Towards A Phylogenetic Taxonomy Of The Genus Diaphanosoma(Crustacea, Ctenopoda, Sididae) Based On The Sixth Trunk Limb And On COI Gene Sequences

1. Diaphanosoma, with more than 40 described species, is the largest genus of the Ctenopoda, and it is in many ways similar to the anomopod genus Daphnia. Here, we offer a comparative morphological analysis of 33 species, and COI analysis of 12 species, aimed at gaining a better insight into the taxonomy and phylogeny of the group. COI(confirmed to some extent by ITS sequences) reveals large genetic distances between morphologically related species(e.g. in the D. branchyurm-group) and cryptic speciation in others(e.g. in D. excisum). The rule in this genus is thus(macro) morphological stasis in the presence of molecular evolution. 2. A search for micro-morphological characters identified the number of setae on the endopodite of P6, and the relative length of the apical setae of the exopodite of P6 as important and consistent markers, dividing the genus in two clades, the hepta-group(with 7 setae) and the hexa-clade(with 6 endital setae). Taxonomically, the two clades should be assigned the rank of subgenera or genera. Valid names for both are available in the published literature(Diaphanosoma Fischer s.s. and Neodiaphanosoma Paggi). 3. A COI-based phylogenetic tree estimate confirms the conclusion of our morphological study. A biogeographic analysis, finally, recovers the hexa-group(containing about 11 species) as restricted to the tropics, with only limited penetration of the subtropics(in contrast to Daphnia). The more species hepta- group(containing about 22 species) lives in the temperate and continental climate zone, with limited extension into sub-polar zones, but considerable penetration of the subtropics and tropics. 4. We recorded two species of Diaphanosoma from Ounianga and Tibesti in Northern Chad, the first ctenopods to be found in the Sahara desert. One species, from a freshwater guelta on the south flanks of the Tibesti(D. excisum) is tropical; the second species, found in a freshwater lake in a largely saline environment(the Ounianga plateau) is new to science(D. bopingi n.sp.) and is here described, with special attention to the postabdomen, which bears four instead of the usual three basal spines. Its relatives are northern species that may transgress into the tropics but largely live outside of them. They are also more salt-tolerant than the tropical D. excisum. The latter is considered a relict of Megachad times, while the new species is considered a relict of more humid but also cooler times in the desert.