Unveiling cryptic diversity: two new species of Dugesia from northern China, including one sibling species (Platyhelminthes, Tricladida, Dugesiidae)

Two new species of Dugesia from the Taihang and Funiu Mountains, including one sibling species, are described by applying an integrative approach, including morphological, karyological, histological, and molecular information. Dugesia cylindrica Chen & Dong, sp. nov. is characterized by the following features, among others: sac-shaped copulatory bursa located immediately posterior to the pharyngeal pocket; stubby, cylindrical penis papilla with a blunt tip; large and pointed diaphragm; a long connecting duct between the seminal vesicle and diaphragm; slightly ventrally displaced ejaculatory duct with a terminal opening; chromosome complement diploid, with 16 metacentric chromosomes.

Dugesia elongata Chen & Dong, sp. nov. is characterized by the following features, among others: barrel-shaped penis papilla; elongated, dumb-bell shaped seminal vesicle; long connecting duct between the seminal vesicle and low-conical diaphragm; ventrally displaced ejaculatory duct opening at the tip of the penis papilla, albeit at its ventral portion; asymmetrical openings of the oviducts; mixoploid karyotype, with diploid metacentric chromosome complements of 2 n = 16 and triploid metacentric complements of 3 n = 24.

In the molecular phylogenetic tree, the two new species belong to two different subclades of the group comprising Eastern Palearctic / Oriental / Australasian species, with the two clades forming sister taxa. The distinct specific status of these two new species is also supported by their genetic distances. The distribution records of the two new species underscore the rich diversity of freshwater planarians in this region, and the discovery of the sibling species highlights the efficacy of integrative taxonomy in delimiting cryptic species.