Genus: Planktonic; colony of 2-8 cells surrounded by cell wall of their mother cell, but sometimes unicellular; cell body broad ellipsoidal, both ends slightly pointed and with a thick cell wall; 1-3 chloroplasts parietal plate-like, with a single pyrenoid (Guide book to the "Photomicrographs of the Freshwater Algae", 1998).
Species: Cell body ellipsoid to broad-ellipsoid, with a slightly projected, thickened cell wall at both ends, 12-18 μm long, 8-16 μm wide; 2, 4, or 8 autospores enclosed within a enlarged mother cellwall; chloroplasts 1-4, parietal laminate, each with a pyrenoid (Photomicrographs of the Freshwater Algae, vol. 18, 1997).

O. marssonii 16 x 11 μm Matto Ishikawa, 2002	O. marssonii or O. borgei ? 16 x 11.5 μm Ho-daisan-koen Yoshikawa Saitama, 2006	O. marssonii 16 x 9 μm Koga museum Koga Ibaraki, 2003	O. marssonii ? μm Fujimitashiro Katashina Gunma, 2007
O. marssonii 16 x 10 μm Ishinazaka Furukawa Miyagi, 2003	O. marssonii ? 15 x 13 μm Higusa-numa Hikari Chiba, 2003	O. marssonii 15 x 13 μm Kawashima or Kawajima Saitama, 2002	O. marssonii 14 x 10 μm Kasuga Fukuoka, 2001
O. marssonii 12 x 8 μm L/W=1.5 Tatebayashi Gunma, 2001	O. marssonii 12.5 x 8 μm L/W=1.6 Koshigaya Saitama, 2001	O. marssonii 12 x 7.5 μm Kejo-numa Furukawa Miyagi, 2003	O. marssonii 11 x 8 μm Showa-machi G.P. Showa Saitama, 2003
O. marssonii 13 x 7 μm L/W=1.9 Kamonoyachi numa Zao onsen Yamagata, 2005	O. marssonii μm Tatsumi-ike Nagano Nagano, 2008

Similar genus -->> Nephrocytium;

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