Choanocystis symna Zlatogursky, 2014

Diagnosis: Cell body ca. 6.7 µm in diameter. Axopodia three–five times longer than a cell diameter. Plate-scales dumbbell-shaped with a median constriction. Circular lines, parallel to the border of the scale sometimes can be seen. The shafts of spine-scales can be straight, but usually it is slightly curved towards the scale base. Length of plate-scales 4.4–5.0 µm (ca. 4.7 µm); width 1.62–1.90 µm (ca. 1.71 µm). Spine-scales with heart-shaped bases. Sometimes a small circular depression is located on the base at the place of the shaft location. Distal ends of spine-scales bear from three to five (usually four) short teeth. Length of spine-scales 3.9–6.7 µm (ca. 5.1 µm). Cells tightly attach to the bottom of Petri dish or, very rarely, float. No rolling movement was observed.

Ecology: Freshwater. Lake Symniakhovskoe, Valamo island, North-Western Russia, 61°22′912″N, 30°58′503″E.

Remarks: There are only two more described species of Choanocystis with dumbbell-shaped plate-scales and teeth-bearing spine-scales: C. pantopoda and C. aculeata. C. aculeata is a much larger heliozoan (cell diameter 23–60 µm) and has very different spine-scales which are notably tapering and have short protrusions (“nodules”) in proximal part of the shaft. This species also has from five to eleven teeth on the spine-scale tip (Dürrschmidt 1985, Siemensma and Roijackers 1988). C. pantopoda spine-scales are three times longer than plate-scales, while in C. symna only two times longer. The length of scales is 25–30 µm in C. pantopoda, while in C. symna it is only about 5 µm. The number of teeth in C. pantopoda is four to six (Siemensma 1991), but six teeth have never been observed in C. symna. (From Zlatogursky, 2014).