Acanthocystis siemensmae Gerasimova & Plotnikov, 2016
Diagnosis: Cells are 17.0–21.0 μm (mean 19.0) in diameter. A cell coat consists of plate-scales of a single type and spine-scales of two types. The long spine-scales have narrow, hollow shafts 6.5–10.8 μm long. The shaft is slightly branching towards the apex and has 14–16 teeth. Extended part of the apex contains four symmetrically oriented longitudinal ridges; each ridge is ended by four teeth of different length. Two central teeth of the ridge usually are longer than lateral ones. Basal plates of the long spine-scales are circular, 0.9–1.2 μm in diameter. The short spine-scales are 1.6–2.9 μm long with narrow shafts and expanded cup-shaped apices up to 1.9 μm in diameter. The apex looks like a maple leaf due to division onto 3–5 ridges of irregular shape. Each ridge has a few teeth of different length. Basal plates of the short spine-scales are circular, 0.6–1.0 μm in diameter. The plate-scales are plane, patternless, elliptical or pearshaped with a thin marginal rim. Length of the plate-scales is 2.5–4.4 μm, width 1.2–2.7 μm.
Habitat: Freshwater. Bottom sediment from the Pismenka River, the South Urals, South-East of European part of Russia.
Remarks: A. siemensmae is slightly similar to A. pectinata Penard, 1889, A. polymorpha Dürrschmidt, 1985 and A. nichollsi Siemensma & Roijackers, 1988, in structure of the cup-shaped apex of the short spine-scales. The short spine-scales of A. pectinata have 4–6 short teeth; the long spine-scales have 2–3 teeth. In contrast, A. siemensmae has the short spine-scales with 3–5 ridges; each of them is ended by a few teeth. Its long scales possess 4 ridges and 14–16 teeth. In addition, plate-scales of A. pectinata are ornamented by granules, whereas those of A. siemensmae are patternless. The short spine-scales of A. polymorpha possess 5–8 ridges ended with one tooth, while A. siemensmae has the short spine-scales with 3-5 ridges, each of them with a few teeth. The long spine-scales of A. polymorpha have slightly dilated distal ends with teeth. In contrast, the distal ends of the long spine-scales of A. siemensmae reveal four ridges with terminal teeth. Short spine-scales of A. nichollsi have a flared terminus consisting of six spine-like ribs joined by a membrane with a scalloped margin, whereas the short spine-scales of A. siemensmae are ended by 3–5 ridges with a few apical teeth. The long spine-scales of A. siemensmae have 4 ridges with teeth, while ones of A. polymorpha have slightly dilated distal ends with teeth. The plate-scales of A. nichollsi are elliptical and ornamented with a medial thickening, radial ribs and a marginal rim, whereas the only ornamentation on the plate-scales of A. siemensmae is the marginal rim.