The polyclads (Platyhelminthes: Polycladida) from the Tropical Eastern Pacific: Commented checklist and description of a new species

In the Tropical Eastern Pacific (TEP), while knowledge of the order Polycladida is longstanding, at the moment no study has synthesized how many and which species are known from this region. In this work, a new species of polyclad from the Oaxaca coast, Callioplana marianae sp. nov., is described. It is characterized by a beige background with light brown and white pigmentation, nipple-shaped nuchal tentacles, cerebral and tentacular eyes, an oval seminal vesicle, a penis papilla armed with a filiform stylet, and a tripartite Y-shaped Lang’s vesicle. An exhaustive review of polyclad studies in the TEP over 173 years (1851–2024) yielded a checklist of 82 species belonging to 53 genera and 22 families have been recorded, with 53 species (66.25%) first described in the region. The countries with the highest number of recorded species are Mexico (45 spp.), Ecuador (18 spp.), Costa Rica (10 spp.), Panama (10 spp.), and Peru (7 spp.). The family with the most species in the region was Leptoplanidae (9 spp.). The best represented genera were Leptoplana (5 spp.) and Paraplanocera (4 spp.). Planocera pellucida, Paraplanocera oligoglena, and Pseudobiceros splendidus are species with pantropical distribution. Koinostylochus burchami and Armatoplana panamensis have the deepest known distributions at 70 and 73 meters, respectively.