A widespread Ponto-Caspian invader with a mistaken identity: integrative taxonomy elucidates the confusing taxonomy of Trichogammarus trichiatus (= Echinogammarus) (Crustacea: Amphipoda)

Amphipods have diversified greatly in the Ponto-Caspian region. Although many of these species are prominent invaders their systematics remains unclear. Taking an integrative approach, we investigate the taxonomy of Trichogammarus trichiatus, a widespread invader in European inland waters. It was initially described from the north-eastern Black Sea coast as Chaetogammarus trichiatus by Martynov in 1932. A similar taxon, Chaetogammarus tenellus major, was described by Carausu from the western Black Sea in 1943 but later synonymized with C. trichiatus. Chaetogammarus trichiatus was itself shuffled between Chaetogammarus and the Atlanto-Mediterranean Echinogammarus, currently being assigned to Trichogammarus. Our analyses (six DNA markers, 60 measurements and scanning electron microscopic imaging) reveal that T. trichiatus and C. tenellus major are distinct species; the former is a Caucasian endemic, whereas the latter invaded Europe. Unexpectedly, T. trichiatus is an incipient species molecularly nested in Chaetogammarus ischnus, despite pronounced morphological and geographical differentiation. We also recover Chaetogammarus as polyphyletic, yet its member species are nested in the Ponto-Caspian radiation, thus distinct from Echinogammarus. Consequently, we reassign T. trichiatus to Chaetogammarus (Chaetogammarus trichiatus), synonymize Trichogammarus with Chaetogammarus and place C. tenellus major in the new genus Spirogammarus gen. nov. (Spirogammarus major comb. & stat. nov.). Chaetogammarus necessitates further systematic refinement.