European regional assessment: Near Threatened (NT)
EU 27 regional assessment: Near Threatened (NT)

The species is assessed as Near Threatened in Europe, based on a population decline of at least 20% inferred from suspected habitat loss and degradation over the last three generations (30 years), which is close to the 30% required to be listed as Vulnerable (A2c). This species is still geographically widespread throughout Europe. 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.

In Europe, Unio pictorum is present in most countries from England and Wales in the west to the Urals in European Russia in the east, from Sweden and Finland in the north to Greece and European Türkiye in the south. It is largely absent in countries on the edges of Europe, such as Ireland and Scotland in the west, Norway in the north, and Portugal, Spain and Italy in the south (Lopes-Lima et al. 2017). Denmark and North Macedonia have not been mapped due to lack of precise location data.

Outside of Europe, is found in western Asia, specifically in the upper tributaries of the Ob River basin in Russia and Kazakhstan, as well as in the Caucasus and Anatolia. (Lopes-Lima et al. 2017, 2021; Babushkin et al. 2021).

This species was, until recently, described as common and abundant with large populations and a wide distribution across Europe (Lopes-Lima et al. 2017). However, an ongoing pattern of decline has become obvious in most European countries despite the species not being monitored and so the population trends have not been quantified. For this reason, U. pictorum is already strictly protected in Czechia and considered Endangered in Switzerland (Ruetschi et al. 2012), Vulnerable in Romania (Sárkány-Kiss 2003), and Near Threatened in Austria (Reischütz and Reischütz 2007), Great Britain (Seddon et al. 2014), Germany (Binot-Hafke et al. 2011), and Sweden (ArtDatabanken 2020). In other countries, such as France, the species is not currently considered threatened, although it is not monitored.

• In England, a 97% decline was observed in the Thames between 1964 and 2020 (Ollard and Aldridge 2022), but other English subpopulations appear to be more stable (D. Aldridge pers. comm. 2022).
• In Ukraine, range contraction has been observed mainly in the south, with an overall population decline of 30% over the last 20 years.
• In Sweden, a population decline of 25% has been observed over the last 40 years (ArtDatabanken 2020).

Overall, a decline of less than 30% over the last three generations is suspected.

Unio pictorum has the typical unique reproductive traits 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 of 200 μm (Wächtler et al. 2001). The species has been reported to use a range of native host fishes for larval dispersal and metamorphosis, although most of these reports have not verified the effective transformation of larvae into juveniles and therefore these hosts require further confirmation (Lopes-Lima et al. 2017).

It can be found in flowing and standing water freshwater habitats mostly in meso- and oligotrophic but with good quality waters (Zettler et al. 2006; Lopes-Lima et al. 2017). It can also be found in eutrophic waters. It may occur in many distinct habitats such as streams, 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 (Lopes-Lima et al. 2017).

It has a lifespan of between 5 and 20 years, depending on latitude, and reaches a maximum length of between 8 and 14 cm, but typically around 8-9 cm (Lopes-Lima et al. 2017). The species is typically dioecious (Lopes-Lima et al. 2017). Larvae are brooded in the gills, usually in multiple broods, and subsequently released in spring/summer (Lopes-Lima et al. 2017).

The main threats to this species include hydromorphological alteration of rivers (or canals), poor river and canal management (dredging and weed control), destruction or over-dredging of large ponds and lakes, degradation of water quality, tourism and development pressures (e.g. marinas), droughts caused by climate change and competition or direct mortality from exotic species. This species is threatened by the continued spread of invasive freshwater bivalves such as Corbicula fluminea, Dreissena polymorpha, and Sinanodonta woodiana.

Although studies in the UK have indicated that this species is relatively tolerant of polluted waters (Killeen et al. 2004), U. pictorum has been shown to have a low tolerance to pollution in France and Sweden (Mouthon 1996). The species has also been shown to be highly sensitive to sediment disturbance that can release toxic levels of nutrients, as occurred in a lake in Poland where a boat accident released high levels of nutrients and toxins from the sediment causing massive mortality of freshwater mussels and about 60% of U. pictorum individuals (Ćmiel et al. 2019).

No use or trade is known for this species.

Due to the observed declines, the conservation status of Unio pictorum is now considered threatened or near threatened in several countries.

• In Czech Republic, it is strictly protected by national legislation.
• In Switzerland, it is listed as Endangered in the Switzerland Red Book of molluscs (Rüetschi et al. 2012).
• In Romania, it is considered as Vulnerable (Sárkány-Kiss 2003).
• In Austria, it is listed as Near Threatened in the Austrian Red List of Molluscs (Reischütz and Reischütz 2007).
• In Germany, it is listed as Near Threatened and it is protected by national legislation. (Binot-Hafke et al. 2011).
• In Great Britain, it is listed as Near Threatened (Seddon et al. 2014).
• In Sweden, it is listed as Near Threatened (ArtDatabanken 2020).
• In France, it is listed as Least Concern (UICN Committee France).

The species should be legally protected in countries where declines have been reported and its conservation status is considered to be threatened, and would benefit from the development of an action plan to guide future conservation efforts. Conservation measures are needed to establish protected areas that include the species' main habitats, such as rivers and streams, as well as to implement habitat restoration with whole river catchments being restored. River management practices should be adapted to avoid harming sessile benthic organisms such as freshwater mussels, and acute and diffuse sources of pollution should also be identified and eliminated. Control of invasive species such as fouling bivalves and macrophytes should also be implemented in many areas.

Research is needed to identify priority populations for conservation, to identify threats, to understand the basic ecology of the species and, in particular, to invest in long-term monitoring of priority populations to provide information on population trends. Outreach and education on the importance of species and ecosystem services would also be beneficial.