Resolution of the Daphnia umbra problem (Crustacea: Cladocera) using an integrated taxonomic approach

The genus Daphnia O. F. Müller (Crustacea: Cladocera) is a model taxon for freshwater biology and a crucial component of freshwater food webs. However, the genus contains commonly studied species groups, such as Daphnia longispina sensu lato, that are taxonomically confused and poorly characterized with respect to morphology. Here we: (1) analyse the taxonomic value of male anatomical characters within the longispina group and (2) integrate taxonomic, morphological, morphometric and genetic (based on COI and 12S rRNA gene sequences) data to resolve the species complex better. We also address a longstanding taxonomic issue in the group, termed the 'Daphnia umbra problem' after a dubious taxon found in the Canadian Arctic. We redescribe the morphology of adult males of Daphnia longispina O. F. Müller, 1785, Daphnia cf. hyalina Leydig, 1860 and Daphnia dentifera Forbes, 1893. As with other cladocerans, we demonstrate that the adult males provide more characters for taxonomic differentiation than the adult females. We also fully redescribe Daphnia umbra Taylor, Hebert et Colbourne emend. nov. and expand its known distribution to the Siberian Arctic and the temperate mountains of Asian Russia. Our results emphasize the value of integrating traditional, long-term, step-by-step taxonomic revisions and routine species redescriptions with morphometrics and genetics to resolve species problems within Daphnia.