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

Sphaerium solidum is very localised throughout the range in Europe, and hence is a species that is considered threatened in some countries. However, it is is extending its range in other countries e.g. Hungary. The principal threat to this mussel is water pollution and river regulation, as well as exploitation of sand and gravel from river beds, alteration of water courses, changes to flow regimes, over-frequent dredging and the effect of invasive species.

Given the reduction in range and habitat quality in the last 10 years it is considered to be Endangered at both the European level and the level of the 27 Member States of the European Union, based on loss of habitat, leading to declines of 50% in the population.

In Europe, Sphaerium solidum is locally distributed in the lowland regions of western, central and eastern Europe. Fauna Europaea (Bank et al. 2006, Welter-Schultes 2012) list the species from Latvia (not mapped), Lithuania, Kaliningrad region (Russia) (not mapped), Poland, England, the Netherlands, Belgium, France (mainland), Germany, Slovakia, Hungary, Spain (mainland), Ukraine, Belarus, and extending through Northern European Russia. Quite comprehensive data are available for S. solidum for some parts of the range of this species, given its easily identifiable thick shell which can be preserved for years in the riverbanks.

In Germany, the species is found in the north and central part with a sporadic occurrence, and a decline in population trend has been observed. In Czechia, the only sub-recent record is from the River Elbe, and it is now considered extinct in this country (Beran and Horsak 1998) (not mapped).

In England, the species was historically known from a short stretch of the River Witham (Lincolnshire), and from the Great Ouse (a fragmented distribution along about 20 km of the Great Ouse/Old West River between Earith and Stretham, and with large populations found in a central area of the New Bedford River between Gault Bridge and Pymore; Willing 2007), however, the species is now thought to be extinct in England, possibly as a result of Corbicula fluminea competition in the case of the Great Ouse and adjacent records, and as a result of increased eutrophication resulting from fertiliser runoff (Killeen et al. 2004) in the case of the Lincolnshire populations.

In France, there are records for most of the northern half of the country. However, all but two records are either very old or consist only of shells. The species has only been recorded three times (in the lower Loire and Seine drainages) since 2000. The use of eDNA surveys throughout the country suggests that the species is now restricted to three locations, one in the Loire and two in the Seine basin (Prié et al. 2023).

In Hungary, it is likely to be a recently spreading species, but presence in this country requires confirmation.

Out of the European region, the range of this species extends to Asian parts of Russia; the Yana River basin and the Ob and Pechora river basins, under different species names that Korniushin (2001) placed in synonymy.

There is no comprehensive data about population trends available, nevertheless, population decline has been observed everywhere over the last decades.

Surveys in 2001 and 2002 in England found a near total collapse of populations of the species in the Witham system. Surveys in 2003 – 2004 show an intermittent distribution along about a 20 km stretch of the Great Ouse but at most sites (sampled to mid-channel of river) only a few freshly dead shells were recovered; seven sites produced in total only seven live mussels. One Great Ouse site sampled in 2003 then showing a healthy population of live individuals produced no live individuals in 2004. A few live individuals were found in a drain adjacent to Witham in 2003, and it was absent from sites in Witham when surveyed 2002 and 2003. There was a moderate population (exact numbers are hard to estimate) reported in a short stretch (perhaps only a few 100 m) of tidal Great Ouse at Earith, Cambridgeshire and very small numbers (a few live individuals in isolated populations) along 19 km of Great Ouse (old channel) downstream of Earith (I. Killeen pers. comm. 2009). There has been no post 2010 records in England (B. Rowson pers. comm.).

In Germany, the population is in decline and the species is considered Critically Endangered (Welter-Schultes 2012).

Sphaerium solidum is one of the rarest molluscs in Poland, with most records being historical, and confirmed recent records in the Narew, Biebrza (one record), Bug and Oder rivers, with records from the Vistula. Relatively large populations persist in the Szczecin Lagoon (Oder drainage), but continued presence in the Zegrzyn reservoir requires confirmation.

In Ukraine, the species was known from 18 sites from the Pripet basin in northern Ukraine, but it has been lost has been lost from 11% of previously known sites there over the past five years.

In France, the species was formerly widespread in the northern and eastern half of the country. Since the year 2000, this species has only been recorded three times, despite extensive eDNA surveys. It was recorded once in 2009 in the Aisne River, once in 2006 in the lower Seine and more recently in two localities in the lower Loire. According to eDNA results, S. solidum appears to be extirpated from the Aisne site by now, but this requires confirmation. Reduction in the area of occupancy (AOO) was estimated to be 88% and the species is considered Endangered in France.

The total population decline in Europe is inferred to be 50% based on the declines in the AOO, extent of occurrence (EOO) and quality of habitat.

The species occurs in rivers with fine sediments, very often in the littoral zone. It also lives in canalised rivers and large, deep drains and backwaters of large rivers, and is very occasionally found in lakes (Killeen et al. 2004, Zettler and Glöer 2006). Favoured habitat is fine sand of large rivers and reservoirs. It is usually found quite deep, but can also be found in shallow waters of sandy oxbows.

Due to the rarity of the species in Britain, the details on its ecology are sparse. However, the habitats in which it was found in Lincolnshire are generally poor in aquatic vegetation but rich in their associated mollusc fauna, and they are generally found in deep water (as for Sphaerium rivicola) (Killeen et al. 2004). In France, the species has been found in large sandy rivers at the bottom. It is also found in the Loire's oxbows and hydraulic appendages.

Water pollution and river regulation, as well as extraction of sand and gravel from river beds, alteration of water courses, changes to flow regimes, over-frequent dredging and the effect of invasive species are the main known threats to this species. Mouthon (1996) showed that Sphaerium solidum was one of the species of freshwater bivalve most sensitive to biodegradable pollution. However, the invasion of the main large rivers by Corbicula species may be the most important cause of decline, probably due to competition.

The recent decline and possible extinction from some of its former known sites in England resulted in its addition to the UK Biodiversity Action Plan list of Priority Species in 2006. There is circumstantial evidence on both Witham and Great Ouse systems suggesting eutrophication as a primary cause, however, excessive sediment clearance could also pose a possible threat to local populations.

There is no known trade or use for this species.

There are localised conservation actions in place for this species. In Germany this species is considered Category 1 (highly threatened) and in most states either 0 or 1, and in Baden Wurtenburg Category 2. In Poland it is considered Endangered. This species was added to the UK Biodiversity Action Plan list of Priority Species in 2006 and considered Endangered (I.Killeen and M.Seddon pers. comm. 2010). However, in most countries S. solidum has been completely ignored by conservation programs although it is likely one of the most endangered species. Future prospects are possibly good if water quality improves under the Water Framework Directive, but some decline is likely through loss of habitat as a result of water course management.

Future conservation measures for this species should focus on habitat management and restoration, as well as invasive species control. Further research into its population and habitat trends would be beneficial.