Pterocystis borysthenica Zagumyonnyi et al, 2022
Diagnosis: Cell diameter 5-18 µm. Spine-scales 1.3–6.1 µm long, with a straight or curved shaft, 0.15–0.22 µm in diameter. Lateral wings short, 0.48–1.79 µm, tapering from the basal wing to the shaft. Basal wing triangular, 0.62–1.55 x 0.391.22 µm in size, with 12–16 parallel ribs on the inner side of the basal wing. Plate-scales elliptical, 1.6-3.1 x 1.1–2.1 µm in size with a medial thickening of 0.65–1.83 x 0.05–0.10 µm in size. Cystic scales 3.3-4.9 x 2.1–3.0 µm are dense and massive, elliptical, bean-shaped or irregular in shape with a medial thickening of 1.2–3.5 x 0.25–0.90 µm.
Encystment was detected in old cultures. Cystic scales were found on some cells. They represent massive dense plate-scales, bean-shaped or irregular in shape. Similar to ordinary plate-scales, cystic scales have a medial thickening.
Type location. Coastal bottom sediments of unnamed pond on Vita Stream, Dnepr basin, Ukraine (50°20022.600 N30° 29032.100 E).
Remarks: The detailed structure of cysts and cyst scales of centrohelids is known only for Raineriophrys erinaceoides, Raphidiophrys heterophryoidea (Zlatogursky 2012; Drachko et al. 2020), R. elongata (Drachko et al. 2021), and Raphidocystis ambigua (Dürrschmidt 1987a,b). Among the members of the genus Pterocystis, the details of cysts and cyst scales morphology are unknown. The closest related species with available data on the morphology of cyst scales is Raineriophrys erinaceoides. However, its polygonal cyst scales have little in common with the cyst scales of P. borysthenica. The knoll in the central part of the scales described for R. erinaceoides is absent in P. borysthenica. Instead, all cyst scales have an oblong medial thickening that resembles the medial thickening of trophic plate-scales. Bean-shaped cyst scales resembling enlarged trophic plate-scales have not been described in R. erinaceoides. The irregular cyst scales of P. borysthenica have a few outgrowths and depressions at the edges, which somewhat resemble the structure of the cyst scales of R. heterophryoidea. It is likely that these scales can be attached to each other like a puzzle, as in R. heterophryoidea.