The identity and number of species within the genus Anodonta from the Western Palearctic region have been always a problem for research. For instance, due to the extensive environmental plasticity, “splitters”, mainly from the French “école nouvelle”, have produced more than 400 nominal taxa (Graf and Cummings 2022). This high number of names and shell forms confused Haas (1969) that decided to merge all of them into a single species, i.e. Anodonta cygnea. Since then, both Anodonta anatina and Anodonta cygnea have been widely recognized as valid species (e.g. Nagel et al. 1998, Falkner et al. 2001) and confirmed by molecular approaches (e.g. Lopes-Lima et al. 2017). Posteriorly, two other species have been recognized, the Adriatic species Anodonta exulcerata and the recently described Anodonta seddonae from the central Anatolian plateau (Riccardi emet al. 2020, Lopes-Lima et al. 2021).

Additionally, using these molecular techniques, some strains of Anodonta anatina seem distinct enough to be considered different species, such as those from the Iberian Peninsula and Northwest Africa, the one from the Adriatic basin, and those in the Azov Sea basin (Froufe et al. 2014, Lopes-Lima et al. 2016, Tomilova et al. 2020). Therefore, a more comprehensive study is necessary to clarify the taxonomy of these lineages.

European regional assessment: Vulnerable (VU)
EU 27 regional assessment: Vulnerable (VU)

The Duck Mussel (Anodonta anatina) is assessed as Vulnerable (VU A2c) for Europe and for the EU27 Member States based on a suspected population decline of at least 30% over the last three generations (45 years) as a result of habitat loss and degradation, although the decline is much greater in parts of its southern European range. The species is still geographically widespread throughout Europe, occurring mainly in lentic habitats. However, the species has been reported to be in decline and is now considered threatened in many European countries due to habitat degradation and loss.

Due to the isolation of the European populations from North Africa and Asia and the thousands of physical barriers (e.g. dams and weirs) in most hydrographic basins of these continents, the transcontinental migration of larvae or fish infested with A. anatina larvae should be minimal or non-existent.

In Europe, this species can be found throughout the continent, from Portugal and Ireland in the west, as far east as the Ural Mountains in Russia, north to Sweden and southern Norway (Lopes-Lima et al. 2017), and south to the European part of Türkiye.

The range of the species extends beyond the European region. In Asia, it can be found in northern Russia, west of the Lena River basin, it can also be found in the countries surrounding the Black, Caspian and Azov Seas, and in Syria (Bolotov et al. 2017, Tomilova et al. 2020). In Africa, the species is very rare and known to occur in Morocco, Algeria and Tunisia (Gomes-dos-Santos et al. 2019, M. Lopes-Lima 2020 pers. data.).

Until recently, this species was described as common, with large subpopulations and a wide distribution across Europe (Lopes-Lima et al. 2017).

The species has not been monitored and population trends have not been quantified in most of its range, but severe declines in abundance and number of subpopulations have been observed in southern France, Italy, Spain and Türkiye, where recent surveys have failed to find the species in many rivers where it was previously common. (V. Prie, N. Riccardi, and M. Lopes-Lima pers. data).

Furthermore, in most European countries, an ongoing pattern of decline has become obvious and it is already considered Endangered in Germany (Binot-Hafke et al. 2011) and Portugal (Lopes-Lima et al. 2023); Vulnerable in Austria (Reischütz and Reischütz 2007), France (UICN French Committee 2021), Romania (Sárkány-Kiss 2003), and Switzerland (Rüetschi et al. 2012), and as Near Threatened in Spain (Verdu et al. 2006).

In Portugal, recent comprehensive surveys across the whole national distribution revealed a decline of 53% in the number of subpopulations and 49% in abundance in the remaining subpopulations over the last 20 years (Lopes-Lima et al. 2023).

In England, the species was found to have undergone a 99% decline in the Thames catchment when resurveyed in 2020 and the data compared to results in 1964 (Ollard and Aldridge 2022). Whilst is still widespread in the UK but has declined more than the Unio species (D. Aldridge pers. comm. December 2022).

In Poland, a 90% decline was found in the Zalew Pińczowie Lake over the 20 years up to 1998 (Zając et al. 2016). Gołąb et al. (2010) found that the routine drainage of reservoirs (in that case, the Czorsztyn reservoir on the Dunajec River) for maintenance purposes led to significant mortality of this species.

Northern (Scandinavia) and some eastern (Ukraine; L. Shevchuk pers. comm. December 2022) populations seem to be stable, but this requires confirmation.

An overall population decline across the European and EU27 regions of at least 30% over the past three generations (45 years) is suspected, but in parts of the southern European range, the decline is much more significant.

Anodonta anatina has the typical unique reproductive features of most Unionidae species including larval parental care (i.e. brooding) and larval parasitism on freshwater fishes (Lopes-Lima et al. 2017).

