This is a new concept of the species that includes the former species Pseudanodonta elongata (Holandre, 1836) and Pseudanodonta middendorffi (Siemaschko, 1849), which are now placed as a synonym of Pseudanodonta complanata (Rossmässler, 1835) by MolluscaBase (2022).

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

Pseudanodonta complanata is assessed as Endangered (EN A2ac) based on a decline in abundance and in subpopulations of at least 50% over the past three generations. Recent data clearly indicate population declines and range fragmentation throughout most of the species' range, and it rarely exists at high densities. With the exception of the north-western part of Russia (Baltic basin), it appears to be quite rare in the country, with a highly fragmented distribution. The species does not occur east of the Caspian Sea as previously thought, and is therefore a European endemic. From the data it can be concluded that the population decline in western, central and south-eastern Europe is well over 50% over a period of three generations (approximately 45 years). While northern and north-eastern populations appear to be less affected, the predicted effects of climate change are likely to be highly detrimental to these populations as well, due to the predicted increase in seasonal extremes such as droughts and floods.

This species is widely distributed throughout northern Europe from Great Britain to Russia west of the Ural Mountains (Lopes-Lima et al. 2017), extending south to the northern Balkans (Croatia). Despite being a widespread species, populations are generally not abundant, unstable and patchy, with scattered subpopulations. It is absent from the Iberian, Italian and southern Balkan peninsulas. It is considered endemic to the Pan-European region. Previous records from the Asian parts of Russia and the former Soviet Union (e.g., Kazakhstan) are all considered erroneous (Bolotov et al. 2020).

Austria: It used to occur in the Danube and its tributaries, but is now extremely rare and listed as Critically Endangered in the country.
Belarus: The species has been recorded from Lake Lepelskoe, Lake Naroch, Lake Dolzha, and Lake Malye Shvaksht in the northwest of the country (Baltic basin) (Burlakova et al. 2000).
England: It was widespread but has declined dramatically in recent decades and is now very rare. It has been lost from some previously known sites, such as the River Thames (Ollard and Aldridge 2022) and the River Great Stour, possibly as a result of changes in agricultural practices.
Czechia: It is known from a few scattered subpopulations (K. Douda 2022 pers. comm.).
France: Formerly widespread, it is now extinct or rare in large parts of France, such as the Garonne, Saone-Rhône and Doubs (Mouthon 2007, Bertrand 2008). The species has recently been listed as Endangered in France due to its restricted range and continuous decline.
Hungary: It is present in the Tisza, Mura, Drava, and Danube (M. Lopes-Lima pers. data).
Norway: It is restricted to the southeast of the country (J. Mageroy et al. unpublished data).
Poland: It is found in much of the country, except in mountainous areas, although in low abundance (K. Zając pers. data).
Romania: It is present in the Mures, Prut, and Danube rivers (M. Lopes-Lima pers. data).
Russian Federation: Older records suggest that the species is found in water bodies of the Baltic, Black, Azov, and Caspian Sea drainage basins. The species is found in the northwest (rivers and lakes of the basin of the Baltic Sea) e.g., Lake Peipsi or Pihkva, and in Lake Ilmen (Andreeva 2010), and in the southwest (Don basin, Voronezh oblast) as well as in the delta of the Volga (Kantor et al. 2010). Its presence further east, i.e. in the Ural River (Russian Federation, Kazakhstan), has not been confirmed (Bolotov et al. 2020). This species was recently considered widespread and common throughout European Russia, except for northern areas with an estimated EOO of more than 2,000,000 km2 (Bolotov et al. 2020).
Slovakia: The species has been recorded in good condition from the Ipoly, Ondava and Danube rivers (M. Lopes-Lima pers. data).
Sweden: It is mainly found in south-central Sweden up to the Dalälven River, with rare records in the north (ArtDatabanken 2020).
Ukraine: Its occurs across the whole country except Crimea (L. Sehvchuk pers. data).

This species does not generally occur in high densities and is often considered rare. Despite being a widespread species, populations are susceptible to decline and are patchy where they occur. Declines have been recorded in response to habitat loss, degradation and the effects of pollution. There is a suspected loss of habitat and number of subpopulations of 50% based on observed losses in many countries. While populations are declining significantly in central, western and south-eastern Europe, populations in the Baltic Sea basin appear to be more stable.

