The Phylogenetic Position of Stylochoposthia Faubel, 1983 (Polycladida, Acotylea) Provides Insights into the Evolution of the Copulatory Apparatus.

Stylochoposthia Faubel, 1983 is a genus of acotylean polyclads, and its family-level assignment is controversial. In this study, we inferred the phylogenetic position of the genus represented by Stylochoposthia sp. using the combined data generated from partial sequences of nuclear 18S and 28S ribosomal DNA, mitochondrial 16S ribosomal DNA, and the cytochrome c oxidase subunit I gene. The phylogenetic analysis revealed that Stylochoposthia sp. was nested in a robust clade comprising the species of four genera within Planoceridae Stimpson, 1857 (Aquaplana Hyman, 1953, Heteroplanocera Oya & Kajihara, 2021, Paraplanocera Laidlaw, 1903, and Planocera Blainville, 1828). Specifically, Stylochoposthia sp. was closely related to Planocera pellucida (Mertens, 1833), with high support values. This study revealed that an eversible cirrus with hard structures (numerous spines, thorns, or teeth) in the male copulatory apparatus, a diagnostic characteristic of Planoceridae, has been lost in the lineage of Stylochoposthia.