Genetic and phenotypic displacement of an endemic Barbus complex by invasive European barbel Barbus barbus in central Italy

Invasions of alien fishes can result in considerable consequences for native biodiversity, including local extinctions of native species through genetic introgression. In Italy, the alien European barbel Barbus barbus was first detected in 1994. It has since undergone range expansion, raising conservation concerns on their impacts on endemic Barbus species, including Barbus plebejus and Barbus tyberinus. Here, the genetic and phenotypic consequences of B. barbus invasion in the Tyrrhenian and Adriatic basins of central Italy were assessed by comparing ‘invaded’ with ‘uninvaded’ river sections that remain free of B. barbus due to barriers preventing their upstream dispersal. In both basins, uninvaded sites were confirmed as B. barbus free, but the endemic populations had low genetic variability. In the invaded sections, haplotype and nucleotide diversity was relatively high, with introgression skewed towards B. barbus genes, with the barbel populations comprising of only 4% and 23% of pure B. tyberinus and B. plebejus respectively. Relatively high morphological differentiation was apparent between pure B. tyberinus and hybrid forms, whilst differences were less apparent between pure B. plebejus and their hybrid forms. Thus, the endemic Barbus species only persist in areas that remain free of invasive B. barbus, with this only due to river structures that impede their upstream movements. As these structures also limit the effective population size of the endemic species, conservation plans must reconcile B. barbus dispersal prevention with the need to increase the population connectivity of the endemics.