Belarus: the situation seems quite stable, with some new subpopulations found.
France: based on the information provided in the distribution section, it is clear that the species is in serious decline, with an estimated 60% decline, and is listed as Endangered (UICN France Committee 2021). The subpopulation in the Garonne basin, considered a subspecies by Falkner et al. (2001), has been lost and the species is now known only from the north-eastern part of France, apart from isolated subpopulations in the Rhône, Charente and Benaize rivers and another subpopulation in Brittany.
Germany: populations have been extirpated in many regions.
Great Britain: it has been lost from at least 50% of the previously known sites over the past 20 years (D. Aldridge pers. comm. 2022).
The Netherlands: the population has declined by more than 50% since 1990 and has disappeared from the north and south-east of the country. However, in the Rhine-Meuse Delta region, where the species was extirpated by pollution in the 1960-70s, populations appear to be recovering (bij de Vaate et al. 2007).
Norway: although the species has not been monitored, it is considered Least Concern (Artsdatabanken 2021) as recent surveys have revealed several new populations (J. Mageroy et al. unpublished data).
Poland: a clear overview of the situation is difficult because in some drainages and some parts of rivers it appears to be stable, while in others, populations fluctuate. However, in general, populations in Poland are small, isolated, and in decline (Głowaciński and Nowacki 2016).
Sweden: current estimates suggest a decline and the species is therefore listed as Near Threatened, although new populations have been discovered in recent surveys (ArtDatabanken 2020).
Ukraine: Yanovich and Pampura (2010) recorded a maximal density of 4 ind./m² for the Dnieper River. Here, the species was only found in eleven out of 100 sampling stations compared with 23 stations in previous surveys (Yanovich and Pampura 2010, Yanovich et al. 2010). This suggests that the species here is rare and decreasing (>50%) over the past 60 years.

The species is generally found in a variety of freshwater ecosystems, including rivers (mainly) and lakes, and prefers silty-sandy substrates (Lopes-Lima et al. 2017). Although not found in lakes in the UK, the species is found in some Norwegian lakes (McIvor and Aldridge 2005).

This species burrows more than some other similar species and anchors itself more firmly than other bivalve species (McIvor and Aldridge 2005, Spikkeland et al. 2022). Pseudanodonta complanata has the typical unique reproductive features of most Unionidae species including larval parental care (i.e. brooding) and larval parasitism on freshwater fishes (and occasionally other vertebrates). Larvae of P. complanata have been found on a wide range of native host fish, but have never been assessed for metamorphic success, so host fish identifications based on these studies should be treated with caution (Lopes-Lima et al. 2017).

It has a lifespan of between 8 and 32 years (although maximum ages in the UK are very close to the lower end of this range; D. Aldridge pers. comm. 2022), depending on the latitude at which they occur, and reaches a maximum length of between 7-11 cm, but typically around 9 cm (Lopes-Lima et al. 2017). The age of sexual maturity is around three years. The species is usually dioecious and the larvae usually overwinter in the gills and are released in early summer (Lopes-Lima et al. 2017).

The full extent of threats to this species is unclear, with a particular lack of information on populations in the eastern and northern parts of its range.

Threats are likely to be mainly related to habitat degradation and loss and include poor river management, water pollution (organic and inorganic), siltation, channelling, water abstraction, drought and poaching for private collections and aquaria (McIvor and Aldridge 2005). Dams, weirs, and other physical barriers also degrade the habitat of P. complanata and severely fragment its populations. The species is very sensitive to environmental disturbance and nutrient loading, for example a boat accident in a lake in Poland which released large amounts of nutrients (e.g. phosphates) and toxins from the sediment caused massive mortality of freshwater mussels, including about 83% of P. complanata individuals. Invasive species such as Corbicula fluminea, Sinanodonta woodiana, and Dreissenid mussel species are also threatening the survival of P. complanata (Ożgo et al. 2020). In France and Luxembourg, Muskrats (Ondatra zibethicus) prey on this species.

With the predicted increase in the number of droughts and other extreme seasonal climate events, and the spread and proliferation of invasive species, the species is likely to become even more threatened in the near future.

There are records of this species being harvested by collectors.

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

Austria: it is listed as Critically Endangered in the Austrian Red List of molluscs (Reischutz and Reischutz 2007).
Belarus: it is listed as Endangered in the Red Book of the Republic of Belarus (Laenko 2006).
Czechia: it is listed as Endangered in the Czech Republic Red List (invertebrates) (Farkač et al. 2005).
Germany: it is strictly protected by national legislation and listed as Critically Endangered in the Red List of German species (Binot-Hafke et al. 2011).
Great Britain: it is listed as Near Threatened in the Great Britain Red List (Seddon et al. 2014).
France: it is listed as Endangered in the French Red List of Species (UICN France Committee 2021).
Hungary: it is protected by National legislation and listed as Endangered (Fehér et al. 2004).
Poland: it is strictly protected by national legislation and listed as Endangered in the Polish Red Book (invertebrates) (Głowaciński and Nowacki 2016).
Romania: it is listed as Endangered (Sárkány-Kiss 2003).
Slovakia: it is listed as Endangered in the Red List of molluscs of Slovakia (Steffek 1994).
Sweden: it is listed as Near Threatened in the Red List of Swedish Species (ArtDatabanken 2020).

Conservation measures are needed to establish protected areas that include the species' main habitats, such as lakes, rivers and streams. This should include habitat restoration, with restoration of the entire river basins, identifying and eliminating acute and diffuse sources of pollution, and implementing control of invasive species such as fowling bivalves and macrophytes in many areas. River management practices should also be adapted to avoid harming sessile benthic organisms such as freshwater mussels. The species should benefit from the development of an action plan to guide future conservation efforts and it should be legally protected in countries where declines have been reported and its conservation status is considered threatened. There is also a need for outreach and education on the importance of the species and the ecosystem services they provide.

Research is needed to identify priority populations for conservation, to understand threats and the basic ecology of the species, and in particular to invest in long-term monitoring of priority populations to provide information on population trends.