The species diversity of the superclass Crustacea only in Lake Baikal has 58 species of the superorder Cladocera and 52 species and subspecies from the subclass Соpepoda, 5 of them belong to the order Calanoida and 47 to the order Cyclopoida. Species that inhabit the waters of Olkhon Island are not included in this list. On the island there are temporary and permanent reservoirs, which by the chemical composition of the water are fresh or brackish. The species diversity of the island crustaceans includes 28 species, of which only 64 % are known for Baikal. This article gives a brief description of the morphology of rare and small species from the family Daphniidae: Ceriodaphnia megops Sars 1985; Diaptomidae: Arctodiaptomus (A.) wierzejskii (Richard, 1888) and Cyclopidae: Eucyclops arcanus Alekseev, 1990. The first two species were found in Lake Baikal, the third species – on Olkhon Island. Morphological analysis of the species was performed by means of a scanning electron microscope (SEM). A detailed description of the morphological features of C. megops, A. (A.) wierzejskii female and male, and E. arcanus female with illustrations is presented. Data on diaptomus and cyclopoid mouth appendages, tables with biometrical parameters of a cyclopoid are provided. Images of cyclopoid Р1–Р4 coxopodite and basal exopodite segment are shown for the first time. C. megops and A. (A.) wierzeskii crustaceans are rare species in the water bodies of the south of Central Asia, including Mongolia. A. (A.) wierzeskii population is the first finding in the water bodies of the south of Eastern Siberia and Baikal.

Sheveleva Natalya Georgievna, Candidate of Science (Biology), Senior Research Scientist, Limnological Institute SB RAS, 3, Ulan-Batorskaya st., Irkutsk, 664033, Russian Federation, e-mail: shevn@lin.irk.ru 

Misharina Evgenia Aleksandrovna, Candidate of Science (Biology), Associate Professor, Irkutsk State University, 1, K. Marx st., Irkutsk, 664003, Russian Federation, e-mail: me603@mail.ru 

Makarkina Natalia Viktorovna, Candidate of Science (Biology), Associate Professor, Irkutsk State University, 1, K. Marx st., Irkutsk, 664003, Russian Federation, е-mail: petrych_m_n@mail.ru

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