Toward a phylogeny and biogeography of Diaphanosoma (Crustacea: Cladocera)

Abstract

Diaphanosoma s.l., with 40+ described species, is the largest genus of the Sididae and the Ctenopoda, similar in many ways to the anomopod genus Daphnia. Here, we offer a c morphological evaluation of 33 species and contrast it with an analysis of the bar coding fragment of the cytochrome c oxidase subunit (COI) gene in order to gain insight into the taxonomy and phylogeny of the group. A search for structure in the genus based on micromorphological characters identified (1) the number of setae on the endopodite of P6 and (2) the relative length of the three apical setae of the exopodite of P6 as consistent markers, recovering two clades, the hexa-clade (with six endital setae) and the hepta-group (with seven setae). A third character is the armament of the posterior zone of the ventral margin of the valves, denticulated in the hexa-group, with fine filaments between the denticles in the hepta-group. Taxonomically, the two clades are assigned the rank of genera. Valid names for both are available in the published literature (Diaphanosoma Fischer s.s. and Neodiaphanosoma Paggi and Da Rocha, amended). A COI-based phylogenetic estimate leads to the same conclusion as morphology, but reveals cryptic speciation as well. COI reveals relatively small genetic distances within and between temperate climate species (e.g., in the D. birgei group) and large differences that form a cline from Australia (the terra typica) across southeast Asia in the tropical Neodiaphanosoma species D. excisum. African populations need more study, but probably represent a separate species. The entire group shows (macro)morphological stasis in the presence of molecular evolution but Neodiaphanosoma is evolving at a much faster rate than Diaphanosoma s.s. that is either a younger group. This hypothesis needs more testing, however. A biogeographic map recovers Neodiaphanosoma (containing slightly over 10 species) as restricted to the tropics, with limited penetration of the subtropics in the southern hemisphere. The more speciose Diaphanosoma group (up to 30 species) lives in the temperate and continental climate zone, with limited extension into subpolar zones, but considerable penetration of the subtropics and tropics. The subtropics, where both groups broadly overlap, is the zone with the highest species richness.