Species [ var. tenuis Agardh ]: Trichome straight, bluegreen, slightly curved at terminal, 2.5-5 μm long, 4-10 μm wide, L/W=1/3 (0.3); septum slightly constricted, granules present along septum; apical cells semispherical, its wall sometimes thickened.
[ var. asiatica Wille ]: 4.5-6 μm long, 10-12 μm wide (Illustrations of The Japanese Fresh-water Algae, 1977).

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O. tenuis v. tenuis 4-6 x 10 μm L/W=0.4-0.6 Yukawa-numa Nishiwaga Iwate, 2007	O. tenuis var. tenuis 2.5-3.5 x 10 μm L/W=0.25-0.35 Saitama, 2000	O. tenuis var. tenuis 2.5 x 8.5 μm L/W=0.3 rice field Ichinoseki Iwate, 2005	O. tenuis var. tenuis 7 μm φ L/W=0.36 Japan, 199?
O. tenuis var. tenuis 3 x 7 μm φ Kitamoto n.o.p. Kitamoto Saitama, 2006
O. tenuis v. asiatica 5.5 x 14 μm L/W=0.4 Sakitama-kofun p. Gyoda Saitama, 2004	O. tenuis v. asiatica 5-10 x 12 μm L/W=0.4-0.8 near Mihashi Park Saitama Saitama, 2006	O. tenuis v. asiatica 3.5-7 x 12.5 μm L/W=0.3-0.6 near Iroha-numa Zao Mt. Yamagata, 2006	O. tenuis var. tergestina ? or O. hamelii ? 5.5 μm φ L/W=0.13-1.1 Japan, 1999

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