As part on an ongoing effort to unravel the taxonomy and phylogeny of Palaearctic freshwater mussels, this paper describes an overlooked Unio species that inhabits the Atlantic rivers of South Iberia and Morocco, based on morphological, reproductive, and molecular characters. Live specimens of Unio gibbus were recently found in the Barbate River in southwest Spain and in the Beth River in the Sebou basin, northwest Morocco.

Phylogenetic relationships inferred from partial COI and 16S rRNA gene sequences point to the recognition of U. gibbus as an evolutionary unit. Genetic divergences with respect to other Unio species ranged from 10.24% and 6.18% (compared to U. tumidus) to 12.91% and 8.89% (compared to U. tumidiformis) for COI and 16S rRNA respectively.

In the Spanish specimens, the entire internal cavity of the external demibranchs (homogeneity) acts as a marsupium (ectobranchy), yet in the Moroccan population, the four gills are occasionally filled with glochidia (occasional tetrageny). Glochidia are rounded triangular, intermediate between the shapes of those in other species of Unio and Potomida. The border of the larval shell bears numerous small lumps, but these fail to form the ventral styliform hook typical of the genus Unio. These two characters, tetrageny and hookless glochidia, both of major significance in phylogenetic studies of the group, are reported here for the first time in the genus Unio.

The Spanish populations of U. gibbus should be a main priority for European invertebrate conservation measures.