Unravelling the cryptic diversity of the low-dispersal land planarian <i>Cephaloflexa bergi</i> to elucidate diversification processes in the Brazilian Atlantic Forest

Abstract The terrestrial flatworm Cephaloflexa bergi (Tricladida: Platyhelminthes) is an excellent model for elucidating the evolutionary processes shaping biodiversity in the Brazilian Atlantic Forest (BAF). Its limited dispersal and sensitivity to environmental changes suggest that its genetic structure may reflect the evolutionary history of the forest biota. Its high interpopulation diversity questions whether it may represent a species complex. Our objective is to conduct a genetic analysis to assess the existence of multiple species, to evaluate population structure at a finer scale than previously reported, and to explore the drivers of its diversification. Specimens were collected from protected areas within the southern BAF, and the COI gene and three newly developed nuclear markers were sequenced. Our phylogenetic and population genetic analyses revealed two clades with high intra- and interpopulation diversity, supporting an ancient origin for the species linked to Neogene geological processes. The contact zone previously observed was restricted to a coastal region, pointing to the Quaternary sea-level fluctuations as the potential driver. Intra-individual diversity analyses provide the first evidence of the mosaic Meselson effect in land planarians. Overall, our species’ delimitation suggests that C. bergi potentially comprises multiple species and highlights the importance of integrating diverse molecular approaches to understand the BAF’s biodiversity.