Due to advances in the study of the genus Potomida, the species Potomida littoralis now strictly refers to the southwest European (France, Spain and Portugal) and northwest African (Morocco, Algeria and Tunisia) Potomida subpopulations.

Traditional conchological characters, which are of limited use for identifying taxonomic units due to the high phenotypic plasticity of unionoid bivalves, are responsible for the more than 90 synonyms described for P. littoralis (Cuvier, 1798) before the mid-20th century (Graf 2010). Of the eight P. littoralis subspecies recognised by Haas (1969), the Iberian P. l. umbonata (Rossmässler, 1844) and the North African P. l. fellmanni (Deshayes, 1848) have been retained as synonyms of P. l. littoralis (Froufe et al. 2016, Araujo et al. 2017).

Based on molecular and biogeographical arguments, the Greek P. l. acarnanica and the Turkish and Middle Eastern P. l. semirugata have recently been considered as distinct species, i.e. P. acarnanica and P. semirugata, respectively (Froufe et al. 2016). P. l. delesserti from the Middle East was synonymised with P. semirugata.

Based on an extensive comparative morphological and anatomical study, P. l. homsensis (Lea, 1865) was correctly reassigned to a different family (Margaritiferidae) and is now Pseudunio homsensis (Lea, 1865) (Smith 2001, Lopes-Lima et al. 2018).

Finally, P. l. komarowi (O. Boettger, 1880) has recently been synonymised under Unio bruguierianus but subsequently reassigned to Unio mardinensis (Lopes-Lima et al. 2021, 2023).

Following taxonomic revision, this species is now restricted to southwest European (France, Portugal, and Spain) and northwest Africa (Morocco, Algeria, and Tunisia).

The species is assessed as globally Endangered (EN A2c+3bc) based on a suspected past reduction of more than 70% in the number of mature individuals based on a 70% decline in the available habitat over the last three generations (21 years), and a suspected future decline of more than 75% of its population over the next three generations (21 years).

In France, only a few isolated and fragmented subpopulations remain and the species is declining dramatically, with a recent estimate of a 73% reduction in available habitat. Subpopulations in Spain and Portugal are highly fragmented and have also been declining dramatically due to lack of water flow in the rivers as the result of increased frequency of drought events combined with water flow management at the dams along the rivers.

In North Africa, it has been considered Critically Endangered in Morocco due to its small area of occupancy and ongoing severe threats. It is likely to have a similar status in Algeria and Tunisia.

Potomida littoralis is a widespread species that occurs in southwestern Europe and northwestern Africa.

Within the European region, this species is recorded from the Mediterranean, Channel, and Atlantic basins of France and Spain and several Atlantic basins of Portugal.

France: It once occurred in the major Atlantic basins from the Seine in the north to the Adour in the south but is now thought to be near-extinct in the Seine drainage. In the Mediterranean part of France, it also occurs in several French basins west of the Rhone drainage (Prié et al. 2014).

Spain: The species was present in the Mediterranean basins from the Muga in the northeast to the Segura in the southeast but it has lost most of its populations in Catalonia, north of Barcelona and it is now extinct in the Segura. In the extreme south of the Iberian Peninsula, it occurs again on the Mediterranean coast in several river basins between the Guadalhorce River and Gibraltar. In the Atlantic basin, it occurs in northeastern Spain in the Minho and Ulla river basins, and in the extreme south, in several river basins between the Portuguese border and Gibraltar. It is absent from the Spanish North African territories (Araújo et al. 2017).

Portugal: The species occurs from north to south in the Minho, Douro, Vouga, Mondego, Lis, Lizandro, Tejo, Sado, Mira, and Guadiana river basins (Froufe et al. 2016).

Beyond the European region, the range of the species extends to northwestern Africa, from the Atlantic river basins of Morocco to its Mediterranean coast and the northern river basins of Algeria and Tunisia (Gomes-dos-Santos et al. 2019).

