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

Following taxonomic revision, the distribution of Potomida littoralis in Europe is now restricted in Europe to France, Portugal and Spain. The species is assessed as Endangered (A2c+3b) for Europe and for the EU27 Member States based on a suspected past reduction of at least 60-70% in the available habitat over the last three generations (30 years) causing population declines of the same magnitude, and also based on a suspected future decline of more than 75% of its population over the next three generations (30 years).

In France, only a few good isolated and fragmented populations persist and the species is declining dramatically, a reduction of 73% of its available habitat has been inferred. The subpopulations in Spain and Portugal are highly fragmented and declining dramatically due to the lack of water. The desiccation, degradation, and fragmentation of freshwater habitats and the expected population decline rates will increase in the near future, challenging the survival of the species in its European range. Due to the isolation of the European populations from North Africa and the thousands of physical barriers (e.g. dams and weirs) in most hydrographic basins of both continents, the transcontinental migration of larvae or fish infested with P. littoralis larvae is likely to be minimal or non-existent.

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).

Potomida littoralis has lost over 70% of its European distribution (area of occupancy (AOO) and the number of subpopulations) in the last two decades based on data projected from the French and Portuguese population trends (Prié et al. 2014, Lopes-Lima et al. 2023). Based on the data from France and Portugal, with similar threats operating in Spain, the overall level of population decline over the past three generation lengths (30 years) exceeds 50% and may be greater than 70%. 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 continued intensification of agriculture and increased water use 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 has probably been extirpated from the Seine basin and many small Mediterranean drainages. The conservation status has not been re-evaluated recently in France but the AOO 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 has 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.

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.

It 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. It 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, and are 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, increased frequency of droughts, 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 is 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 France, the main threats 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 the 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).

The species is not known to be used or traded.

There are no known conservation actions in place for Potomida littoralis at the European 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 3 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).

Conservation measures are urgently needed to reverse the rate of decline of P. littoralis and an action plan for the species should be prepared to guide the implementation of conservation measures. 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 essential to improve the status of river basins for P. littoralis, especially in the Mediterranean region. Site protection for the most important populations should be implemented at national level and strengthening the implementation of existing legislation to protect P. littoralis and its habitats and reduce the impact of invasive species is recommended across the region. 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.

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.