Amended diagnosis, validity and relationships of the genus Acrolichanus Ward, 1917 (Digenea: Allocreadiidae) based on the 28S rRNA gene, and observations on its lineage diversity.

Adult specimens of the allocreadiid trematode Acrolichanus auriculatus (Wedl, 1858) were collected from Acipenser schrenckii Brandt from the River Amur and Amur Estuary, Acipenser ruthenus L. from the Rivers Irtysh and Oka (Asian and European parts of Russia, respectively), and Acipenser fulvescens Rafinesque from Lake Winnebago, Wisconsin, USA, and used for phylogenetic analyses based on 28S rDNA sequence data. The results supported the monophyly of Acrolichanus populations from both continents and a clear separation of A. auriculatus from other allocreadiid taxa, thus supporting the validity of the genus Acrolichanus Ward, 1917. This study formally reestablishes the genus Acrolichanus. Species of this genus differ from other allocreadiids by the following combination of morphological features: triangular mouth opening with rounded posterior corners, simple ventral, dorsolateral and dorsomedian muscular lobes on the oral sucker, stout cirrus-sac, pars prostatica with dorsal pocket, and massive cirrus with conspicuous triradiate lumen when invaginated. Specimens of the trematode from North America and the Rivers Irtysh and Oka were more closely related to each other than to the specimens from River Amur and Amur Estuary. This reflects the evolutionary divergence among sturgeons into Pacific and Atlantic clades. The phylogenetic analyses also confirmed the paraphyly of the genus Crepidostomum Braun, 1900.