A highly permeable species boundary between two anadromous fishes.

Meristic identification, mitochondrial DNA and a suite of microsatellite markers were employed to estimate the incidence of hybridization in wild populations of anadromous Allis shad Alosa alosa and twaite shad Alosa fallax in southern Irish riverine and estuarine waters. It was shown that 16% of the fishes examined were misclassified using meristic count of gill rakers. Next, a significant proportion of fishes that were robustly assigned to a species using nuclear markers were shown to possess the mtDNA of the other. The genomes of A. alosa and A. fallax in Ireland are extensively introgressed, which suggests a complex history of hybridization between these species, which can only partially be explained by recent man-made habitat changes.