Morphology and phylogeny of two acotylean flatworms (Platyhelminthes, Polycladida) from the genera Cryptophallus and Limnoplana in the South China Sea

We describe two new species of the genera Cryptophallus Bock, 1913 and Limnoplana Faubel, 1983, which have not been reported for nearly a century. Herein, the genus Limnoplana represents a rare group of polyclads inhabiting brackish waters. Cryptophallus sinensis sp. nov. is characterized by: i) a grayish-brown dorsal surface with a slightly darker median region, ii) numerous cerebral eyespots forming two scarcely separate clusters, and iii) an ejaculatory duct joining the prostatic duct at the distal end of the penis.

Limnoplana obscuriviridis sp. nov. is distinguished from its congeners by: i) the presence of a true seminal vesicle, ii) a heart-shaped brain, iii) gonopores positioned closer to the posterior end of the body, and iv) a U-shaped Lang’s vesicle with two swollen ends. We provide the first partial DNA maker sequences for these two new species and employ them to analyze the phylogenetic relationships of their corresponding genera.

Our molecular phylogenetic analyses show that Cryptophallus and Limnoplana are nested within the Stylochoidea clade. In addition, we have revised the genus Cryptophallus; its familial affiliation remains unresolved, and additional species are required to clarify its phylogenetic position.