Acanthocystis costata Zlatogursky, 2014
Diagnosis: Cell body ca 9.5 µm in diameter. Axopodia 2–3 times longer than cell diameter. Length of spine-scales 2.1–9.5 µm (on average 5.1 µm). There are straight scales as well as curved ones. Spine-scales with four, five or six teeth. Plate-scales with concave or, sometimes, parallel edges and rounded ends. Length of plate-scales 1.3–3.7 (ca. 2.6) µm, width 0.9–1.9 (ca. 1.4) µm. Periphery of scale with a thickened border. Axial thickening and about fifty radial slits are present. Some scales with numerous small granules, probably present on one side of the scale only and therefore visible not on all scales. Cells actively move at the bottom of the culture dish or float in the culture medium. Sometimes fusions of two or three cells with a common food vacuole were observed. Large particles of unknown nature were observed in vacuoles of some individuals.
Ecology: Freshwater pool in the botanical garden of St. Petersburg State University, St. Petersburg, North-Western Russia.
Remarks: The sculpture of the plate-scales is a very characteristic feature of this species. Plate-scales with similar radial slits are also described in A. myriospina and A. nichollsi. In A. myriospina these slits are observed only by Roijackers (cit. from Siemensma 1991) and only line drawing of the plate scale is published. But the quantity of slits on this drawing is nineteen, while in A. costata there are about fifty slits per scale. Probably this finding of Roijackers represents not an A. myriospina, but a new not yet described species. Anyway, even assuming, that slit-bearing scales are characteristic for A. myriospina, this species differs from A. costata in the quantity of slits and the morphology of spine-scales. The similar quantity of slits is characteristic for scales of A. nichollsi. The size of plate-scales is comparable, but not the same: 1.4–2.2 × 2.3–3.8 (A. nichollsi from Siemensma 1991) vs 1.3–3.7 × 0.9–1.9 and appearance is alike, except of the presence of granules in A. costata. But morphology of spine-scales between those two species is clearly different, because in A. nichollsi short spine-scales with apexes, separated into arms and connected with flarings of scale shaft are present (Nicholls 1983). Contrary, there is no any flarings on spine-scales of A. costata. A. mylnikovi is similar to the new species in having spine-scales with four, five or six teeth (however two or three teeth are also reported for A. mylnikovi). But spine-scales of A. costata are much shorter: 2.8–11.3 µm vs 5–17 µm for A. mylnikovi and plate-scales of A. mylnikovi have no slits (Leonov 2010). A. pectinata, A.polymorpha and several other species have granules on plate-scales similar to granules in A. costata. But in these species there are no slits on plate-scales and morphology of spine-scales is very different. are completely different. A. valdiviense differs from the new species in absence of hook-bearing scales. (From Zlatogursky, 2014).