A new divergent lineage of Daphnia (Cladocera: Anomopoda) and its morphological and genetical differentiation from Daphnia curvirostris Eylmann, 1887

The systematic biology of the subgenus Daphnia s.s. remains confused. Prior attempts at resolution used chiefly postabdominal claw morphology, chromosome numbers and rRNA gene sequences as characters for higher-level relations. Still, several taxa, such as Daphnia curvirostris Eylmann, 1878, have unclear affiliations. We addressed the position of D. curvirostris in this genus by estimating phylogenetic trees from a rapidly evolving protein coding gene (ND2), conducting broad geographical comparisons and carrying out detailed morphological comparisons. The Japanese 'curvirostris' was found to be a new divergent lineage in the subgenus Daphnia, and to possess distinctive morphological characteristics from D. curvirostris. We described this new species as Daphnia tanakai sp. nov., and redescribed D. curvirostris. The polymorphic postabdominal claw morphology and the distinctive chromosome number of D. tanakai sp. nov. provided evidence for rapid evolution of these traits. Our new morphological, chromosomal and genetic assessment of Daphnia weakened the argument for division of the subgenus Daphnia (Daphnia) O. F. Müller, 1785 sensu Johnson, 1952, into two further subgenera.