Ceriodaphnia (Cladocera: Daphniidae) in China: Lineage diversity, phylogeography and possible interspecific hybridization.

The distribution and species/lineage diversity of freshwater invertebrate zooplankton remains understudied in China. Here, we explored the species/lineage diversity and phylogeography of Ceriodaphnia species across China. The taxonomy of this genus is under-explored. Seven morphospecies of Ceriodaphnia (C. cornuta, C. laticaudata, C. megops, C. pulchella, C. quadrangula, C. rotunda and C. spinata) were identified across 45 of 422 water bodies examined. Rather little morphological variation was observed within any single morphospecies regardless of country of origin. Nevertheless, we recognized that some or all of these morphospecies might represent species complexes. To investigate this, phylogenetic relationships within and among these morphospecies were investigated based on mitochondrial (partial cytochrome c oxidase subunit I gene) and nuclear (partial 28S rRNA gene) markers. The mitochondrial marker placed these populations in nine lineages corresponding to the morphospecies: C. laticaudata and C. pulchella were each represented by two lineages, suggesting that both are species complexes. The remaining five morphospecies were each represented by a single mtDNA lineage. Three of the nine mitochondrial lineages (belonging to C. pulchella, C. rotunda and C. megops) are newly reported and exhibited a restricted distribution within China. The nuclear-DNA phylogeny also recognized seven Ceriodaphnia taxa within China. We detected occasional mito-nuclear discordances in Ceriodaphnia taxa across China, suggesting interspecific introgression and hybridization. Our study contributes to an understanding of the species/lineage diversity of Ceriodaphnia, a genus with understudied taxonomy.