Formerly placed as a synonym of Unio mancus Lamarck, 1819 (Haas 1969), U. elongatulus has been split from U. mancus by numerous studies (e.g. Prié and Puillandre 2014, Froufe et al. 2017, Marrone et al. 2019).

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

Unio elongatulus is still geographically widespread throughout Italy and the Western Balkans as far south as Albania. However, the species has declined and populations are now rare and isolated throughout its range. It is assessed as Endangered (A2bce), based on a population size reduction loss of at least 50% over the last three generations (45 years) caused by a suspected decline in habitat quality, abundance and to the introduction of competitive invasive species.

This European endemic species occurs only in river basins flowing to the Adriatic Sea in Italy, north and east of the Apennines, extending into Slovenia, Croatia, Bosnia and Herzegovina, Montenegro and northern Albania (Froufe et al. 2017, Marrone et al. 2019, Lyubas et al. 2022).

Some subpopulations reported in Italy, west of the Apennines from the Arno River basin, are likely the result of human-mediated introductions (M. Lopes-Lima pers. obs. 2022).

In Italy, riverine subpopulations are disappearing at an alarming rate due to habitat loss and degradation. Recent comprehensive surveys (2017-2022) failed to find the species in more than 70% of previously recorded localities, especially in the western subpopulations (N. Riccardi pers. data 2022). Lake populations are also declining and are now mostly restricted to deeper parts due to the presence of invasive freshwater bivalve competitors such as Corbicula fluminea, Sinanodonta woodiana, and Dreissena polymorpha.

There are no data on population trends in the Balkans, but recent surveys (2018-2022) along the Adriatic coast have shown that the species is declining and is now extremely rare, occurring in only a few remaining rivers (M. Lopes-Lima pers. comm. 2022). Overall, during the last 45 years (three generations), given the observed decline of between 50% and 70% of the Italian populations and the suspected decline of 50% of the Balkan populations, in terms of area of occupancy, habitat and abundance (N. Riccardi and M. Lopes-Lima pers. comm. 2022), it is suspected that there has been an overall population size reduction of more than 50%.

Unio elongatulus has the typical unique reproductive traits of most Unionidae species, including larval parental care (i.e. brooding) and larval parasitism on freshwater fish (Lopes-Lima et al. 2017). The larvae (glochidia) are triangular and hooked with a typical length of 200 μm (M. Lopes-Lima pers. obs. 2022). The species has been reported to use a range of host fish 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 freshwater habitats, mostly in mesotrophic and oligotrophic waters, but with good water quality (N. Riccardi pers. data 2022). It can occur in many different habitats such as streams, floodplains, 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). It is generally found in areas with sandy, gravelly, or muddy oxygenated substrates (Lopes-Lima et al. 2017).

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

This species has suffered from the inefficient management of lakes and rivers, physical alterations such as dams, levees, and channelisation, water shortage, and pollution from agriculture, industry and urban effluents. Droughts arising from recent climate change have also impacted the species. More recently, the introduction and proliferation of alien invasive species such as Corbicula fluminea, Sinanodonta woodiana, and Dreissena polymorpha is thought to be one of the major reasons behind the disappearance of many populations of Unio elongatulus from most rivers, streams and lake shores (N. Riccardi pers. obs.).

No use or trade information is known for this species.

There are no species-specific conservation strategies in place for Unio elongatulus, however due to the reported declines, it is listed as a protected species in the Bern Convention and as a species of community interest in the Habitats Directive, and therefore legally protected in EU countries. In Switzerland, the species is listed as Endangered (as Unio mancus) in the Red Book of molluscs of Switzerland (Rüetschi et al. 2012).

The species should benefit from the development of an action plan to guide future conservation efforts. It is essential to identify priority populations for conservation in order to establish protected areas, and then to invest in long-term monitoring of these populations using standardised surveys to provide information on population trends. Habitat restoration is also required, with the restoration of connectivity in the lower reaches of the rivers Aliakmonas, Axios and Pinios, where the species occurs. These areas should be included in order to achieve CBD Target 2 at the national level in the countries where it occurs. It would be beneficial to adapt river management practices that involve excavation and disturbance of the substrate, banks and key hydromorphological elements of the channel to avoid harming sessile benthic organisms such as freshwater mussels. In addition to this, control of invasive species such as fouling mussels and macrophytes should also be undertaken in many areas with the hulls of boats and other aquatic vessels being inspected for invasive fouling organisms when they are transported across different waterbodies.

Research is needed to determine its tolerance to contaminants, its detailed distribution, to assess threats, and to understand the basic ecology of the species. Tolerance to traditional and emerging pollutants should be studied in detail and point and non-point sources of pollution identified and eliminated. Outreach and education to relevant stakeholders such as environmental and water management agencies, farmers and farmers' associations, and the general public on the importance of the species and the ecosystem services it provides is also needed.