Acanthocystis nichollsi Siemensma & Roijackers 1988

Acanthocystis nichollsi Siemensma & Roijackers, 1988 (Fig. 1A–E) Description. The cell is spherical, enclosed in a periplast 37–43 μm in diameter, comprising two types of siliceous elements (Fig. 1A–B): radial, presented by two types of spine-scales, and tangential, presented by plate-scales. Spine-scales have a hollow, cylindrical shaft 0.2–0.3 μm in diameter, centrally set on a centrally depressed circular basal part 0.6–1.3 μm in diameter. Plate-scales and the basal part of spine-scales are surrounded by a smooth marginal rim 0.02–0.06 μm thick. Long spine-scales are straight or slightly curved, 4.8–11.9 μm long. The apex of the shaft (Fig. 1C) is slightly expanded (0.4–0.7 μm in diameter) with a varying number of apical teeth (7–8). Short spine-scales are 1.3–3.1 μm long; the apex is funnel-like shaped (Fig. 1D), 0.7–1.5 μm in diameter, with 5–9 diverging sharp teeth interconnected by a thin membrane. Plate-scales (Fig. 1B, E) are elliptical-shaped (1.9–3.8 × 1.4–2.1 μm) with the median constriction 1.2–1.9 μm wide and a marginal rim 0.03–0.04 μm thick. Plate-scales are ornamented with radial slits (0.07–0.22 μm long), and the lower surface has axial rod-shaped thickening 0.53–0.58 μm long. Remarks. In the Siemensma & Roijackers’ publication (1988), where the authors distinguish A. nichollsi as a new species, the morphometric data of the periplast are not indicated in the species description. For comparison, we used data from Kosolapova & Myl’nikov (2015), reporting the first record of this species in East Asia. In our study, morphometric data of the periplast of A. nichollsi are similar to those indicated from examinations of Mongolian specimens. However, the larger variation in length of spine-scales is found in Japanese specimens: 4.8–11.9 μm (for long spine-scales) and 1.3– 3.1 μm (for short spine-scales). Its value, according to literature, is 6.8–12.0 and 1.6–2.7 μm, respectively. Additionally, the plate-scales have a larger variation in sizes—1.9–3.8 × 1.4–2.1 µm, while in Kosolapova & Myl’nikov’s description, the scales’ dimensions are 3.8 × 2.1 µm. Distribution. Acanthocystis nichollsi has been recorded in Africa,Asia, Europe, North and South America (Siemensma & Roijackers, 1988; Wujek et al., 2010; Zlatogursky & Klimov, 2016; Prokina & Myl’nikov, 2019). It is known that this species is distributed throughout all Europe (Siemensma & Roijackers, 1988; Cavalier-Smith and von der Heyden, 2007; Zlatogursky & Klimov, 2016), whereas in Asia, it is known only from a few localities. Apart from Mongolia, A. nichollsi has also been reported from India and Vietnam (Gerasimova, 2021). A. nichollsi is frequently found in freshwater habitats. However, it has also been reported to be found in continental brackish waters (Gerasimova, 2021). We recorded A. nichollsi in a freshwater channel, representing the first documented occurrence of this species in Japan.