Haas (1969) recognised eight races/subspecies of Unio crassus. Subsequently, all of these subspecies were considered junior synonyms of U. crassus. More recently, however, integrative approaches, including molecular data, have changed the taxonomy and systematics of several of these races of the U. crassus complex.

The Iberian subpopulations of U. crassus batavus were considered to be the valid species U. tumidiformis, separate from U. crassus (Reis and Araujo 2009).

Unio crassus mongolicus was placed in a separate genus as Middendorffinaia mongolica (Lopes-Lima et al. 2020).

U. crassus bruguierianus, U. crassus carneus, Unio crassus cytherea, Unio crassus ionicus, and Unio crassus gontierii were considered as valid species U. bruguierianus, U. carneus, U. ionicus, U. nanus, and U. gontierii, respectively, separate from U. crassus (Araujo et al. 2018, Lyubas et al. 2022, Lopes-Lima et al. 2024).

U. crassus crassus sensu Prié and Puillandre (2014) belongs to U. nanus, while U. crassus courtillieri belongs to U. crassus (Lopes-Lima et al. 2024).

Global and European regional assessment: Endangered (EN)
EU 27 regional assessment: Endangered (EN)

Unio crassus is a European endemic species and is assessed as Endangered (A2ac) based on losses throughout its range. This species has suffered widespread declines and extirpations as a result of several major threats including habitat destruction, water pollution, dam construction, drainage, sedimentation and increased predation. The decline of U. crassus over the last three generations (ca. 60 years) is estimated to be more than 50% and the threats to this species are likely to continue to increase.

Unio crassus is endemic to Europe in most Atlantic and Baltic river basins from France to Russia, including Sweden and Finland. It is also present in most Eastern European river basins draining into the Caspian and Black Seas, from the Danube to the Ural basins (Lopes-Lima et al. 2024).

Prior to the recent taxonomic revision, the previous species concept was more widespread. Haas (1969) recognised eight races/subspecies of Unio crassus which were subsequently considered to be U. crassus (e.g. Graf and Cummings 2007). More recently, however, integrative approaches incorporating molecular data have changed the taxonomy and systematics of these races of the U. crassus complex.

• The Iberian populations of U. crassus batavus were considered to belong to the valid species U. tumidiformis separate from U. crassus (Reis and Araujo 2009).
• The Amur and Far East Asian populations represented by Unio crassus mongolicus were placed in a separate genus as Middendorffinaia mongolica (Lopes-Lima et al. 2020).
• The Turkish, Greek and Balkan populations formerly included in U. crassus bruguierianus, U. crassus carneus, and U. crassus ionicus were split into five valid species, U. bruguierianus, U. vicarius, U. ionicus, U. desectus, and U. carneus separate from U. crassus (Araujo et al. 2018, Lyubas et al. 2022, Lopes-Lima et al. 2024).
• The U. crassus gontierii populations from Georgia, Armenia, and upper Euphrates in Asia represent a unique old lineage and was recently recognised as a separate valid species, i.e. Unio gontierii (Lopes-Lima et al. 2024).
• U. crassus crassus sensu Prié and Puillandre (2014) belongs to U. nanus, while U. crassus courtillieri belongs to U. crassus.
• Finally, populations of Unio crassus cytherea found in western and central Europe have also been separated from U. crassus crassus and considered as Unio nanus. Unio nanus occurs in sympatry with Unio crassus in several Atlantic river basins in Europe, from the Seine in the west to the Vistula in the east.

Throughout its distribution, the number of subpopulations has declined considerably and Unio crassus is now considered to be highly threatened at the European Union level and in numerous European countries (Lopes-Lima et al. 2017).

The evidence presented in this assessment and in the multiple red books across Europe shows a dramatic decline in the number of subpopulations and individuals that started in the most industrialised regions of Europe, with the highest population densities and a developed agro-industrial sector (i.e., in Western European countries), leading to extirpated or threatened subpopulations since the 1970s. A second wave of population decline seems to have taken place during the period 1980–2022, when many European countries of the former Eastern Bloc were also affected by modernisation (with a large increase in domestic wastewater discharges) and intensified agro-industrialisation.

