Recent evolution of ancient Arctic leech relatives: systematics of Acanthobdellida

Abstract Acanthobdellida gnaw into the sides of salmonid fishes in frigid Arctic lakes and rivers, latching on with fearsome facial hooks. Sister to leeches, they are an ancient lineage with two described species. Unfortunately, Acanthobdellida are rarely collected, leading to a paucity of literature despite their unique morphology. Populations range from Eurasia to Alaska (USA), but few specimens of Acanthobdella peledina are represented in molecular studies, and no molecular data exist for Paracanthobdella livanowi, making their taxonomic position difficult to assess. We use phylogenetics and morphology to determine whether allopatric populations of A. peledina are distinct species and assess the current classification scheme used for Acanthobdellida. We produce a new suborder, Acanthobdelliformes, to match the taxonomy within Hirudinea. Scanning electron micrographs indicate species-level differences in the anterior sucker and facial hooks; molecular phylogenetics mirrors this divergence between species. We assign both species to the family Acanthobdellidae and abandon the family Paracanthobdellidae. Alaskan and European A. peledina populations are morphologically similar, but appear phylogenetically divergent. Our data strongly suggest that members of the order Acanthobdellida diverged relatively recently in their ancient history, but based on genetic distance, this divergence appears to pre-date the most recent cycles of glaciation.