Morocco: In Morocco, it occurs in the Atlantic Draa, Souss, Massa, Oum-er-Rbia, and Sebou river basins, and the Mediterranean Martil and Laou river basins

Algeria: The species was recorded in the major river basins and wetlands of northern Algeria from the Moroccan to the Tunisian border, although these records are very old (>100 years). No recent surveys or published records of the species are available.

Tunisia: The species was only recorded recently in four river basins, i.e. El Maaden, Ziatine, Sejenane, and Joumine, all in the extreme north of the country.

The population is now considered to be highly fragmented, as it was once widespread throughout most of the Atlantic and Mediterranean basins of Western European and northwestern African countries, but is now restricted to small isolated pockets remaining within its original river basins.

It is estimated that the species has lost over 70% population over the whole distribution during the last two decades based on data projected from the French and Portuguese population trends, taken from recent site survey data.  It is suspected that the species will lose more than 75% of its habitat, and therefore its global population, in the near future due to the ongoing intensification of agriculture and the increase in water consumption for agricultural practices, coupled with projections of extreme droughts.

In France, the species was historically known from all the main drainages, and shells are still found in most of them. However, living specimens are becoming increasingly rare and the overall population is thought to be in sharp decline (V. Prié pers. comm. 2022). It is probably extirpated from the Seine basin and many small Mediterranean drainages. The conservation status has not been re-evaluated recently in France but the area of occupancy has decreased by 73% based on the inferred available habitat (Prie et al. 2014).

In the Iberian Peninsula, the number of subpopulations and individuals has decreased substantially over the last two decades with many remaining populations having only a small number of old individuals and no signs of recruitment.

In Spain, it was listed as Vulnerable (Verdú and Galante 2009) due to the predicted loss of a significant proportion of its distribution in that country. Araujo et al. (2009) considered that the decline of this species had accelerated since the beginning of the 21st century.

It has also declined significantly in Portugal, where most populations are very isolated and fragmented. All sites where P. littoralis was recorded in Portugal 20 years ago were recently surveyed, and the results showed that the species had been extirpated from 68% of the sites, and in sites where the species was still present there was a decline in abundance of more than 70% (Lopes-Lima et al. 2023). The recent construction of two large dams (on the Tua River in 2011 and on the Sabor River in 2016) has extirpated more than 70% of the two larger subpopulations in northern Portugal (Lopes-Lima et al. 2023). Many populations in southern rivers and streams have also been extirpated due to lack of water and changes in hydrological regimes. The species has recently been listed as Endangered in Portugal due to its small range and a continuous decline in area, habitat quality and number of subpopulations.

The species is also considered Endangered in North Africa due to population losses (Van Damme et al. 2010). The species is now extirpated from most of the lower reaches of the major river basins in southern Morocco (e.g., Souss, Massa, Tensift, and Draa) and survives in these basins only in isolated small permanent pools (Gomes-dos-Santos et al. 2019). Due to water scarcity and lack of wastewater treatment, many rivers in southern Morocco dry up completely in summer or have low flows with high levels of pollution (Gomes-dos-Santos et al. 2019). In Tunisia, half of the habitat has already been lost due to the construction of dams where the water is pumped to the main cities. The remaining habitats are almost dry and the species survives with difficulty in restricted shallow stretches during summer. There is no information on the Algerian subpopulations.

Potomida littoralis occurs in lotic or slow-moving stretches of streams and rivers, generally in the middle to lower river sections, but occasionally appearing in the upper parts of the river systems rich in calcium. The species seems to be quite sensitive to pollution and generally prefers clean waters.

Potomida littoralis has the typical unique reproductive features of most Unionidae species including larval parental care (i.e. brooding) and larval parasitism on freshwater fish (and occasionally other vertebrates). The larvae (glochidia) are elliptical and unhooked with a length of ∼200 μm.

This species broods glochidia in both inner and outer demibranchs (tetrabranchy), is dioecious and probably a short-term brooder (Nagel 2004).

Gravid animals and released glochidia are recorded to occur between March and October, more typically in July and August (Nagel 2004, Araújo et al. 2009), and maturity is reached after 4-5 years (size 4 cm).

