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. (Eurycer-cus) s.str., E. (Bullatifrons) Frey, 1975 and E. (Teretifrons) Frey, 1975. We conducted a revision of the subgenera Eurycer-cus (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. lamel-latus (O. F. Müller, 1776); E. macracanthus Frey, 1973; E. pompholygodes Frey, 1975; E. microdontus Frey, 1978; E. lon-girostris 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. ber-ingi 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. (Eu-rycercus), 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 demonstrat-ed that E. macracanthus, E. pompholigodes, E. longirostris and E. nipponica have much broader distributional ranges than previously known.