The larvae (glochidia) are triangular and hooked with a typical length between 300-400 μm (Lopes-Lima et al. 2016). The species can use a range of native host fish for larval dispersal and metamorphosis but is unable to use most of the non-native fish species (Douda et al. 2013), suggesting that the changes in the fish fauna pose a serious threat to local populations of this bivalve.

It can be found inhabiting flowing and standing water freshwater habitats in both oligotrophic and eutrophic waters without chemical pollution (Zettler et al. 2006). It may occur in many distinct habitats such as streams, floodplains, rivers, and lakes, and is also able to live in artificial freshwater habitats such as reservoirs, flooded gravel pits, and fishponds (Lopes-Lima et al. 2017, Sousa et al. 2021). It is generally found in areas with sandy, gravelly, or muddy oxygenated substrates (Bauer and Wächtler 2001, Lopes-Lima et al. 2017).

It has a lifespan between 10 and 30 years, depending on the latitude, reaching a maximum length between 8-19 cm but typically of around 10 cm (Lopes-Lima et al. 2017). The species is usually dioecious although hermaphroditic populations have been recorded mainly in standing water populations (Lopes-Lima et al. 2016). Larvae usually overwinter in the gills and are released in early spring (Lopes-Lima et al. 2017).

Habitat loss and degradation, invasive species, water pollution and high levels of eutrophication have been reported throughout the species' distribution, leading to its global decline (e.g. Lopes-Lima et al. 2017, Beran 2019).

This species has been severely affected by the Zebra Mussel Dreissena polymorpha and has suffered local declines in areas where this species has invaded its habitats (Byrne et al. 2009). Heavily infested Duck Mussels were found to have lower physiological conditions than those that were not infested (Sousa et al. 2011, Urbańska et al. 2019). In the absence of suitable substrates, Anodonta species are known to be a preferred settlement site for Zebra Mussels, and heavy infestations can affect the feeding, respiration, and reproduction of Unionid bivalves, causing mortality and eliminating entire populations (Rosell et al. 1999). The continued range expansion of D. polymorpha into suitable habitats is likely to further impact on population numbers of this species (Byrne et al. 2009). Other rapidly expanding invasive bivalve species, such as the Asian Clam Corbicula fluminea and the Chinese Pond Mussel Sinanodonta woodiana may harm A. anatina through competition for food, space, and host fish resources (Donrovich et al. 2017, Urbańska et al. 2019, Modesto et al. 2021). Competition for calcium has also been reported in Lake populations (N. Riccardi pers. comm). Other introduced species such as the Muskrat Ondatra zibethicus and North American crayfish species, e.g. Procambarus clarkii and Pacifastacus leniusculus, may also prey on A. anatina populations and severely impact them (Lopes-Lima et al. 2017, Meira et al. 2019).

Water scarcity in southern European countries, caused by increasing consumption of water resources for agricultural and urban purposes, is a growing threat that is now being exacerbated by ongoing climate change trends, in particular changes in precipitation patterns, increased global warming and the increasing intensity of extreme events such as floods and droughts (Sousa et al. 2012).

The species is known to be consumed as food locally but no trade has been reported.

This species is now considered as Threatened or Near Threatened in several countries.

• In Germany, it is listed as Endangered (Binot-Hafke et al. 2011) and is protected under the BArtSchV (Annex I) legislation (Federal Species Regulation) (Zettler et al. 2006).
• In Ireland, the species is classified as Vulnerable under criterion A4ce on the Irish Red List No. 2 (Non-Marine Molluscs) (Byrne et al. 2009).
• In Switzerland it is listed as Vulnerable in the Switzerland Red Book of Molluscs (Rüetschi et al. 2011).
• In France, it is listed as Vulnerable in the French Red List of Threatened Species (UICN French Committee 2021).
• In Portugal, it has been recently listed as Endangered and included in the National Catalogue of Protected Species (Lopes-Lima et al. 2023).
• In Spain, it is listed as Near Threatened in the Atlas of Threatened Invertebrates of Spain (Verdú et al. 2006).
• In Austria, it is listed as Near Threatened on the Austrian Red List of Molluscs (Reischütz and Reischütz 2007).

Anodonta anatina would benefit from the development of an action plan to guide future conservation efforts. Protected areas that include the main habitats of the species, such as rivers and streams should be established. Habitat restoration should focus on the  restoration of entire watersheds, and river management practices should be adapted to avoid harming sessile benthic organisms such as freshwater bivalves. Acute and diffuse sources of pollution sources should also be identified and eliminated. Control of invasive species such as fowling bivalves and macrophytes should also be implemented in many areas and would benefit this species. Anodonta anatina should be legally protected in countries where declines have been reported and its conservation status is considered threatened.

Research is needed to identify priority populations for conservation, to define threats, to understand the basic ecology of the species and, in particular, long-term monitoring of priority populations to provide information on population trends. More research is currently needed on the global population of the species to assess the impact of the documented threats. Outreach and education on the importance of the species and ecosystem services is also needed.