THE FIRST DATA ON MICROTURBELLARIANS (PLATHELMINTHES, RHABDITOPHORA) OF THE BOGUCHANSKY RESERVOIR. 2. THE FAMILIES MACROSTOMIDAE, MICROSTOMIDAE AND PROVORTICIDAE, WITH THE DESCRIPTION OF BAICALELLIA NASONOVI SP. N.

This is the second contribution in series devoted to the study of the microturbellarian fauna (Plathelminthes, Rhabditophora) of the Boguchansky Reservoir, lower section of the Angara River, eastern Siberia. It provides information on the taxonomy, the structure of the male copulatory organ, and zoogeography of one species each from the families Provorticidae Beklemischev 1927, Macrostomidae Benden 1870, and Microstomidae Luther 1907. The faunas of these worms in the Angara River and Lake Baikal are still poorly studied. Three species from these families were recorded and described more than 90 years ago from the Angara River before our study, but the original descriptions were accompanied by highly schematic and non-informative sketches which do not allow us to perform exact species identifications and comparisons, as a rule. In the research process of the lower section of the Angara River, we found representatives of further three species: Macrostomum johni Young 1972, Microstomum rogozini Timoshkin et Krivorotkin 2023, and Baicalellia nasonovi Timoshkin et Krivorotkin, sp. n. The stylets of M. johni from the Angara River and Lake Baikal were revealed to be almost identical in structure and size to those of specimens of the type series from Great Britain. M. rogozini is one of the most common species of Microstomidae that inhabits the coastal zone of Lake Baikal. The stylet structure of Baicalellia species resembles that of Baicalellia baicali Nasonov 1930 and Baicalellia nigrofasciata Nasonov 1930, but their available descriptions do not allow us to make reliable comparisons with the species found. Therefore, we propose to consider both these names as nomina nuda and attribute those individuals to B. nasonovi, a species new to science. Despite rhe significant geographic isolation, the stylets of Baikal and Boguchan specimens are identical in structure and size. Similarly to Kalyptorhynchian species (see contribution 1), the discovery of provorticids and microstomids of Baikal origins in such a remote section of the Angara River considerably expands their distributions and demonstrates their capacities to colonize water bodies hydrographically connected to Lake Baikal. Illustrated descriptions of Boguchan and Baikal specimens of M. johni, M. rogozini and B. nasonovi, comparisons with the most similar species and zoogeographic information are given. In addition, a complete checklist of the Angara River microturbellarian fauna known so far, one currently including 35 species from nine families, is presented.