Unio crassus 

Scope: Global
Language: English

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Taxonomy [top]

Kingdom Phylum Class Order Family
Animalia Mollusca Bivalvia Unionoida Unionidae

Scientific Name: Unio crassus Philipsson, 1788
Regional Assessments:
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 A2ce ver 3.1
Year Published: 2014
Date Assessed: 2013-04-04
Assessor(s): Lopes-Lima , M., Kebapçı, U. & Van Damme, D.
Reviewer(s): Numa, C., Seddon, M.B., Collen, B. & Böhm, M.
Contributor(s): Van Damme, D., Dyer, E., Soulsby, A.-M., Seddon, M.B., Whitton, F., McGuinness, S., De Silva, R., Milligan, H.T., Kasthala, G., Herdson, R., Thorley, J., McMillan, K. & Collins, A.
The species is assessed as Endangered (EN A2ace) based on losses across the range. This species has suffered localised declines as a result of several major threat processes including water pollution, dam construction, drainage, sedimentation and increased predation. The latest evidence suggests that the decline of U. crassus within Europe over the last three generations (ca. 45–60 years) is estimated as being greater than 50% and the threats to this species are known and will continue to increase.  If the three subspecies were to be assessed separately, then based on the population losses over the last 60 years, Unio crassus crassus (>50 %) and Unio crassus gontieri (>50 %) should be listed as Endangered and Unio crassus mongolicus (>80 %) as Critically Endangered.

In Greece and Turkey, the status of some populations are uncertain as there are still unresolved taxonomic problems that require consideration to improve the quality of the conservation assessment, given that within some Unionid species there are previously unrecognised cryptic lineages. 
Previously published Red List assessments:

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).

Another subspecies Unio c. bruguierianus has a distribution that encompasses all Asian Turkey until the Tigris and Euphrates basin from Turkey to Syria and Iraq (Lopes-Lima, pers. comm., 2013).
Countries occurrence:
Albania; Armenia; Austria; Azerbaijan; Belarus; Belgium; Bosnia and Herzegovina; Bulgaria; Croatia; Czech Republic; Denmark; Estonia; Finland; France (France (mainland)); Georgia; Germany; Greece; Hungary; Iran, Islamic Republic of; Iraq; Latvia; Liechtenstein; Lithuania; Luxembourg; Macedonia, the former Yugoslav Republic of; Netherlands; Poland; Romania; Russian Federation (Central European Russia); Serbia (Serbia); Slovakia; Slovenia; Sweden; Switzerland; Turkey; Ukraine (Ukraine (main part))
Additional data:
Upper elevation limit (metres):900
Range Map:Click here to open the map viewer and explore range.

Population [top]

Population:In the past, Unio crassus was assessed as Endangered, however the old assessments included literature which pre-dated the 1980s, and the quality of some of this data is questionable due to taxonomic issues. 

The evidence presented in this current assessment is based on recent changes and shows 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 subpopulations since the 1970s.

The second wave of population decline in Europe seems to have taken place during the period 1980–2000 when many countries of the formerly Communist Eastern Block became equally intensified as the result of modernisation (large increase of discharged 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 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, although it needs to be demonstrated that these sub-populations are Unio crassus. 

Only in the Baltic countries and European Russia does 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) is estimated as being  greater than 50% and the threats to this species are known and increasing.
Current Population Trend:Decreasing
Additional data:
Population severely fragmented:No

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 ichthyofauna 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 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).
Generation Length (years):15-20

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 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, Endangered in the Czech Republic, Poland and Sweden, Vulnerable in Albania and Belarus, Finland and Latvia and Extinct in Netherlands (using IUCN criteria, Schulte 2010) and Lithuania. This species is listed on the EC Habitats Directive (92/43/ECC) in Appendix II and IV (Bachmann 2000) although further research is required on population trends and distribution. Active site protection and management may help in reversing population declines. Eight European Community Life projects for habitat conservation and restoration actions have included this species in the following countries: Denmark, Germany, France, Luxembourg, Sweden and Poland. Five of the projects are still ongoing.

Citation: Lopes-Lima , M., Kebapçı, U. & Van Damme, D. 2014. Unio crassus. The IUCN Red List of Threatened Species 2014: e.T22736A42465628. . Downloaded on 21 April 2018.
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