A revision of the genus Baltoplana Karling, 1949 (Rhabdocoela: Schizorhynchia: Cheliplanidae): Phylogenetic Insights and Seven New Species

Schizorhynchia is a taxon of marine free-living platyhelminthes that is generally characterized by having a divided proboscis; however, current taxonomy fails to accurately represent the evolutionary history of species within Schizorhynchia. Here, we present an integrative taxonomic revision and phylogenetic analysis of the genus Baltoplana Karling, 1949 (Cheliplanidae). We collected specimens from various locations around the world, including Australia, Brazil, Spain, France, Italy, and the United States. The specimens were extracted from sandy substrates on beaches. After collection, the specimens were placed in 500 mL wide-mouth flasks and processed using the MgCl₂ decantation method. Live specimens were observed under a microscope, and we took photographs, videos, and detailed anatomical drawings to support subsequent species identification. Each observed specimen was fixed in lactophenol and sealed for later observations under differential interference contrast (DIC) microscopy. Phylogenetic work based on newly acquired 18S and 28S rDNA sequences confirms that the family Cheliplanidae is monophyletic, for the first time positions Baltoplana within Cheliplanidae, and reveals Baltoplana as the sister taxon of Cheliplana. We describe seven new species and provide new morphometric data of known species. The new species of Baltoplana are mainly distinguished by the shape and size of the (un)armed cirrus, the presence or absence of accessory cirri, and the presence or absence of sclerotized tissue (cap) in the distal region of the male copulatory bulb.