Species [var. setaceum]: Cell body quickly attenuated toward both ends (cf. C. ralfsii), each extended part longer than spindle-shaped part at middle (cf. C. kuetzingii), 150-(252-528)-600 μm long, 6-(8.6-17) μm wide, L/W=30-33; cell wall transparent or slightly yellowish, finely striated (Illustrations of The Japanese Fresh-water Algae, 1977).

Similar species -->> C. kuetzingii;

50 μm 100 μm 150 μm; x 400 : 31 μm 63 μm 94 μm; x 640

Closterium kuetzingii ?, x 400, x 640, August 7-13, 1999, Vermont-Maine U.S.A., by Y. Tsukii

Closterium setaceum Ehrenberg var. setaceum f. setaceum: Cell body small in size, elongate, L/W= 30-42, 252-528 μm long, 6-16 μm wide; both termini slightly curved, spindle-shaped at middle; cell wall transparent or brownish with thin lines (8-13 lines/10 μm wide); 2-3 pyrenoids in each half-cell (Photomicrographs of the Fresh-water Algae, vol. 6, 1997).

* Closterium kuetzingii Brébisson var. kuetzingii: Cell body moderate in size, elongate, L/W= 18-30, 270-512 μm long, 13-22 μm wide; both termini slightly curved, spindle-shaped at middle; cell wall transparent or brownish with thin lines (8-11 lines/10 μm wide); chloroplasts composed of 5 lamellae; 4-6 pyrenoids in each half-cell (Photomicrographs of the Fresh-water Algae, vol. 4, 1985).