Molecular Phylogenetic Reexamination of Trigonostomidae Graff, 1905 (Platyhelminthes, Rhabdocoela), with the Resurrection of Messoplana Den Hartog, 1966, and the Description of Four New Species from the United States

Abstract

The rhabdocoel family Trigonostomidae has been the subject of recent phylogenetic revisions. However, due to the limited number of sequenced taxa, questions regarding intrafamilial and intrageneric relationships remain unresolved. To address these gaps, we collected trigonostomids from the North Sea, the Mediterranean, and the Atlantic Ocean, and examined them using an integrative approach. Nineteen specimens, representing ten species, were sequenced for phylogenetic analyses. Eight species were sequenced for the first time, including the first molecular data for Lutheriella. All genera were recovered as monophyletic; however, Ceratopera falcata (type species of Messoplana) and a new closely related species were found to cluster with species of Beklemischeviella, while the remaining species of Ceratopera formed a distinct clade. Based on these findings, we propose the resurrection of Messoplana, which had previously been synonymised with Ceratopera. Notably, genetic divergence and differences in the bursal mouthpiece between specimens of M. falcata suggest the possible presence of cryptic diversity or geographical isolation within this taxon. The inclusion of Trigonostomum breitfussi provided molecular support for its morphologically defined group. Furthermore, L. diplostyla was found to be closely related to species of Ptychopera. In addition to phylogenetic analyses, we describe four new species: Ceratopera bransoni sp. nov., Messoplana judithae sp. nov., Parapharyngiella scotti sp. nov., and Ptychopera woodyi sp. nov. The discussion of the new species of Parapharyngiella and Ptychopera motivated the reclassification of Pt. scutulifera as Pa. scutulifera comb. nov. We record for the first time eight species from Florida, seven from the Indian River Lagoon, and two from the Florida Keys National Marine Sanctuary, in the USA. This study refines the phylogenetic framework and internal classification of Trigonostomidae by providing new molecular evidence for previously unsampled taxa and by re-evaluating generic boundaries within the family.