Unio crassus is already extinct in one country and population decline rates exceed 50% in several countries as in the following examples.

• In the Netherlands it is extinct (Nienhuis 2012).
• In Finland, a population decline of at least 30% has been estimated over the last three generations (Hyvärinen et al. 2019).
• In France, a reduction of 42% was recorded in the extent of occurrence (EOO) based on modelled suitable habitat (Prié et al. 2014).
• In Germany, the number of subpopulations has declined by more than 65% in some regions and now has the highest extinction risk at the national level (Zettler and Jueg 2007; Binot-Hafke et al. 2011).
• In Poland, a decline of over 60% in the number of historical subpopulations and individuals has been estimated in the 1990s (Piechocki and Dyduch-Falniowska 1993).
• In Sweden, the population is still declining, although stable in some areas (M. Osterling, pers. data).
• In Ukraine, a decline of more than 50% in the number of historical subpopulations has been reported (L. Shevchuk pers. data).

Based on this evidence, the population decline of U. crassus over the last three generations (approximately 60 years; 1962-2022) is estimated to be greater than 50%, and the threats to this species are known and increasing.

Habitat requirements
This species inhabits clean river ecosystems with flowing water and various substrates, including sand, gravel, and fine sediment bottoms (Zając 2009, Schultes 2010).

It prefers the lentic parts of streams and rivers on the banks or under the cover of obstacles in the channel, such as roots, boulders, etc., but is not threatened by flood pulses (Zając et al. 2019). However, if the sediment is stable, they can occur even in the middle or deeper parts of the channel, usually in small streams (K. Zając and T. Zając pers. data).

It is vulnerable to changes in water chemistry, especially to high nitrate and phosphate concentrations, to the species composition of the surrounding ichthyofauna and the degradation of natural river systems (Zając and Zając 2009).

The biology of the species and its distribution are consistent with the metapopulation model, with mussel shoals extirpating and recolonising local habitat patches (Zając et al. 2018).

Life history
Unio crassus has the typical unique reproductive features of most Unionidae species including larval parental care (i.e. brooding) and larval parasitism on freshwater fish (and occasionally other vertebrates). U. crassus has been described as an intermediate host generalist as it is able to parasitise on a variety of fish species (Bauer 2001). Primary hosts are suggested to be Phoxinus phoxinus, Cottus gobio, Squalius cephalus, Gasterosteus actuleatus, Scardinius erythrophthalmus and Chondrostoma nasus (Hochwald 1997; Douda et al. 2012; Stoeckl et al. 2015; Lamand et al. 2016). However, the host compatibility of these fish species may vary between fish species and geographical areas (Douda et al. 2012; Taeubert et al. 2012). As U. crassus has recently been separated from the co-occurring Unio nanus (for which morphological identification is not possible), previous host fish trials using mussel larvae originated from sympatric populations of U. crassus and U. nanus should be re-evaluated.

The species has a variable lifespan and growth pattern depending on the latitude, climatic and hydrological conditions of the habitat. Lifespan can vary between 8 and 90 years, but is typically between 20 and 30 years (Lopes-Lima et al. 2017). Maximum length also varies between 5 and 11 cm for the same reasons, but typically does not exceed 8 cm (Lopes-Lima et al. 2017).

Larvae (glochidia) are typically incubated in females in successive broods with subsequent larval release between spring and summer, but with variation between subpopulations (Lopes-Lima et al. 2017).

Females of this species lay eggs on marsupia as early as mid-March in southern Poland, but much later in northern Europe. Eggs are fertilised by spermatozeugmata (sperm balls), which flow freely or are filtered from the water to the female marsupia. In Poland, females usually lay three broods, but may lay as many as 5, and exceptionally 7. In early spring the incubation period lasts about 20-30 days, but later in the year it lasts only 10 days. Spawning is prolonged until the beginning of August (K. Zając and T. Zając pers. data).

Natural mortality is highest in spring and minimal later in the year. The species usually hibernates in sediments, covered by sediment or leaves and debris, individuals usually descend near the bottom of the channel, but they remain on the bank slope (K. Zając and T. Zając unp. data).

