A very large number of names have been synonymised to this species (MolluscaBase 2022).

European regional assessment: Least Concern (LC)
EU 27 regional assessment: Not Applicable (NA)

Within its restricted native range in the Pan European region, this species is assessed as Least Concern. It is Not Applicable (NA) for the EU27 Member States as it occurs as a recent introduction there.

In the European region, the species occurs as a native in the river basins of the Black, Azov and Caspian Seas, in isolated and semi-isolated relic estuarine reservoirs along the coasts, and in freshwater influenced areas of these seas in Russia, Bulgaria, Hungary, Romania, Moldova, Ukraine, and Türkiye. However, D. polymorpha has been found as fossils in central and western Europe (Birnbaum 2011).

It is a highly invasive bivalve that has continued to spread across Europe since the early 19th century, colonising virtually all European countries by 2019 (reviewed in Karatayev and Burlakova 2022a). In 2021, D. polymorpha was first found in the Pyshma River in the West Siberian Plain (Babushkin et al. 2022). In 1986 D. polymorpha was found in North America, where by 2023 the species had been recorded in 31 states and three Canadian provinces (Benson et al. 2023).

The main pathways of spread include downstream drift of mussel larvae, mussels attached to boat hulls or wood transported by ships through canals and oceans, spread of larvae by ballast water discharge, and overland transport of boats and fishing gear from infested waters (Karatayev and Burlakova 2022a). Major geographic expansions have been associated with changes in the pace of human activities that provided previously unavailable means of spread, such as the construction of shipping canals for trade, the building of reservoirs for water storage and energy production, political boundary shifts and changes in political systems that affected the position or permeability of national borders, changes in the nature and volume of international trade, and new industrial practices and environmental regulations (Karatayev et al. 2007).

Outside of the European region tt is also likely that the native range of D. polymorpha included northern Iran, Turkmenistan, Kazakhstan and Uzbekistan (reviewed in Karatayev and Burlakova 2022a).

Historically, this species has dominated benthic communities in its native range (Vorobiev 1949, Markovskiy 1954), but there is no recent information on population size or trends.

In the invaded area it can attain high densities (Karatayev and Burlakova 2022a). D. polymorpha is known to be declining in lakes and reservoirs as a result of replacement by D. rostriformis bugensis, but the extent of replacement depends on the morphometry of the waterbody: in deep stratified lakes D. r. bugensis almost completely outcompetes D. polymorpha, whereas in shallow polymictic lakes both species usually coexist (Karatayev et al. 2015, 2021; Karatayev and Burlakova 2022b).

This species occurs in a range of habitats from freshwater to oligohaline (up to 5-6 parts per thousand) waterbodies, including lakes, reservoirs, ponds, estuaries and brackish coastal lakes, with the highest densities in canals and the lowest in rivers and streams (Karatayev et al. 1998; Karatayev and Burlakova 2022a). Dreissena polymorpha requires a hard surface for attachment (rocks, shells, coarse sand and the submerged part of macrophytes) and usually avoids silt. The highest densities of D. polymorpha occur on various artificial substrates (Karatayev et al. 1998, Karatayev and Burlakova 2022a). In the littoral zone of small polymictic lakes, D. polymorpha usually reaches maximum population densities at depths between 1 and 6 m. In large stratified European lakes, D. polymorpha spreads down to 50-55 m (reviewed in Karatayev et al. 1998; Karatayev and Burlakova, 2022a), and in the North American Laurentian Great Lakes this species has been reported down to 109 (Watkins et al. 2007) and 128 m (Nalepa et al. 2014). Despite these few unusually deep records, D. polymorpha is largely restricted to the well-mixed littoral zone and, even when suitable substrates are present, never forms high densities below the thermocline due to low growth temperatures, food scarcity and occasionally low oxygen concentrations (reviewed in Karatayev and Burlakova, 2022a).

Environmental factors controlling D. polymorpha include substrate and food, eutrophication, pollution, oxygen depletion, competition and predation (reviewed in Karatayev and Burlakova, 2022a). Several cases have been documented where D. polymorpha disappeared from entire lakes or rivers due to severe pollution (bij de Vaate et al. 1992, Jantz and Neumann 1992, Karatayev et al. 2003). The upper temperature limit for D. polymorpha observed in the field is 31-33°C (Karatayev et al. 1998, Allen et al., 1999). In brackish waters, D. p. polymorpha thrives in the least saline areas, from freshwater and up to 5-6% (reviewed in Karatayev and Burlakova 2022a).

