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

Anodonta cygnea is assessed as Vulnerable (VU A2c) for Europe and the EU27 Member States, based on a suspected population decline of at least 30% over the last three generations (45-60 years) as a result of habitat loss and degradation. The species is still geographically widespread throughout Europe and small areas of Western Asia, occurring mainly in lentic eutrophic habitats. However, the species is declining and is now considered threatened in many European countries due to habitat degradation and loss.

Due to the isolation of the European populations from those in 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. cygnea larvae is likely to 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 European Russia and north to southern Sweden and Norway (Lopes-Lima et al. 2017).

The range of the species extends beyond the European region. In Asia, it has only been confirmed in Georgia (Bikashvili et al. 2022) and northwest Anatolia in Türkiye (Lopes-Lima et al. 2021). It should also be present in the lower sections of the Volga River basin in Kazakhstan, although there are no published records available from this region. It had been previously recorded in Northern Iran (e.g. Pourang et al. 2009) but recent molecular studies consider this to be a misidentification with the congeneric Anodonta anatina (Lopes-Lima et al. 2021).

In Africa, A. cygnea had also been previously reported but recent molecular data on Moroccan and Tunisian populations confirms the inclusion of African lineages of Anodonta within the species A. anatina, therefore, excluding the presence of A. cygnea in this continent (Lopes-Lima M. 2022 pers. comm., Gomes-dos-Santos et al. 2021).

Although this species is widespread and considered to be stable in some places (Zettler et al. 2006), few studies have been carried out to assess population size and structure. There are almost no monitoring programmes and therefore no quantifiable studies of population trends.

This species is highly specialised and only occurs in lentic habitats such as slow-flowing sections of large rivers, ponds, channels, and especially lakes. Most of the recorded populations in Europe occur in lakes, especially oxbow lakes (Zając 2005). There is an increasing loss of lateral hydrological connectivity in river floodplain ecosystems worldwide and especially in Europe due to poor river management and the increasing human demand for freshwater and land resources (Stoffels et al. 2022). This has affected the quality and number of oxbow lakes, other permanent lakes, and ponds, especially during recent extreme droughts in Europe. For this reason, a decline of at least 30% in habitat availability and quality has led to a suspected decline of the same magnitude in Anodonta cygnea, both in numbers of individuals and subpopulations across its distribution.

In some countries, the species is considered threatened due to observed and suspected declines.

• In Central Europe, populations are declining and it is already considered as Vulnerable in Austria (Reischütz and Reischütz 2007), Czechia (Farkač et al. 2005) and France (UICN French Committee 2021), and as Critically Endangered in Germany (Binot-Hafke et al. 2011).
• In Poland and Ukraine, there have been significant declines, with a 50% decline in localities over the last 50-60 years (Zając 2005), mainly from oxbow lakes due to habitat degradation. As a result, the species is listed as Endangered (Głowaciński and Nowacki 2016).
• In Great Britain, the river management strategy includes river dredging, which may remove 20% of the subpopulation within dredged rivers (Aldridge 2001).
• In Ireland, the species is assessed as Vulnerable due to past and predicted future population declines (Byrne et al. 2009).
• At the southeastern edge of its range, subpopulations have disappeared from Spain and are now restricted to three small lagoons in Portugal, where the species has been recently assessed as Critically Endangered (Lopes-Lima et al. 2023).

This species mainly inhabits lentic eutrophic waterbodies such as ponds and peatlands, oxbow lakes, lakes, and slow-flowing lowland rivers (Lopes-Lima et al. 2017). It is also found in canals, drainage dikes and dam reservoirs, and other man-made lentic structures (Lopes-Lima et al. 2017, Sousa et al. 2021). The species appears to prefer lentic waterbodies characterised by high dissolved oxygen concentrations (a likely consequence of the species' fast growth and large size), free of floating vegetation, and with fertile bottom sediments (Zając 2002, Rosińska et al. 2008). The species is intolerant of poor environmental conditions and can be used as a bioindicator of clean water (Rosińska et al. 2008). When present, it is often the only mussel species in these areas (Zettler et al. 2006).