This species has shown local declines as a result of several major threat processes, including water pollution, dam construction, drainage, poor river management, sedimentation and increased predation (Lopes-Lima et al. 2017). This species is vulnerable to changes in water chemistry and requires clean water in flowing rivers, but is tolerant of moderate levels of eutrophication in naturally clean rivers (Hus et al. 2006). Endobenthic juveniles are particularly sensitive to changes in water chemistry and reproduction is prevented in adults at nitrate concentrations above 10 mg/l (Schultes 2010).

The native predator of this species was originally the Eurasian Otter (Lutra lutra), but the introduction of other mammalian predators from America to Europe, such as the Muskrat (Ondathra zibethicus), Coypu (Myocastor coypus), Raccoon (Procyon lotor) and American Mink (Neovison vison), has inevitably contributed to the decline of U. crassus (Lopes-Lima et al. 2017). Declines in host species due to habitat degradation and increased predation have reduced recruitment success, negatively impacting local subpopulations (Lopes-Lima et al. 2017).

The species is used as a poultry feed in Romania. Human consumption also occurs in some parts of its range.

Unio crassus is protected in the European Union where it is listed in annexes II and IV of the EC Habitats Directive. Multiple EU projects have been implemented in several countries to reverse the decline of this species, including habitat restoration and ex situ propagation and population augmentation programmes.

Due to the observed declines, the conservation status of the species is now considered as threatened or Near Threatened in several countries.

• In Austria, it is listed as Critically Endangered in the Austrian Red List of Molluscs (Reischutz and Reischutz 2007).
• In Belarus, it is listed as Vulnerable in the Red Book of the Republic of Belarus (Laenko 2012).
• In Belgium, it is strictly protected by national legislation.
• In Bulgaria, it is protected by national legislation.
• In Croatia, it is strictly protected by national legislation.
• In Czechia, it is listed as Endangered in the Czech Red List (Farkac et al. 2005) and protected by national legislation.
• In Finland it is listed as Vulnerable in the Red List of Finnish species (Hyvärinen et al. 2019).
• In France, it is listed as Vulnerable in the French Red List of Species and protected by national legislation.
• In Germany, listed as Critically Endangered in the Red List of German species (Binot-Hafke et al. 2011) and it is strictly protected by national legislation.
• In Hungary, it is protected by National legislation and considered as Rare (Fehér et al. 2004).
• In Latvia, it is listed as Vulnerable in the Red Book of Latvia (Rudzīte et al. 2018) and it is protected by national legislation.
• In Lithuania it is considered as Endangered in the Red Book of Lithuania (Rašomavičius 2021).
• In the Netherlands, it is considered as Extinct (Nienhuis 2012).
• In Poland, it is strictly protected by National legislation and listed as Endangered in the Polish Red Book (Invertebrates) (Zając 2009).
• In Romania, it is considered as Endangered (Sarkany-Kiss 2003).
• In Slovakia, it is listed as Vulnerable in the Red List of molluscs of Slovakia (Steffek 1994) and it is protected by national legislation.
• In Sweden, it is listed as Near Threatened in the Red List of Swedish Species (ArtDatabanken 2020) and it is protected by national legislation.
• In Ukraine it is protected and listed as Vulnerable in the 2021 Red Data Book of Ukraine and it is protected by national legislation.

The species should benefit from the development of an action plan to guide future conservation efforts. Measures should include changes to river management practices that involve excavation and disturbance of the substrate, banks and key hydromorphological elements of the channel to avoid harming sessile benthic organisms such as freshwater mussels, identifying and eliminating acute and diffuse sources of pollution, and habitat restoration, with restoration of entire river basins. Control of invasive species such as fowling bivalves, macrophytes and fish should also be undertaken in many areas.

Research is needed to identify priority populations for conservation, to understand threats, the basic ecology of the species and, in particular, long-term monitoring of priority populations to provide information on population trends. It is essential to identify priority populations for conservation in order to establish protected areas, and then to invest in long-term monitoring of these populations using standardised surveys to provide information on population trends. Outreach and education to relevant stakeholders such as environmental and water management agencies, farmers and farmers' associations, and the general public on the importance of the species and the ecosystem services it provides is also needed.

The species should be legally protected in countries where declines have been reported and its conservation status is considered to be threatened.