Family [ Sciadiaceae ]: Unicells; planktonic; cell body ellpsoidal or cylindrical in shape; both ends rounded or slightly swelled, with spiny projections in some species; 2 or more chloroplasts plate-like, without pyrenoids; many leucosin granules present (Illustrations of The Japanese Fresh-water Algae, 1977).
Genus: Unicellular; planktonic; cell body elongated cylindroid, curved or twisted; both ends rounded, with spiny projections; cell wall not separated into two parts (compare to Centritractus); several chloroplasts plate-like, without pyrenoids (Guide book to the "Photomicrographs of the Freshwater Algae", 1998).

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O. arbuscula 5 samples 10-80 x 4.5 μm 2001-2005	O. arbuscula 5 samples 10-80 x 4.5 μm 2001-2005	O. capitatum 6 samples 62-210 x 7-10 μm 2001-2008	O. bicuspidatum or O. captatum ? 3 samples 65-80 x 6-8 μm 2005-2006
O. gracillimum 77 x 7 μm Suga-daira Ueda Nagano, 2006	O. parvulum 12 samples x 5-9 μm 2002-2006	O. maius 90 x 5.5 μm Dojo-ike Norikura highland Nagano, 2006

Similar genus -->> Centritractus

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