Centrohelid heliozoans of Ukraine with a description of a new genus and species (Haptista: Centroplasthelida)

New data on the species diversity and morphology of centrohelid heliozoans in freshwater, marine, and soil habitats of Ukraine were obtained. Cell coverings (scales and spicules) were observed using scanning and transmission electron microscopy. Eighteen species from seven genera of centrohelids and unidentifiable Heterophrys-like organisms were revealed. The micrographs and detailed morphological descriptions of observed species and their comparison with previously found centrohelids are provided. A new genus and three new species Acanthocystis tyrasiana sp. nov., Pterocystis borysthenica sp. nov., and Khitsovia mutabilis gen. et sp. nov. were described. Molecular phylogenetic analyses based on 18S rRNA sequences, obtained for three strains, expand our knowledge on the diversity and evolution of centrohelids within the Pterista clade. The novel data on the morphology of studied scales supplement available information on the intraspecific variability of centrohelid heliozoans.