82. Unio crassus Philipsson, 1788 Findings in Serbia. Unio stevenianus: Tadić (1960). Unio dokici: Tadić (1960). Unio piscinalis: Droüet (1881, 1884); Tadić (1960). Unio cytherea: Droüet (1881, 1884); Dokić (1882); Tadić (1960). Unio crassus: Dokić (1882); Tadić (1960); Marković (1999); Živić et al. (2001); JDS-ITR Report (2002); Simić et al. (2006); Novaković (2012, pers. comm. 2012, 2014, 2018); Savić (2012); Raković (2015); Lucić et al. (2015); Tomović et al. (2010 b); Tomović (2016). Common name. Thick shelled river mussel. Morphology. Large bivalve. Shell is distinctly rounded in the posterior end, with a completely rounded dorsal margin. It reaches 70 mm (rarely up to 120 mm) in length, 45 mm in height and 35 mm in depth. For more details see Welter-Schultes (2012: 12, figure on the top of the page). Distribution and ecology. Western Palearctic species preferring clean waters (rivers and smaller watercourses) in lowland regions. It usually could be found on a sandy or stony substrate, avoiding polluted muddy rivers. In Serbia it has been found in the Sava, Tisza, Velika and Zapadna Morava, Nišava and Kolubara Rivers and catchments. There are also sporadic findings of the species in the Danube River (Tadić 1960; Tomović et al. 2016). This mussel rarely reaches altitudes over 300 m. a.s.l. in Serbia, while in the neighboring Bulgaria, for example, an abundant population has been recorded as high as nearly 900 m. a.s.l. (Kenderov & Sashov 2017). Other remarks. Despite its relatively wide distribution range it is one of Europe’s most endangered freshwater mussel taxa, being assessed as EN on the IUCN Red List (Lopes-Lima et al. 2014 a) and listed in the 92/43 EEC Directive (annexes II and IV; Directive, 1992). The species is protected on national level as well by Serbian national regulative (Annex 1-strictly protected species; Official Gazette of RS 5/2010, 47/2011, 32/2016, 98/2016). One of the limiting ecological factors for this species are juveniles, which are extremely sensitive to any pollution and require well-oxygenated waters (Welter-Schultes 2012). There are 318 nominal species as synonyms (Graf & Cummings 2021)., Published as part of Marković, Vanja, Gojšina, Vukašin, Novaković, Boris, Božanić, Milenka, Stojanović, Katarina, Karan-Žnidaršič, Tamara & Živić, Ivana, 2021, The freshwater molluscs of Serbia: Annotated checklist with remarks on distribution and protection status, pp. 1-64 in Zootaxa 5003 (1) on page 39, DOI: 10.11646/zootaxa.5003.1.1, http://zenodo.org/record/5120059, {"references":["Tadic, A. (1960) Najvazniji predstavnici skoljaka roda Unio iz Save, Dunava i Kopackog jezera. Prirodnjacki muzej, Beograd, 46 pp. [in Serbian]","Dokic, L. (1882) Skoljke u Srbiji. Grada za faunu Kraljecine Srbije. Kraljevsko-srpska drzavna stamparija, Beograd, 36 pp. [in Serbian]","Zivic, I., Markovic, Z. & Brajkovic, M. (2001) Macrozoobenthos in the Pusta Reka river, left tributary of the South Morava river. Archives of Biological Sciences, 53 (3 - 4), 109 - 122.","JDS-ITR National Tisza Report (2002) Report on the joint investigation of the Tisza River on the territory of the FR Yugoslavia within the international JDS-ITR Program. Federal Hydro- Meteorological Institute, Belgrade, 52 pp.","Simic, V., Simic, S., Petrovic, A., Paunovic, M., Soric, V. & Dimitrijevic, V. (2006) Biodiversity in aquatic ecosystems in Serbia (BAES ex situ online database). Faculty of Sciences, Kragujevac. Available from: http: // baes. pmf. kg. ac. rs (accessed 12 April 2015)","Novakovic, B. (2012) Indicative ecological status assessment of the Juzna Morava River based on aquatic macroinvertebrates. Water research and management, 2 (4), 45 - 50.","Savic, A. (2012) Ecological analysis of macrozoobenthos communities in the Nisava River. University of Belgrade, Belgrade, 172 pp. [PhD. Thesis, in Serbian]","Rakovic, M. (2015) Diversity of Molluscs in the Danube River (1260 - 863.5 rkm) and Taxonomic Analysis of the Genera Planorbarius, Radix, Physella and Ferrissia (Pulmonata: Basommatophora). University of Belgrade, Belgrade, 147 pp. [PhD. thesis, in Serbian]","Lucic, A., Paunovic, М., Tomovic, J., Kovacevic, S., Zoric, K., Simic, V., Atanackovic, A., Markovic, V., Kracun-Kolarevic, M., Hudina, S., Lajtner, J., Gottstein, S., Milosevic, D., Andus, S., Zganec, K., Jaklic, M., Simcic, T. & Vilenica, M. (2015) Aquatic macroinvertebrates of the Sava River. In: Milacic, R, Scancar, J. & Paunovic, M. (Eds.), The Handbook of Environmental Chemistry. Vol. 31. The Sava River. Springer, Heidelberg, New York, Dordrecht, London, pp. 335 - 359. https: // doi. org / 10.1007 / 978 - 3 - 662 - 44034 - 6 _ 13","Tomovic, J., Vrankovic, J., Zoric, K., Borkovic Mitic, S., Pavlovic, S., Saicic, Z. & Paunovic, M. 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