Haas (1969) believed that the species Unio elongatulus C. Pfeiffer, 1825 (= Unio mancus Lamarck, 1819, see below) consisted of a group of 17 regional subspecies distributed mainly throughout the Mediterranean and Asia Minor.

The taxonomy of these subspecies was first unravelled by Araujo (2005), who considered the Iberian populations of U. elongatulus to be U. mancus, a separate and valid species. This was later revalidated by Prié et al. (2012), who also included the French populations in U. mancus and separated them from the true U. elongatulus for the populations in northern Italy.

Since then, both U. mancus and U. elongatulus have been recognised as valid with mutually exclusive distributions in Europe (Araujo et al. 2018, Froufe et al. 2017, Marrone et al. 2019).

Unio mancus is still geographically widespread throughout western Spain, France and Italy, however, the species has declined considerably and populations are now rare and isolated throughout its range. It is assessed as Endangered (A2ac), based on an observed habitat loss and degradation estimated to have caused population declines of at least 50% over the last three generations (45 years). The species is almost extinct in Sicily and is quite rare in Italy. In Spain, most subpopulations have few individuals and a lack of recruitment. In France, there has been a significant decline in the number of subpopulations, which are also becoming increasingly rare.

Unio mancus is a western Mediterranean species. It is known to occur in the Mediterranean river basins of Spain from the Serpis basin in the south, north to the Pyrenees and into France. In France it occurs in most of the mainland Atlantic and Mediterranean basins and on Corsica. In continental Italy, it occurs south and west of the Apennines and also on the islands of Sardinia and Sicily (Araujo et al. 2005, Prié et al. 2012, Froufe et al. 2017, Marrone et al. 2019).

Most of the data on population trends have been collected at the country level, but overall the species has been in sharp decline since the 1970s.

In Spain, it is a species in considerable decline, especially in the middle sections of the Ebro, where its populations are disappearing at an alarming rate (Nakamura et al. 2023). This disappearance is due to changes in the environment caused by anthropogenic activities. It is also disappearing from the rest of the Iberian Mediterranean rivers, as well as from the Albufera de Valencia. A precise estimate of the overall rate of decline in Spain is currently not possible.

In France, the species has declined drastically over the last 45 years (three generation lengths), with the number of known subpopulations declining by more than 50% (Prié et al. 2012, 2014).

In Italy, populations in rivers and streams are disappearing at a dramatic rate due to habitat loss and degradation. In a recent comprehensive survey (2017-2022), the species was not found in more than 70% of the previously recorded sites (N. Riccardi pers. data 2022). In Sicily, a decline in abundance of over 99% is estimated for the last twenty years, with only one individual recently found in the River Belice, despite recent extensive surveys (M. Lopes-Lima pers. data 2022). In Sardinia and Corsica, populations may be more stable (M. Lopes-Lima and V. Prié pers. obs. 2022).

Based on country-level observations, an overall decline of at least 50% over the last 45 (three generation lengths) years is estimated.

Unio mancus 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; Modesto et al. 2018). The larvae (glochidia) are triangular and hooked with a typical length of 217 μm (Araujo et al. 2005). The species has been reported to use a range of Iberian host fishes for larval dispersal and metamorphosis (Lopes-Lima et al. 2017).

It is a typical riverine species that, with the exception of high mountain areas, is able to occupy all types of rivers, both first-order and minor tributaries, as well as irrigation channels that maintain a natural substrate. Although not very common, it can also be found in large reservoirs and lakes (e.g. Albufera de Valencia and Lake Bourget) (Araujo et al. 2005; Prié et al. 2012). It generally lives semi-buried in the gravel bottoms of secondary arms of the river, in the middle of the channel in areas with little current,  on well-preserved banks in the shade of riparian vegetation and even among the roots of trees. Taking into account the historical distribution of this species and the characteristics of the water where live specimens were collected, it seems that U. mancus is a species typical of hard waters with calcium concentrations above 100 mg/l (Araujo et al. 2009).

The species is usually dioecious (Lopes-Lima et al. 2017). Larvae are usually brooded in the gills in the spring and released in the summer (Lopes-Lima et al. 2017).

The species has suffered from poor river management policies and physical changes such as the construction of dams, embankments and urban areas, as well as from increased levels of channelling and water abstraction. Pollution has increased from a variety of sources including agriculture, industry and urban run-off. More recently, the introduction and proliferation of invasive species such as Corbicula fluminea, Sinanodonta woodiana and Dreissena polymorpha is thought to be a major cause of the disappearance of many populations of Unio mancus from most rivers, streams and lake shores throughout its range (Araujo et al. 2005, Prie et al. 2012, N. Riccardi pers. data). Finally, the increased frequency and severity of drought events has escalated over the last 10 years as a result of climate change, severely affecting the already poor status of populations.

No use or trade information is known for this species.

Unio mancus is protected (under the name U. elongatulus) by the following protection figures: Habitats Directive (Annex V): Animal and plant species of community interest and the Berne Convention (Annex III): Protected species that may be subject to controlled exploitation, and it occurs in protected areas in parts of its range in France, Italy and Spain.

In Spain it is listed as Near Threatened in the Red Book of Invertebrates (Verdu and Galante 2006) and proposed for inclusion as Vulnerable in the Catalogue of Threatened Species of Spain. Regionally, it is also included in the Valencian Catalogue of Threatened Fauna Species (Annex I) as Endangered and is protected in Catalonia in the category of Protected Species of Autochthonous Wild Fauna b. Invertebrates (Legislative Decree 2/2008).

The species has been the subject of a propagation programme based on a European LIFE project, ref: LIFE 08NAT/E/000078 "Estany Project". The programme produced more than 100,000 juveniles in 2015, of which a few hundred were reintroduced into Lake Banyoles in Spain, increasing the depleted population by 40% (Araujo et al. 2015). However, no recent data are available on the status of this population.

The species should benefit from the development of an action plan to guide future conservation efforts. Future actions that need to be taken include control of invasive species, changes to river management practices and habitat restoration. Control of invasive species such as fouling mussels and macrophytes should 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. River management practices that involve excavation and disturbance of the substrate, banks and key hydromorphological elements of the channel should be adapted to avoid harming sessile benthic organisms such as freshwater mussels. Habitat restoration, with the restoration of connectivity in the rivers where the species occurs, should be carried out. These areas should be included in order to achieve CBD Target 2 for France, Italy, and Spain.

Research is needed to determine its tolerance to contaminants, its detailed distribution, to assess threats, and to understand the basic ecology of the species. 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. 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 are also needed.