Three new freshwater species of centrohelid heliozoans: Acanthocystis crescenta sp. nov., A. kirilli sp. nov., and Choanocystis minima sp. nov.

Three new species of centrohelid heliozoans Acanthocystis crescenta, A. kirilli, and Choanocystis minima from two freshwater lakes of Valamo Island (North-Western Russia) were studied using light- and scanning electron microscopy. The main apomorphy of A. kirilli are slightly branched radial scales. Acanthocystis crescenta has characteristic radial scales with crescent-like structures on their tips. Cell diameter of these two species does not exceed 10 μm. Radial scales of C. minima have truncated tips and lack apical teeth; plate scales of this species are oviform, without lateral notches or central rib. The latter organism is the smallest heliozoan species known; the diameter of its body is about 3 μm. The possible domination of small heliozoans in the cryptic diversity of centrohelids is discussed.