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SummaryThe species composition and morphology of centrohelid heliozoa collected from mire water bodies of different types in the North of the European part of Russia were studied. Eighteen species from five genera and four families, two species with an uncertain systematic position and some unidentified Heterophrys-like organisms were found (Arkhangelsk Region -9 species, Republic of Karelia -7, Vologda Region -9). Three species (Pterocystis paliformis, P. striata, P. tropica) are new for Russia. Nine species are new for Arkhangelsk Region (Acanthocystis penardi, A. trifurca, A. turfacea, Choanocystis symna, P. pinnata, P. tropica, Raineriophrys echinata, R. kilianii). Eight species are new to the centrohelid diversity of the Vologda Region (Acanthocystis lyra, A. aff. takahashii, A. trifurca, Polyplacocystis symmetrica, Pterocystis pinnata, P. tropica, Raphidiophrys intermedia). Four species are new for the Republic of Karelia (Pterocystis paliformis, P. striata, P. tropica, Raphidiophrys minuta). The species composition of the mires of the Arkhangelsk Region and the Vologda Region has much more in common in comparison with the centrohelid diversity of the Republic of Karelia. The most favourable conditions for the presence of high centrohelid species diversity occurred in the minerotrophic mires (15 species, incl. 5 -in mire streams, 7 -in space between hummocks, 9 -in flarks of the aapa mires), in comparison with the water bodies of ombrotrophic mires (4 species, incl. 2 -in laggs and 2 -in hollows of the raised bogs).

Species diversity of centrohelid heliozoans in different types of freshwater bodies (forest lakes, floodplain lakes, sphagnum bogs, and river) in the Middle Russian forest-steppe was studied. The morphology of cell coverings (scales and spicules) were observed using scanning and transmission electron microscopy. A total of 20 species of centrohelid heliozoans from five families (Acanthocystidae, Pterocystidae, Raphidiophryidae, Raphidocystidae, and Choanocystidae) and six genera (Acanthocystis, Raineriophrys, Pterocystis, Raphidiophrys, Raphidocystis, and Choanocystis) as well as an unidentified Centrohelea species and a Heterophrys-like organism were found. Morphological descriptions and electron micrographs are provided. The most common species were Acanthocystis nichollsi, Raphidiophrys capitata, and a Heterophrys-like organism. The highest species diversity was observed in terrace forest lakes, whereas the lowest was seen in sphagnum bogs. Four species (Acanthocystis costata, A. elenazhivotovae, A. mikrjukovii, and A. spinosa) were found for the first time since their original description. Three species (Acanthocystis spinosa, Raphidiophrys capitata, and Pterocystis pulchra) are new records for Russia. The distribution of the observed morphospecies confirms the cosmopolitanism of many centrohelids and highlights the importance of the type of water body and microbiotope in forming the species composition. The obtained data on the morphology of studied scales supplements our knowledge of the intraspecific variability of centrohelid heliozoans.

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