D. polymorpha is considered one of the most notorious freshwater invaders in the Northern Hemisphere with tremendous ecological and economic impacts. The ecological effects of D. polymorpha are associated with their role as ecosystem engineers, as they can physically change bottom substrates creating new three-dimensional structures. These structures provide refuge from predation and other stressors (waves, currents, desiccation) for benthic organisms that would otherwise be absent from this environment (Karatayev et al. 1997, 2002; Sousa et al. 2009, Burlakova et al. 2012). The ecosystem effects of D. polymorpha are associated with their role as suspension feeders. Suspension feeding not only affects planktonic communities, it also transfers materials from the water column to the benthos, enhancing the coupling between the planktonic and the benthic components of the ecosystem, which can trigger a suite of changes including, but not limited to, an increase in water transparency and macrophyte coverage, and a reduction in the concentration of seston, phosphorus, chlorophyll, and phytoplankton (reviewed in Karatayev et al. 1997, 2002; Ibelings et al. 2007, Higgins and Vander Zanden 2010, Goedkoop et al. 2011, Mayer et al. 2014, Noordhuis et al. 2016, Karatayev and Burlakova 2022a).

Heavy shell infestation can cause mass mortality of host unionid bivalves, and this impact is one of the best-documented negative ecological consequences of dreissenid invasions (reviewed in Karatayev et al. 1997, Lucy et al. 2014). The severity of D. polymorpha impacts depends on their density in the waterbody, time since invasion and sediment type. Extensive unionid overgrowth resulting in mass mortality is typical of early invasion periods when D. polymorpha populations are growing rapidly. Later in the invasion, D. polymorpha can coexist harmlessly with native bivalves, and although overgrowth may cause some host mortality, unionids not only survive but can maintain high densities (Burlakova et al. 2000, Lucy et al. 2014).

D. polymorpha has severe negative impacts on raw water-dependent infrastructure, including power plants, drinking water treatment plants, industrial facilities, navigation locks and dam structures, and disrupts the operation of pumps, forbays, holding tanks, trashracks and condenser units (reviewed in Mackie and Claudi 2010, Karatayev and Burlakova 2022a). Connelly et al. (2007) estimated the average total economic cost of US power generation and water treatment facilities from 1989 to 2004 to be $267 million (range: $161 - $467 million). These costs do not include those associated with other impacts on industry and navigation, natural resources (e.g. fisheries), or impacts related to recreational boating and tourism. The most recent assessment of the global economic costs of dreissenids between 1980 and 2020 was $51.1 billion (2017 US$) (Haubrock et al. 2022), but this estimate suffers from a number of shortcomings, including overlapping costs extracted from different sources (Diagne et al. 2021), the fact that 98% of the data collated were from North America, and the fact that 'cost' categories include several controversial expenditures, such as research, management, detection, surveillance, monitoring, education, communication and information, and risk assessment. Unfortunately, these cost estimates ignore the many benefits of dreissenids, including those to drinking water treatment plants (Wang et al. 2021), water purification, property values, fisheries, etc. (Burlakova et al. 2022, Boltovskoy et al. 2022).

In its native range, the species is impacted in the Volga Delta by agricultural activities and pollution. The species could be impacted in its native range by other invasive bivalves (e.g. Corbicula fluminea and Dreissena rostriformis bugensis).

This species is widespread and highly invasive out of its native range and is unlikely to be impacted by any major threats in these areas.

Dreissena polymorpha is widely used as a sentinel organism for the assessment and biomonitoring of contaminants and pathogens (Binelli et al. 2015), for bioremediation (Noordhuis et al. 1992), and is an important food resource for fish and waterfowl (reviewed in Burlakova et al. 2022).

Conservation

It is unknown if any conservation actions are required for this species within its native range.

Research needs

Invasion dynamics:
Population explosions are well documented for D. polymorpha early in the invasion, but there is insufficient long-term data to predict which environmental factors determine whether the population will fluctuate widely, remain relatively stable for long periods, or decline later in the invasion.

Life history and ecological traits:There is a lack of information on the reproductive behaviour, fecundity and longevity of deep-water populations of D. polymorpha, where mussel growth is limited by both low temperatures and low food concentrations. It is not known whether D. polymorpha grows faster and lives shorter in the early stages of invasion, when food resources are usually sufficient, than later in the invasion.

Impact on food webs:
Further studies of the effects of D. polymorpha on energy flow through food webs in different continents and water types are needed to fully understand and predict their effects on recipient fish communities.

Impact of diseases, predators, and parasites:
Additional research is needed to understand if diseases, predators, and parasites can cause long-term, system-wide declines in mussel populations.

Ecosystem services and economic benefits:
Currently, the damage caused by D. polymorpha attracts more attention than its benefits in cost-benefit analyses. Whilst dreissenids provide considerable benefits, many of which are economically significant, their ecosystem and economic benefits are usually ignored, minimised or considered 'non-monetised'. Given the widespread distribution of D. polymorpha, it is important to quantitatively assess their positive ecological effects and economic benefits as an opportunity to provide additional information to scientists, managers and policy makers.