Species: 250-(396-576)-790 μm long, 25-(33-84) μm wide; cell body large, slightly curved; both termini tapered; pyrenoids 7-15 in each semicell (Illustrations of The Japanese Fresh-water Algae, 1977). Cell body slightly curved, gradually attenuated toward the truncated ends, which somewhat recurved due to the covexity of the inner margin near the ends; inner margin straight or slightly concave, outer margin convex, 300-500 μm long, 32-50 μm wide, L/W=9-14; cell wall smooth and colorless or sometimes delicately striated and yellowish-brown in color; chloroplasts with ridges containing 7-11 pyrenoids in central row (Photomicrographs of the Freshwater Algae, vol. 1, 1984).

100 μm 200 μm 300 μm; x 200

Closterium acerosum (Schrank) Ehrenberg var. acerosum, cell body 450 μm long, 48 μm wide, x 640, Maruyama Park, Ageo city, Saitama Pref., Japan, March 2002 by Y. Tsukii

Closterium acerosum (Schrank) Ehrenberg var. acerosum, cell body 400 μm long, 40 μm wide, x 640, Maruyama Park, Ageo city, Saitama Pref., Japan, March 2002 by Y. Tsukii

Closterium acerosum, cell body μm long, μm wide, x 640, Maruyama Park, Ageo city, Saitama Pref., Japan, March 2002 by Y. Tsukii

* C. acerosum (Schrank) Ehrenberg var. acerosum: 250-(396-576)-790 μm long, 25-(33-84) μm wide; cell body large, slightly curved; both termini tapered; pyrenoids 7-15 in each semicell (Illustrations of The Japanese Fresh-water Algae, 1977).

C. acerosum var. elongatum Brébisson: 650-(773-780)-915 μm long, (30-38)-40 μm wide; L/W = 15-33 (Illustrations of The Japanese Fresh-water Algae, 1977).

C. acerosum var. minus Hantzsch: 200-300 μm long, 18-25 μm wide; L/W = 7-11 (Illustrations of The Japanese Fresh-water Algae, 1977).

C. acerosum var. tumidum Borge: Slightly swell at center; 336-(384-494) μm long, 35-(42) μm wide; L/W = 8-12 (Illustrations of The Japanese Fresh-water Algae, 1977).