Unio populations from western Iberia were previously thought to belong to the widespread and more common European Unio pictorum and the species name Unio delphinus to be its junior synonym (Haas, 1969). However, subsequent integrative molecular and morphological work recognised U. delphinus as a valid species and distinct from U. pictorum (Araujo et al. 2009, Lopes-Lima et al. 2017).

Unio populations from Morocco, previously thought to be U. delphinus, belong to a separate species, the Moroccan endemic Unio foucauldianus Pallary, 1936 (Froufe et al. 2016).

Unio delphinus is a European endemic species and is globally assessed as Endangered (A2bc), due to an observed population decline of more than 50%, based on declines in the number of subpopulations (66.7%) and total abundance (71.5%) over the last 20 years in Portugal, and projected over the Spanish range, where the population declines are expected to be similar or worse due to increasing aridification and decreasing water quality and quantity of freshwater habitats.

This species is restricted to Portugal and Spain in many western Atlantic basins of the Iberian Peninsula, from the River Ulla in the north to the Guadalquivir River basin in the south and then east to the River La Vega basin near Gibraltar. It also occurs in a few Mediterranean coastal basins east of Gibraltar to the Guadalhorce River basin near Málaga (Lopes-Lima et al. 2020).

Populations are declining dramatically in both Portugal and Spain. The species was abundant in some watercourses until the 2000s, mainly in the Douro River basin, but has suffered a sharp decline in the last two decades (Lopes-Lima et al. 2020, 2023).

In Portugal, a recent survey of the entire country's distribution reported a 67% decline in the number of subpopulations and a 71% decline in the number of individuals over the last two decades (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 continuous decline in area, habitat quality and number of subpopulations.

In Spain, the situation is likely to be even worse due to the lack of water in most of the basins where the species occurs, and therefore the decline rates of the species are predicted to be the same or worse (Lopes-Lima et al. 2023).

The species occurs in permanent and perennial rivers and streams on sandy, gravelly or muddy substrates, generally along banks shaded by trees, but occasionally on more exposed sandy banks (Araujo et al. 2009, Lopes-Lima et al. 2020). During the dry season, specimens may survive in intermittent Mediterranean rivers and streams in isolated subpopulations, sheltering in pools with fish (Lopes-Lima et al. 2020).

It is a typically fluvial species, but can also be found in lakes (e.g. Ruidera Lakes in Spain and Mira Lagoons in Portugal). It is usually found buried in sand or, less frequently, in finer sediments on the banks, associated with aquatic vegetation, roots of riparian vegetation or stones. It lives in both oligotrophic and eutrophic rivers, but is generally not very tolerant of pollution (Reis 2006).

Unio delphinus grows rapidly and is a relatively short-lived species (up to 11 years), reaching sexual maturity at two years of age (Lopes-Lima, et al. 2020). 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 triangular and hooked with a length of ∼200 μm (Lopes-Lima et al. 2020). Larval release occurs continuously from May to August, with two peaks in May and August (Lopes-Lima et al. 2020). The host fish are the Brown Trout Salmo trutta fario and the native Iberian cyprinids belonging to the genera Achondrostoma, Luciobarbus, Pseudochondrostoma, and Squalius (Lopes-Lima et al. 2017, 2020).

Urban pollution, especially in coastal areas, was the main cause of localised extinction until the beginning of the 21st century. The species has been threatened by habitat loss and fragmentation due to the construction of dams and reservoirs, which continues to this day. In southern Portugal and Spain, the main threat is water scarcity due to increasing demand for agricultural and urban use, leading to higher water temperatures, oxygen depletion and ultimately desertification. This situation is exacerbated by global warming scenarios, which are likely to cause irreversible desertification trends.

As Unio delphinus appears to be unable to use non-native fish species as hosts, biotic homogenisation and fish introductions may become a major threat to the species in the near future (Lopes-Lima et al. 2020). Other threats come from invasive alien bivalve species, such as the Asian clam Corbicula fluminea, which may harm U. delphinus by competing for food and space (Ferreira-Rodriguez et al. 2018, Modesto et al. 2021). Other introduced alien species such as North American crayfish species, e.g. Procambarus clarkii and Pacifastacus leniusculus, may also prey on U. delphinus subpopulations and severely impact them (Lopes-Lima et al. 2017, Meira et al. 2019).

This species is consumed locally but no trade is known.

There are no known global or national conservation actions in place for Unio delphinus.

In Spain, it is listed as Near Threatened (Verdú and Galante 2009) and regionally in Andalusia as Vulnerable A4ce;B1ab(iii)+2ab(iii) in the Andalusian Red List of Invertebrates: (Barea-Azcón et al. 2008). In Portugal, the species has recently been assessed as Endangered (EN A2ac) and included in the Catalogue of Protected Species due to a low area of occupancy, with a continuous decline in area of occupancy, habitat quality and number of subpopulations (Lopes-Lima et al. 2023).

Conservation actions are needed for the establishment of protected areas that include the species' main habitats, such as rivers and streams, and this species would benefit from the development of an action plan to guide this. Habitat restoration would be beneficial, with 30% of the total river basin area where it occurs restored, and this would meet the Convention on Biological Diversity (CBD) Global Biodiversity Framework (GBF) 2030 Target 2 in both Portugal and Spain. Future river management practices should avoid excavation and severe alteration of riparian areas, banks and channel hydromorphology, and should be adapted to avoid harming sessile benthic organisms such as freshwater mussels. The control of invasive species such as the bivalves Dreissena polymorpha, Dreissena rostriformis bugensis and Sinanodonta woodiana, and macrophytes such as the Water Hyacinth Pontederia crassipes should be carried out throughout the range of Unio delphinus by controlling the movement of boats and other vessels through the water bodies. Restocking host fish populations and ex situ breeding to provide juveniles for restocking populations in the rivers that are declining in agreement with Target 4 of the CBD GBF should be considered for this species.

Research is needed to identify priority populations for conservation, assess threats, understand the basic ecology of the species and, in particular, carry out long-term monitoring of priority populations to provide information on population trends. Levels of tolerance to both classical and emergent novel pollutants should be researched urgently and acute and diffuse pollution sources should then be identified and eliminated. Outreach and education of relevant stakeholders such as environmental agencies, local authorities, water management agencies and the general public on the importance of the species and ecosystem services is also needed.