The species was recognised as a synonym of one of the Unio pictorum races (i.e. Unio pictorum delphinus) by Haas (1969). Since then, U. delphinus has been elevated to the species level (Araujo 2009) and placed within the pictorum lineage, one of the four recognised clades within the Unio genus (Lopes-Lima et al. 2017). Subsequently, U. gibbus specimens were collected for molecular analyses and the species was redescribed in the early 2000s for populations in the Atlantic basins of Morocco and southern Spain based on an integrative, morphological and molecular approach (Araujo 2009). The species was recognised as distinct from U. delphinus and placed in a different lineage (i.e. the gibbus lineage) together with another North African species, Unio durieui. Since then, the species has also been reported from Tunisia, extending its distribution to the Mediterranean basins of northwest Africa.

European regional assessment: Critically Endangered (CR)
EU 27 regional assessment: Critically Endangered (CR)

Unio gibbus is assessed as Critically Endangered under criteria A3c and B1ab(ii,iii,iv,v) for Europe and for the EU27 Member States based on predictions of total population loss over the next three generations (30 years) due to habitat loss and degradation. It is also based on an extent of occurrence (EOO) of 16 km² in a single location with a continuing decline in the extent of occurrence, area of occupancy, area and quality of habitat, and in the number of subpopulations.

In Europe, the species is known from one river in Spain, where very few individuals have been found in the last two decades. It is considered to be one of the most imperilled Iberian naiad species. Its limited distribution, population status and threatened habitat make it a species of great conservation importance. The increasing number of annual droughts could rapidly eradicate the European population in the near future. Water abstraction and pollution from increasing industrial and agricultural development near the river where the species lives is a constant threat. Urgent action is needed to establish ex-situ captive breeding to conserve the species in Europe, as well as catchment management plans to conserve its habitats.

Within the European region, this species is restricted to a single location on the Barbate River near Cádiz in Andalusia, southwest Spain (Araujo et al. 2009). This species may be a historical (before 1500 AD) introduction from North Africa, but further work is required to confirm this.

Outside of Europe, the species occurs in northwest Africa where it is known from six Atlantic river basins in Morocco from the Oued (River) Noun in the south to the Oued Mda in the north (Gomes-dos-Santos et al. 2019), and from six Mediterranean rivers in northern Tunisia, from the Oued El Kébir in the east to the Oued Medjerda in the west (Khalloufi and Boumaiza 2009). The species should also occur in Algeria, although its current presence in the country is uncertain.

Unio gibbus was once more widespread occurring also in the basin of River Guadalquivir. At present, however, this species can only be found in a section of the River Barbate just above the large Barbate dam and up to 10 km below the dam (Barea-Azcón et al. 2008). It has been unsuccessfully studied in other parts of this basin and in neighbouring basins (Guadalete, Palmones, Guadarranque, Guadiaro and Guadalquivir) (Barea-Azcón et al. 2008).

There is no information on population trends, as the species is only known from a few specimens collected in the last 10 years, although its habitat is being degraded and water is becoming more scarce. Based on habitat quality and availability, the species should be declining rapidly and may soon become extinct in Europe due to the scarcity of individuals (Barea-Azcón et al. 2008).

Unio gibbus is mainly found in middle and lower sections of temporary Mediterranean-type streams in a variety of substrates from silt to sand and gravel, close to the banks at pool areas that do not dry up during the dry season (Gomes-dos-Santos et al. 2019). The section of the River Barbate where this taxon is known to occur belongs to the Thermo-Mediterranean bioclimatic unit and is located at an altitude of 40 metres above sea level passing through a mosaic landscape where pastures for livestock (mainly cattle) alternate with Mediterranean scrub (Barea-Azcón et al. 2008). The riparian forest is well preserved and consists of oleanders (Nerium oleander), poplars (Populus alba), wild olives (Olea europara var. syvestris), reeds and willows (Salix sp.). In summer the water level is extremely low and the riverbed is reduced to a few small pools. During the rest of the year the river maintains a more abundant flow.

Unio gibbus has the typical unique reproductive traits of most Unionidae species including larval parental care (i.e. brooding) and larval parasitism on freshwater fish (Araujo et al. 2009, Lopes-Lima et al. 2017). From studies in North Africa, the larvae (glochidia) are triangular and hooked with a typical length of 210 μm (Araujo et al. 2009). The host fishes used by U. gibbus for larval dispersal and metamorphosis are still unknown (Lopes-Lima et al. 2017). Its lifespan is not known but, using other Unio species as a surrogate, should not exceed 20 years. The species is usually dioecious (Khalloufi and Boumaiza 2009). Larvae are brooded in the gills typically from late winter to late spring, and released from early spring and more intensively in the summer until July (Khalloufi and Boumaiza 2009, Lopes-Lima et al. 2017).

The main threat to this species is water scarcity due to increasing demand for agricultural and urban uses. This situation is exacerbated by global warming scenarios, which are likely to cause irreversible desertification trends (Barea-Azcón et al. 2008, Gomes-dos-Santos et al. 2019). Other additional threats include habitat degradation and loss, increased pollution and eutrophication due to changes in agricultural practices, the regulation of natural watercourses, the presence of introduced molluscs and fish species, and the decline of native fish populations (Barea-Azcón et al. 2008).

No use or trade is known for this species.

There are no species-specific conservation strategies in place for this species.

Due to observed habitat degradation and loss, the conservation status of the species has been recommended for inclusion in the Spanish Red List as Endangered, based on its single location, low population density and threats from habitat degradation (Verdu and Galante 2009), but without success to date. In the Autonomous Region of Andalusia, Spain, it is listed as Endangered (A4c, B2ab(i,ii,iii,iv,v)c(iv)+C2b) in the Andalusian Red List of Invertebrates (Verdú and Galante 2009).

Conservation action is needed to establish protected areas that include the species' main habitats, such as rivers and streams. In the specific stretches where U. gibbus has been reported, there is an urgent need to control water level fluctuations and to rehabilitate some stretches (regulating cattle stocking rates and restoring riverbanks) where the species could potentially occur upstream of the Barbate dam. Extensive livestock farming plays an important role in the ecosystems of these mountain ranges, although it is recommended that a moderate stocking rate be maintained and that cattle access to a large part of the riverbed be restricted. Discharges into the river from livestock facilities also need to be controlled. River management practices should be adapted to avoid harming sessile benthic organisms such as freshwater mussels, and acute and diffuse sources of pollution should also be identified and eliminated. Control of invasive species such as fowling bivalves, fish and macrophytes should also be implemented in the area. Urgent action is also needed to establish an ex-situ captive breeding programme to conserve the species. The species should benefit from the development of an action plan to guide future conservation efforts and legal protection in Spain should be considered.

Research is needed to improve knowledge of the basic ecology of the species, and investment in long-term population monitoring is needed to provide information on population trends. Outreach and education on the importance of the species and ecosystem services are also needed.