia (Serbia, Serbia); Slovakia; Slovenia; Spain; Sweden; Switzerland; Syrian Arab Republic; Turkey; Ukraine (Ukraine (main part)); United Arab Emirates; United Kingdom (Great Britain); Yemen
Range Map: Click here to open the map viewer and explore range.

Population [top]

Population: Angelov and Georgiev (2000) cite an average density of 80–90 ind/m² in Bulgarian rivers prior to 1990. In three Polish lowland rivers studied by Jabukik (2008) U. crassus was found in only one river with a density of 4–20 ind./m². Babko and Kurmina (2009) cite a average density of 4±2 ind/m² in the Psel River (Ukraine) upstream from an hydroelectrical dam and 0 ind/m² in the reservoir and downstream of it.

In 1997, Sárkány-Kiss (1997) cites a maximal density of 135 ind/m² and a mean density of 62 ind/m² in the Crişul Alb/Fehér-Körös River at Ineu (Romania). The same author (Sárkány-Kiss 1983) reports that in the Târnava Mică River (Romania) the Unio crassus population is continuously spread between Ghindari and Târnăveni. Twenty years later, Sîrbu (2005) states that in the Târnava Mică only one live specimen was found in 2001 at Sângeorgiu de Pădure and after more intense research a few scattered relict populations were rediscovered upstream of Târnăveni. In the Târnava Mare River only empty shells were found along its whole course. Populations can hence crash suddenly without recovery.

In the eastern part of its range, densities of Unio crassus (U. crassus mongolicus sensu Graf = Middendorffinaia spp. of Russian authors) are extremely low. Thus Middendorffinaia dulkeitiana occurs in densities of 5–8 ind/m², M. mongolica in densities of 3–5 ind/m² and M. weliczkowkii in densities of 3 ind/m².

Localised declines have thus been documented (Schultes 2010), and the decline in population can be estimated at between 50% and 75% over the last three generations (D. Van Damme pers. comm. 2011). The following gives a country-based assessment of the population status of this species.

Albania: The species occurs in the Shkodra Lake-Buna-Drin Basin, but it is stated as rare in the Buna River which is polluted by urban sewage and agricultural run-off and siltation due to deforestation (Dhora et al. 2001).

Austria: Here, population reduction has been estimated at 90% (Welter-Schultes 2010). The species is not found any more in the Austrian part of the Danube River (Bernerth et al. 2002).

Belarus: Though the parameters (complete lack of pollution, sandy sediment, high oxygen content) were suitable for this species in the river Pripyat near Turov, it was absent; however, U. pictorum, U. tumidus and A. anatina were found (Oosterbaan 2001).

Belgium: The species was formerly abundant here, but presently is extirpated in the lower, central and most of the upper part of the country. It is only still found south of the Meuse River (some small rivers in the basins of the Meuse, Hermeton, Viroin, Lesse, Semois, Ourthe, Our, Moselle and Sûre). It is still abundant in the Haut-Sûre (S. Belgium – Luxemburg)  where the population is estimated at 30,000–35,000 individuals (Schmitz and Krippel 2007). However, according to these authors this population is also threatened with extinction mainly due to predation by Muskrat (Ondatra zibethicus). Population loss for Belgium can be estimated around 80–90% (D. Van Damme pers. comm. 2011).

Bulgaria: Angelov and Georgiev (2000) cite an average density of 80–90 ind/m² in Bulgarian rivers and state that Unio crassus is common in the whole of the Bulgarian stretch of the Danube, the lower parts of its tributaries, the Iskar River, Maritz River, Tundzha River, and the Srebarna Biosphere Reserve. But the datum of the Angelov and Georgiev paper, namely 2000, may be misleading. In an earlier publication on the molluscs of Srebarna biosphere, Angelov (1998) writes that the mollusc data from this reserve used by Frank et al. (1990) were copied from an earlier unpublished manuscript containing his data on the Bulgarian malacofauna. The latest field inventories of the Bulgarian bivalves hence pre-date 1990 and are more than 20 years old. The most recent data are those obtained during the International Danube Survey in 2001. From the results of a detailed dredging campaign Bernerth et al. (2002) conclude that U.crassus is not found any more in the Bulgarian part of the Danube River. This conclusion contradicts the statement of Angelov and Georgiev (2000) that the species is common in the Bulgarian Danube and supports the trend of a severe population decrease. Hubenov (2007) also considers the species as rare.

Czech Republic: According to Douda and Beran (2009) the species is severely threatened and based on their distribution map, it is reduced to ten populations whose present area of occupation is probably less than 100 km².

Denmark: The most recent distribution map shows that Unio crassus only survives in four locations (D. Van Damme pers. comm. 2011).

