A new species of the genus Plagiostomum (Platyhelminthes, Prolecithophora, Plagiostomidae), with description of its reproductive and developmental characteristics

A new species of marine flatworm of the genus Plagiostomum is described from the intertidal zone along the coast of South China, based on comprehensive morphology, histology, reproductive biology, and molecular phylogenetic analyses. Plagiostomum aestuarium has a blunt anterior end with a small tentacle-like structure and a dark-brown patch covering the brain region. A pair of testes and a pair of ovaries are separated.

The testes are located ventrally, while the ovaries are distributed from the vitellaria towards the testes. The vitellaria are separated at the anterior end, merge at the dorsal part of the intestine, and separate again at the posterior end on both sides of the intestine. The penis is encased in a well-developed penis sheath.

The proximal part of the penis is coiled within the distal sac, and the distal part of the penis is relatively short. In light of the distinctive characteristics that distinguish this new species, we have classified it within the genus Plagiostomum (Platyhelminthes: Prolecithophora: Plagiostomidae). The phylogenetic tree constructed based on the concatenated datasets of 18 S rDNA and 28 S rDNA revealed that P. aestuarium is closely related to other Plagiostomum species.

Furthermore, this study provides a comprehensive account of the distinctive reproductive biology of P. aestuarium and, for the first time, elucidates the function of the penis sheath structure during hypodermic insemination. This new finding could potentially serve as a new classification criterion for species within the family Plagiostomidae.