The host fishes of P. littoralis are known only for Iberia, i.e. the native cyprinid species Achondrostoma oligolepis, Luciobarbus bocagei, Peudochondrostoma duriense, Squalius alburnoides, Squalius carolitertii, and the salmonid Salmo trutta fario (Lopes-Lima et al. 2017).

The main threats to Potomida littoralis are habitat degradation and fragmentation. These include poor river management, such as the channelling of rivers and streams for flood control and irrigation, the construction of physical barriers such as dams and weirs, and water quality degradation due to various sources (domestic, industrial and agricultural) of pollution and eutrophication. Changes in the fish fauna due to the introduction of exotic fish species are also considered a major threat, as these species require native fish hosts as an intermediate stage in their development. Native fish populations are declining throughout the distribution of P. littoralis due to the increasing addition of invasive species.

In Europe, there are different sources of threats in different countries:

In France, the main causes are thought to be habitat degradation and fragmentation, and alteration of hydrological regimes by dams, which promote eutrophication and lentification of rivers and streams.

In Spain and Portugal, the main threats to species are also changes in hydrological regimes and the lack of sufficient ecological flows in rivers and streams due to poor river management, as well as increased water consumption for intensification or conversion to more water-intensive agricultural practices (Araujo 2011; Lopes-Lima et al. 2023).

In North African countries, major threats to the species are the lack of water due to increasing water extraction activities for agriculture, urban development as well as pollution due to the lack of wastewater treatment (Gomes-dos-Santos et al. 2019).

In all Mediterranean climate countries, the increase in the last decades of extreme events, such as floods but mainly prolonged droughts, is already impacting P. littoralis. The increase in extreme events in recent decades, such as floods but especially prolonged droughts, is already affecting P. littoralis. Moreover, the predicted increase of these events in the future will seriously challenge the survival of the species in this region (Gomes-dos-Santos et al. 2019).

The species is not known to be used or traded.

There are no known conservation actions in place for Potomida littoralis at the global level.

In France, it is considered to be a declining species and was recently assessed as Endangered A2ace (IUCN Committee France).

In Spain, it is protected and listed as Vulnerable in the Atlas and Red Book of Threatened Invertebrates, based on population declines over the last three generations (30 years) and it is considered to be declining (Verdú and Galante 2006).

In Portugal, the species has been assessed recently as Endangered due to a low area of occupancy presenting a continuous decline in area of occupancy, habitat quality, and the number of subpopulations (Lopes-Lima et al. 2023).

In Morocco, the species has been assessed as Critically Endangered given its estimated extent of occurrence (EOO) of less than 100 km² and area of occupancy (AOO) of less than 10 km² (Lopes-Lima in Gomes-dos-Santos et al. 2019). The species also shows a high degree of fragmentation and a continuing decline in terms of habitat, extent of occurrence, and number of populations.

The status of subpopulations in Algeria is not known, but is likely to reflect the same trends as in neighbouring countries.

In Tunisia, the status of the species has never been assessed, but its current distribution has been reduced to less than half due to the construction of dams and water extraction activities, and the species should now be highly endangered (Lopes-Lima M. pers. comm. 2022).

Conservation measures are urgently needed to reverse the rate of decline of P. littoralis. The maintenance of ecological flows and river rehabilitation measures that increase the persistence of water in watercourses, such as the replanting of riparian buffers, the restoration of original meandering channels and the restriction of water extraction activities, are urgently needed to improve the status of river basins for the species, especially in the Mediterranean region. Site protection for the most important populations should be implemented at national level and an action plan for the species should be prepared to guide the implementation of conservation measures.

Future distribution and population density surveys are needed followed by the implementation of population monitoring programmes. Further research should also be carried out on the life history, ecology and habitat requirements of the species.

Strengthening the implementation of existing legislation to protect P. littoralis and its habitats and reduce the impact of invasive species is recommended, as well as outreach activities to all relevant public and private stakeholders should be undertaken to raise awareness of the rarity, threatened conservation status and ecological importance of the species.