A new species of Iberian cyprinid fish, Chondrostoma almacai, from the southwest of Portugal is described. Distribution area includes the Mira, Arade and Bensafrim drainages, previously considered to be inhabited by another phylogenetically related taxon, Chondrostoma lusitanicum. Chondrostoma almacai sp. nov. and C. lusitanicum present important morphological similarities, though significant differences were found for meristic and morphometric characters. The main diagnostic characters for C. almacai are the higher number of scales on the lateral line and above the lateral line, the lower number of right pharyngeal teeth, the longer snout and the smaller head depth. Furthermore, the new species has a small number of gill rakers and a larger predorsal distance, pelvic fin length and eye diameter. Previous molecular data, both mtDNA and nuclear markers (allozymes), suggested that C. almacai and C. lusitanicum are strongly differentiated sister-species. The morphological variability and geographic distribution of C. lusitanicum are reviewed and the most southerly populations from the southwest of Portugal are now recognized as the new species, C. almacai. It is recommended that both species be considered Critically Endangered according to the IUCN Red List Categories and Criteria [B1ab(ii,iii,iv)c(iv)+2ab(ii,iii,iv)c(iv) for C. almacai and A2ce+3ce+4ce for C. lusitanicum].
Analysis of the hybridisation events between two Iberian Chondrostoma species in the Távora River (Douro Basin) suggests different levels of trait introgression. Nuclear traits studied showed different introgression levels, whereas mitochondrial DNA introgression was not found. Lack of mtDNA introgression suggests that male and female hybrids may not equally fit or that possibly backcross matings may not be random. This could be contributing to the maintenance of a relative morphologic cohesion of hybridizing species, in spite of differences relative to allopatric populations. The hybrid zone was possibly originated by secondary contacts between populations of the species involved, motivated by connectivity between adjacent basins. Reanalysis of the hybridizing taxa revealed that Chondrostoma macrolepidotum is the species involved in the interspecific crosses with C. duriense, instead of C. arcasii as previously proposed.