New and Rare Species of Microturbellarians of the Genus Opisthocystis Sekera, 1911 (Plathelminthes, Kalyptorhynchia, Polycystididae) from Lake Baikal

Descriptions of four new species of the genus Opisthocystis Sekera, 1911 are given, including Opisthocystis zaytsevae Krivorotkin et Timoshkin sp. n., Opisthocystis strictostylus Krivorotkin et Timoshkin sp. n., Opisthocystis sekeri Krivorotkin et Timoshkin sp. n., and Opisthocystis tenuistylus Krivorotkin et Timoshkin sp. n., all of them are endemic to Lake Baikal. The first three species were found in the coastal zone of the lake: O. zaytsevae in the shallow waters of Bolshoy Ushkaniy Island; O. strictostylus in the shallow waters of Ayaya Bay (northern basin) and opposite the village of Khuzhir (Olkhon Island, Maloye More Strait); and O. sekeri in the shallow waters of Onokochanskaya Bay (northern basin). O. tenuistylus was discovered on the underwater Akademichesky Ridge at a depth of about 350 m. Illustrated descriptions, differential diagnoses, and brief information on their distribution and ecology are provided for all new species. New extensive information is provided on the morphology and zoogeography of three rare and poorly studied endemic Baikal species of the same genus. These are Opisthocystis multifidus (Nasonov, 1935), Opisthocystis octopropagostylus Lukhnev et Timoshkin, 2014, and Opisthocystis cf. umylinae Timoshkin, 2010. Brief illustrated descriptions and generalized information on their distribution are provided for all species. The taxonomic status of five Baikal species—Opisthocystis bilobatus Nasonov, 1935, Opisthocystis bicurvatus Nasonov, 1935, Opisthocystis campanulatus Nasonov, 1935, Opisthocystis irregularis Nasonov, 1935, and Opisthocystis trifida Nasonov, 1935—is discussed. The diagnoses of these species include only very brief descriptions of the distal stylet ends, with schematic drawings provided for three of them. It was impossible to reliably identify the species listed by N.V. Nasonov. To confirm this, we provide the author’s original drawings and descriptions. As part of the revision of the Opisthocystis fauna in Baikal, we have to assign the status of nomen nudum to these five species. The paper provides their up-to-date list that includes 41 species, with the origin of 40 of them associated with Lake Baikal.