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

Unio tumidus is assessed as Near Threatened in Europe, based on suspected habitat loss and degradation resulting in a population decline of at least 20% over the last three generations (30 years) and, therefore almost reaching the 30% required to be listed as Vulnerable (A2c). This species is still geographically widespread throughout Europe, however it has been reported to be in decline and is now considered threatened in many European countries due to habitat degradation and loss.

Unio tumidus is widely distributed throughout Europe, from Great Britain in the west to the Ural River in European Russia in the east (Lopes-Lima et al. 2017). It is present in most European countries from England and Wales to Russia, and 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 in the west, Norway in the north, and Portugal, Spain, and Italy in the south (Lopes-Lima et al. 2017).

The range of this species extends beyond the European region into western Asia (east of the Urals), where it occurs in the countries surrounding the Black Sea, in Kazakhstan and Russia, in rivers flowing to the northern shores of the Caspian Sea, and in tributaries of the Ob River basin. It has also been reported in the Transbaikalia region of eastern Russia (Klishko et al. 2017), but this is most likely due to human introductions on fish translocated to this region (Babushkin et al. 2021).

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

The species has not been monitored and population trends have not been quantified, but an ongoing pattern of decline has been reported in most European countries. For this reason, U. tumidus is already strictly protected and listed as Endangered in Germany (Binot-Hafke et al. 2011) and Switzerland (Rüetschi et al. 2012), Vulnerable in Austria (Reischütz and Reischütz 2007), Czechia (Farkač et al. 2005), Romania (Sárkány-Kiss 2003) and Slovakia (Steffek 1994), and Near Threatened in Great Britain (Seddon et al. 2014) and France, where a historical decline in extent of occurrence (EOO) of 70% has been estimated (Prié et al. 2014). In England, no decline was observed in the Thames between 1964 and 2020 (Ollard and Aldridge 2022).

For the European region and the EU27 as a whole, an overall decline of less than 30% but more than 20% is estimated for the last three generations (about 30 years).

Unio tumidus 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 have been reported to use a range of native host fishes for larval dispersal and metamorphosis, although most of these reports did not verify the effective transformation of larvae into juveniles and therefore these hosts still need further confirmation (Lopes-Lima et al. 2017).

It is mostly restricted to lowlands and to the middle and lower reaches of medium and large rivers, where it inhabits meso- and eutrophic waters (Zettler et al. 2006, Welter-Schultes 2010). However, it can occur in many different habitats such as streams, floodplains, rivers and lakes, and is also able to live in artificial habitats such as reservoirs, flooded gravel pits and fishponds (Lopes-Lima et al. 2017, Sousa et al. 2021). It is generally concentrated in marginal zones where the substrate is firm and muddy, and is less common on sand, fine gravel, and soft mud substrates (Aldridge 1999).

It has a lifespan between 5 and 20 years, depending on the latitude where it occurs, and reaches a maximum length between 6-12 cm, but typically of around 8-9 cm (Lopes-Lima et al. 2017). The species is usually dioecious (Lopes-Lima et al. 2017). Larvae generally are typically brooded in the gills in the spring and released in the summer (Lopes-Lima et al. 2017).

The main threats to Unio tumidus are habitat loss and disturbance due to habitat modification for river and canal management, agriculture and industrial development, dredging and pressure from invasive species (Weber 2005).

In the absence of suitable substrate, Unio species are known to be a preferred settlement site for the Zebra Mussel Dreissena polymorpha. Heavy infestations can disrupt the feeding, respiration and reproduction of unionid bivalves, causing mortality and wiping out entire populations (Rosell et al. 1999). Other rapidly expanding invasive bivalve species, such as the Asian Clam Corbicula fluminea and the Chinese Pond Mussel Sinanodonta woodiana, can harm U. tumidus by competing for food, space and host fish resources (Donrovich et al. 2017; Modesto et al. 2021).

Other introduced species, such as the Muskrat Ondatra zibethicus and North American crayfish species, may also prey on Unio populations and severely impact them (Lopes-Lima et al. 2017; Meira et al. 2019).

In Germany, the mussel is also threatened by overexploitation, where it is collected in large numbers from small streams near villages and boiled as feed for pigs and chickens, and the shells are crushed and used as fertiliser on crops (Welter-Schultes 2010).

Affected by increasing water pollution, habitat destruction and invasive species, U. tumidus is declining rapidly in many countries.

In Germany, the species was harvested in the past for animal feed and fertilisers, and numbers rapidly declined as a result (Welter-Schultes 2010). The species is used as an indicator of water quality.

There are no species-specific conservation strategies in place for this species, however it does occur in protected areas throughout its range.

Due to 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 Vulnerable on the Austrian Red List of Molluscs (Reischütz and Reischütz 2007).
• In Czechia, it is listed as Vulnerable on the Czech Red List of Invertebrates (Farkač et al. 2005).
• In France, it is listed as Near Threatened (UICN France Committee).
• In Germany, it is listed as Endangered and it is strictly protected by national legislation (Binot-Hafke et al. 2011).
• In Great Britain it is listed as Near Threatened (Seddon et al. 2014).
• In Romania, it is considered as Vulnerable (Sárkány-Kiss 2003).
• In Slovakia, it is listed as Vulnerable in the Red List of molluscs of Slovakia (Šteffek 1994).
• In Sweden, it is listed as Least Concern (ArtDatabanken 2020).
• In Switzerland, it is listed as Endangered in the Switzerland Red Book of molluscs (Rüetschi et al. 2011).

The species would benefit from the development of an action plan to guide future conservation efforts, and protected areas that include the species' main habitats, such as rivers and streams should be established. Habitat restoration is also needed and should  include the restoration of entire river basins. 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 fowling bivalves and macrophytes should also be implemented in many areas.

Unio tumidus should be legally protected in countries where declines have been reported and its conservation status is considered to be threatened. Research is needed to identify priority populations for conservation, to determine 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 the species and ecosystem services are also needed.