Morphological and molecular identification of an echinostomatid digenean Pegosomum asperum ex Ardea alba (Aves: Pelecaniformes: Ardeidae) from the Volga River delta

Pegosomum is a genus of aberrant echinostomatid digeneans that parasitize ardeid birds as adults in the biliary system.Phylogenetic relationships of this genus are not entirely clear.Two specimens of Pegosomum were collected from the Great egret Ardea alba in the Volga River delta (Russia), and identified based on morphological characteristics as P. asperum.Morphological identification of these specimens was confirmed by phylogenetic analysis based on markers of nuclear (28S rRNA gene, ITS2 region) and mitochondrial (nad1 and cox1) genes.It is the first comprehensive molecular and morphological evidence of the occurrence of this species in Eastern Europe.The results of our phylogenetic analyses show that Pegosomum spp.share the most recent common ancestor with the type species of the genus Petasiger, P. exaeretus, and morphologically undescribed cercariae Petasiger sp. 1 of Laidemitt et al. (2019) from Radix natalensis (Krauss, 1848), Kenya.The genus Petasiger appears to be a non-monophyletic taxon, and its revision is necessary.