Estonia, Latvia, Lithuania: Populations of Unio crassus in Estonia still seem to be in good condition, despite severe pollution during the Soviet period. It should be pointed out that the area of wetlands keeps diminishing markedly since Estonia's independence in 1991 and accession to the European Union in 2004 (Kimmel et al. 2009). In Latvia, populations are still found over the whole of the territory; however, Rudzīte et al. (2010) plead for an increase of protected areas (presently only 23 of the 674 indicated areas contain U. crassus) to maintain a sufficient number of populations considering the increasing water pollution. In Lithuania, the species is still widely spread in rivers and watercourses, particular in the Nemunelis and Zeimena rivers (Ivinskis and Dagys 2003).

Finland: The species' range is restricted to the south and south-west of the country, forming the northern edge of its natural range. The species is declining from habitat  loss and pollution (Valovirta, cited in Wells and Chatfield 1992).

France: The distribution map of its occurrence in Nature 2000 sites shows that the species presently is still relatively common but certainly not frequent in the central region (region du Centre), and rare or extirpated everywhere else. In the Lorraine region, sizeable populations are only found in a few rivers (mainly Longeau River and Seigneulle River) (Muller 2010). In Auvergne, the population decrease compared to the period before 2000 is estimated at 90% (Brugel et al. 2009). In the Vosges, dead shells are found in all rivers, but live populations have become restricted to the few rivers not receiving urban waste water (e.g., in parts of La Lanterne) (D. Van Damme pers. obs. 2011). In Picardie, only four populations are remaining (Cucherat 2009). In the Département Nord-Pas de Calais empty shells are common but only one live population was found (Helpe River) (Cucherat 2003). It can hence be assumed that population reduction in the whole of France is between 50 and 90%.

Germany: A 90% population decline has been estimated for the species (Welter-Schultes 2010). It is not found in the German part of the Danube River (Bernerth et al. 2002).

Georgia: No recent data are available on the status of the populations in this country. Most of the species of the same genus, placed by Russian taxonomists in the genus Eolymnium, appear to be restricted to western Georgia (Kantor et al. 2010).

Greece: The species is mentioned from one small stream in northeastern Greece by Reischütz (1988) but this author states ‘in dried out river-bed’. It is also mentioned from the Trichonia-Lisimacheia Lake complex and the Koroneia-Volvi Lake complex in the northern part of the country (D. Van Damme pers. comm. 2011). Both lake systems are polluted by waste waters from villages, pig farms and olive oil refineries and by run-off from arable lands.

Hungary: In the Danube, populations were only found in the Delta and in the Tisza River (Bernerth et al. 2002). The species was not found in any of the smaller tributaries. While relatively abundant in the Hungarian part, the species was rare or absent in the Yugoslavian part of the Tisza (Csanyi 2002).

Kazakhstan: Crassiana fuscula and C. irgizlaica have been recorded from the Ural drainage in this country that according to Graf (2007) belong to U. crassus crassus, as well as Crassiana stevenianus that belongs to U. crassus gontieri sensu Graf.

Macedonia: The only reliable recent mention is from Lake Ohrid. It is absent in Lake Presba (Korniushin 2004).

Moldova: Here, the species is apparently rare (Popescu and Davideanu 2009), as far as recent data are available.

Netherlands: The species was formerly distributed over the whole country, but has not been found since 1968 (D. Van Damme pers. comm. 2011). It is thought to be extirpated.

Romania: Sárkány-Kiss (1983) reported that in the Târnava Mică River (Small Târnava) the population was continuously spread between Ghindari and Târnăven, i.e., over a distance of more than 50 km. Twenty years later, Sîrbu (2005) states that in this river stretch only one live specimen was found in 2001 (at Sângeorgiu de Pădure) and after more intense research, a few scattered relict populations were rediscovered upstream of Târnăveni. Downstream Târnăveni in the Târnava Mare river (Big Târnava), only empty shells were found along its whole course. The species is not found any more in the Romanian part of the Danube River except in the Delta (Bernerth et al. 2002). The population decrease in Romania is confirmed by Sîrbu et al. (2010) who cite only empty shells from the Danube at Cozla.

Russian Federation: No recent detailed data are available on the status of the species in European Russia. Studies such as Starobogatov et al. (2004), Hubenov (2007) and Kantor et al. (2010) are overviews based on older literature. Anecdotal information (e.g., Popov 2008) do suggest that the species is still widespread in European Russia but that populations are decreasing. Popov (2008) states that it is rare and in the St. Petersburg region restricted to two river basins.

Serbia, Slovakia: The species is not found any more in the Serbian or Slovakian part of the Danube River (Bernerth et al. 2002) but it still occurs in the Tisza (rare).

Switzerland: The species has disappeared from most cantons in the country (Welter-Schultes 2010).

Sweden: The species reaches the northern edge of its range in Sweden, namely in the south-east, from Scania to Dalaina and the populations are very scattered (von Proschwitz, cited in Wells and Chatfield 1992).

Syria: The species has been cited from the Orontes River as Unio orontesensis and as U. syriacus, but all records are more than 100 years old. It is considered as extirpated in this country.