Anodonta cygnea has the typical unique reproductive features of most Unionidae species including larval parental care (i.e. brooding) and larval parasitism on freshwater fish. The larvae (glochidia) are triangular and hooked with a length of between 300-350 μm (Pekkarinen and Englund 1995). The species can use a wide range of host fish (both invasive and native fish) for larval dispersal and metamorphosis (Huber and Geist 2017, Lopes-Lima et al. 2017).

It has a lifespan between 10 and 40 years (giving an estimated generation length of 15-20 years) depending on the latitude, reaching a maximum length between 9-26 cm but typically of around 14 cm (Lopes-Lima et al. 2017).

The species is usually hermaphroditic (Lima et al. 2012). Larvae generally overwinter in the gills and are released in early spring (Lopes-Lima et al. 2017).

The main threats affecting Anodonta cygnea are the decline and degradation of high-quality lentic freshwater habitats such as slow sections of large rivers, lakes, and especially oxbow lakes (Zając 2002). The increasing loss of the natural formation and regeneration of oxbow lakes is now completely disrupted by the loss of lateral hydrological connectivity in river floodplain ecosystems worldwide and especially in Europe (Stoffels et al. 2022). Habitat degradation is mainly due to water pollution, eutrophication, hydrological alterations, and physical transformation of banks and watercourses (e.g. Dyduch-Falniowska 1993, cited in Mills and Reynolds 2004).

The species is also threatened by poaching for supply in artificial basins and garden ponds (Rosińska et al. 2008).

This species has been severely impacted by the Zebra Mussel Dreissena polymorpha and has suffered localised declines in areas where this species has invaded its habitats (Byrne et al. 2009). In the absence of suitable substrate, Anodonta species are known to be a preferred settlement site for Zebra Mussels. 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 habitat is likely to further impact this species' population numbers (Byrne et al. 2009). Other freshwater bivalves such as Corbicula fluminea and Sinanodonta woodiana also compete with native freshwater bivalves for food, substrate and hosts and severely affect their growth and survival (Donrovich et al. 2017, Modesto et al. 2021).

The invasion of Water Hyacinth (Eichornia crassipes) into the few freshwater lagoons of Iberia threatens the entire population in this edge of the range (M. Lopes-Lima pers. comm.). In Great Britain, poor river management is a major threat to the species: dredging occurs approximately every ten years and can remove 20% of the unionid populations (Aldridge 2001). This species is also sensitive to drought (D. Aldridge pers. comm. 2022).

More research is currently needed on the global population of the species to assess the impact of the documented threats.

This species is known to be used for sale for large garden ponds and aquariums.

The conservation status of the species is now considered as threatened in many countries.

• In Austria, it is listed as Vulnerable on the Austrian Red List of Molluscs (Reischütz and Reischütz 2007). It is also legally protected as a specially protected animal by Act No. 114/1992 Coll. on Nature and Landscape Protection.
• In Germany, it is listed as Highly Endangered (Binot-Hafke et al. 2011) and is protected under the BArtSchV (Anlage 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 Poland, this species has been strictly protected by law since 2011 and is listed as Endangered on the Polish Red List (Zając 2005).
• In Portugal, it was recently listed as Critically Endangered and proposed to be included in the national catalogue of protected species (Lopes-Lima et al. 2023).
• In Spain, the species is considered extinct (Araujo et al. 2009).
• The species is protected in Ukraine and it is listed as Vulnerable.
• The species is now listed as Vulnerable in Norway (Artsdatabanken 2021).
• It is maintained as Least Concern in Sweden (ArtDatabanken 2020), and Finland (FinBif 2020).

Conservation actions are needed for the establishment of protected areas that include the species' main habitats, such as lakes and oxbow lakes. Habitat rehabilitation with the restoration of lateral hydrological connectivity and the natural replenishment of water of lentic habitats in floodplain rivers is required. Acute and diffuse pollution sources should also be identified and eliminated, and river management practices should be adapted to avoid harming sessile benthic organisms such as freshwater bivalves. Invasive species control such as fowling bivalves, introduced crayfish, and macrophytes should also be implemented in many areas.

The species should benefit from the production 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.

Research is needed for the establishment of priority populations for conservation, threat determination, the basic ecology of the species, and especially long-term monitoring of priority populations to inform about population trends. Outreach and education on the species' importance and ecosystem services would also be beneficial.