Vol. 60, 2021

(update: 2021.04.14, 06.24)

Description of a New Species of the Marine Flatworm Prosthiostomum (Platyhelminthes: Polycladida) and its Three Known Congeners from Misaki, Japan, with Inference of Their Phylogenetic Positions within Prosthiostomidae

Aoi Tsuyuki^1,*, Hisanori Kohtsuka², and Hiroshi Kajihara³

¹Graduate School of Science, Hokkaido University, N10W8, Sapporo, Hokkaido 060-0810, Japan. *Correspondence: E-mail: tykamsp0430@gmail.com (Tsuyuki)
²Misaki Marine Biological Station, The University of Tokyo, Koajiro 1024, Misaki, Miura, Kanagawa 238-0225, Japan. E-mail: kohtsuka@g.ecc.u-tokyo.ac.jp (Kohtsuka)
³Faculty of Science, Hokkaido University, N10W8, Sapporo, Hokkaido 060-0810, Japan. E-mail: kajihara@eis.hokudai.ac.jp (Kajihara)

Received 22 September 2020 / Accepted 6 April 2021
Communicated by Benny K.K. Chan

The present study provides morphological descriptions of four species of Prosthiostomum (Polycladida, Prosthiostomidae)—P. auratum Kato, 1937; P. hibana sp. n.; P. cf. ostreae Kato, 1937; and P. vulgare Kato, 1938—based on specimens collected among branching coralline algae and kelp holdfasts in Misaki, Japan. The new species P. hibana sp. n. is characterized by i) the dorsal surface of the body covered with numerous orange maculae, some of which coalesce together to form larger ones; ii) a pair of linear cerebral-eyespot clusters, each consisting of relatively few (7–9) cerebral eyespots; iii) 3–4 pairs of ventral eyespots embedded in parenchyma: iv) the inner wall of the male atrium deeply ruffled; v) the lumen of the seminal vesicle being narrow and elongated in shape; and vi) a large sucker situated in the center of the body. We remark on some morphological characters that were not mentioned in the original description of P. auratum. We infer the phylogenetic positions of these four species within Prosthiostomidae using the maximum-likelihood analysis based on partial 28S rRNA and COI gene sequences determined de novo, in addition to those that are currently available in public databases. In the resulting tree, the four species—P. auratum, P. hibana sp. n., P. cf. ostreae, and P. vulgare—were nested in a clade that was composed of all the other Prosthiostomum species included in the analysis.

Key words: Cotylea, Phylogeny, Species inquirenda, systematics, taxonomy.

Citation: Tsuyuki A, Kohtsuka H, Kajihara H. 2021. Description of a new species of the marine flatworm Prosthiostomum (Platyhelminthes: Polycladida) and its three known congeners from Misaki, Japan, with inference of their phylogenetic positions within Prosthiostomidae. Zool Stud 60:29. doi:10.6620/ZS.2021.60-29.