Turkey: Records of the species in the last 50 years are extremely scarce. Çetìnkaya (1996) cites Unio stevenianus ( = U. crassus gontieri sensu Graf 2007) from the lower part of the Karasu River, and Yarsan et al. (2000) state that they collected it at different locations in Lake Van itself. This is highly surprising since this lake is saline and suggests that the Unio in Lake Van is not U. crassus. Other records appear from the Dicle River (i.e., Turkish name for the Tigris) in the region of the Ilisu Dam in Anatolia (northeastern Turkey) and from eastern Anatolia, but some of these are probably based on an old reference (D. Van Damme pers. comm.).

Ukraine: The species is present in the Delta of the Danube River (Bernerth et al. 2002). It seems to be absent from southern Ukraine, but still is found widely scattered over the northern part with the greatest concentration in the Pripyat Marshes/Pripyat River in northern Ukraine (Polesia). This marshland also extends in Belarussia and Russia. In their study on the malacofauna of the Psel and Vorskla River (Ukranian part of the Dnieper basin), Babko and Kurmina (2009) also refer to the species as being rare in Ukraine.

Iraq: The most recent  record dates from 1955 when it was collected in the Tigris Basin at Sarchinar (northeast of Mosul, northern Iraq) (Najiji 1999). The species is most likely extirpated.
Population Trend: Unknown

Habitat and Ecology [top]

Habitat and Ecology: This species inhabits clean river ecosystems and lakes, with flowing waters and sandy or sandy-gravel bottoms (Zajac 2009, Schultes 2010). As a result, it is vulnerable to changes in water chemistry, species composition of the surrounding ichtyofauna and the degradation of natural river valleys (Zajac and Zajac 2009). Juveniles are particularly sensitive to water pollution and reproduction is prevented in adults with nitrate levels that exceed 10 mg/l (Schulte 2010).

This species requires a host fish in order for successful recruitment. The eggs are released in small packets and ingested by species including Cottus gobio, Phoxinus phoxinus, Leuciscus cephalus, Scardinuis erythrophthalmus, Gymnocephalus cernua and Perca fluviatilis. The glochidia are then reliant upon a parasitic period of five weeks in the gills of the fish (Bachmann 2000). Different populations may have different fish hosts (Van Damme, pers. comm. 2011). Declines in fish species as a result of pollution, water abstraction and predation by non-native species have ultimately lead to localised declines of U. crassus (Nordsieck 2010).

The native predator of this species was the Otter (Lutra lutra), but the introduction of other mammalian predators from America such as the Muskrat (Ondathra zibethicus), the Raccoon (Procyon lotor) and the American Mink (Neovison vison) has inevitably contributed to declines of U. crassus due to direct predation of the mussel and also as a result of increased predation of the host species (Bachmann 2000, Nordsieck 2010).
Systems: Freshwater

Threats [top]

Major Threat(s): This species has demonstrated localised declines as a result of several major threat processes including water pollution, dam construction, drainage, sedimentation and increased predation. This species is vulnerable to changes in water chemistry, requiring clean waters of flowing rivers. Endobenthic juveniles are particularly sensitive to changes in water chemistry and reproduction is prevented in adults with nitrate levels that exceed 10 mg/l (Schulte 2010). Juveniles have shown increased mortality above concentrations of 2.3 mg NO3-N/l (Klink 2004, Köhler 2006).

Reduction in dissolved oxygen concentration is a major threat to this species. In Siberia, populations are threatened by the transport of logged trees via rivers and overgrazing of the riverbanks. In the Baltic countries pollution from urban wastewater stations, the construction of hydroelectric plants (D. Van Damme pers. comm. 2011), and increased beaver populations (Rudzīte 2010) are major threats to U. crassus populations.

The native predator of this species was the Otter (Lutra lutra), but the introduction of other mammalian predators from America into Europe such as the Muskrat (Ondathra zibethicus), the Raccoon (Procyon lotor) and the American Mink (Neovison vison) has inevitably contributed to declines of U. crassus. Declines in their host species as a result of habitat degradation and increased predation has reduced recruitment success, therefore negatively impacting local populations.

Conservation Actions [top]

Conservation Actions: There are no known conservation measures in place for this species outside the European range. This species has been assessed on national Red Lists as Critically Endangered in Germany, Switzerland and Austria, Vulnerable in Albania and Extinct in Netherlands (using IUCN criteria, Schulte 2010). It has also been assessed as Endangered in Poland (Zajac 2009). This species is listed on the EC Habitats Directive (92/43/ECC) in Appendix II and IV (Bachmann 2000). It is therefore possible that conservation strategies may be in place, but this information is unclear. Further research is required on population trends and distribution. Active site protection and management may help in reversing population declines.
Citation: Van Damme, D. 2011. Unio crassus. In: IUCN 2013. IUCN Red List of Threatened Species. Version 2013.2. <>. Downloaded on 20 April 2014.
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