A revision of the subgenus Eurycercus (Eurycercus) Baird, 1843 emend. nov (Cladocera: Eurycercidae) in the Holarctic with the description of a new species from Alaska

Frey (1975) subdivided the genus Eurycercus Baird, 1843 (Cladocera: Eurycercidae) into three subgenera: E. (Eurycercus) s.str., E. (Bullatifrons) Frey, 1975 and E. (Teretifrons) Frey, 1975. We conducted a revision of the subgenera Eurycercus (Eurycercus) and E. (Bullatifrons) in the Holarctic based on the morphology of parthenogenetic females and a phylogeny of cytochrome c oxidase subunit I (COI) sequences. The following six species are found to be valid: E. lamellatus (O. F. Muller, 1776); E. macracanthus Frey, 1973; E. pompholygodes Frey, 1975; E. microdontus Frey, 1978; E. longirostris Hann, 1982; E. nipponica Tanaka & Fujuta, 2002. The separation of E. vernalis Hann, 1982 from E. longirostris lacks morphological and genetic justification, so E. vernalis is a junior synonym of E. longirostris. A new species, E. beringi sp. nov., was found in several localities in Alaska, U. S. A. Its characters are intermediate between two subgenera sensu Frey (1975): a median keel is expressed, but only in the posterior portion of the carapace dorsum (while it is absent in E. (Bullatifrons) and passes through all the dorsum in Eurycercus s.str.); the dorsal head pores are located on the bubble-like projection (a character of the subgenus E (Bullatifrons), but the latter is sitting on a prominent transverse fold (character of the subgenus Eurycercus s.str.). The COI tree also does not support separation of the subgenus E. (Bullatifrons) from E. (Eurycercus), while separation of E. (Teretifrons) is well-supported. So, we propose to avoid a separation of E. (Bullatifrons) and regard all the species previously placed there as belonging to the subgenus E. (Eurycercus) emend. nov. We also demonstrated that E. macracanthus, E. pompholigodes, E. longirostris and E. nipponica have much broader distributional ranges than previously known.