biostudies-literatureUnknown64Oya Y<i>Stylochoposthia</i> 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 <i>Stylochoposthia</i> sp. using the combined data generated from partial sequences of nuclear 18S and 28S ribosomal DNA, mitochondrial 16S ribosomal DNA, and the cytochrome <i>c</i> oxidase subunit I gene. The phylogenetic analysis revealed that <i>Stylochoposthia</i> sp. was nested in a robust clade comprising the species of four genera within Planoceridae Stimpson, 1857 (<i>Aquaplana</i> Hyman, 1953, <i>Heteroplanocera</i> Oya & Kajihara, 2021, <i>Paraplanocera</i> Laidlaw, 1903, and <i>Planocera</i> Blainville, 1828). Specifically, <i>Stylochoposthia</i> sp. was closely related to <i>PlaZoological studiese47https://www.ebi.ac.uk/biostudies/studies/S-EPMC12813612biostudies-literatureThe Phylogenetic Position of <i>Stylochoposthia</i> Faubel, 1983 (Polycladida, Acotylea) Provides Insights into the Evolution of the Copulatory Apparatus.PMC12813612Tsuyuki AOya YfalseThe Phylogenetic Position of <i>Stylochoposthia</i> Faubel, 1983 (Polycladida, Acotylea) Provides Insights into the Evolution of the Copulatory Apparatus.<i>Stylochoposthia</i> 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 <i>Stylochoposthia</i> sp. using the combined data generated from partial sequences of nuclear 18S and 28S ribosomal DNA, mitochondrial 16S ribosomal DNA, and the cytochrome <i>c</i> oxidase subunit I gene. The phylogenetic analysis revealed that <i>Stylochoposthia</i> sp. was nested in a robust clade comprising the species of four genera within Planoceridae Stimpson, 1857 (<i>Aquaplana</i> Hyman, 1953, <i>Heteroplanocera</i> Oya & Kajihara, 2021, <i>Paraplanocera</i> Laidlaw, 1903, and <i>Planocera</i> Blainville, 1828). Specifically, <i>Stylochoposthia</i> sp. was closely related to <i>Pla2025-01-01T00:00:00Z20252026-06-06T17:49:32.909Z2026-06-03T03:10:50.445ZS-EPMC128136124156187910.6620/zs.2025.64-4710.6620/ZS.2025.64-47