S. acuminatus (Lagerheim) Chodat var. acuminatus: Colonies of 4 or 8 cells; in the 4-cell type, cells attached at center; in the 8-cell type, cells attached in a zigzag pattern; cell body long spindle in shape; cells at both end curved; cell wall smooth; 12-30 μm long, 2.5-6 μm diam. (Illustrations of The Japanese Fresh-water Algae, 1977).
S. acuminatus f. maximus Uherkovich: Larger than var. acuminatus; 28-50 μm long, 7-9 μm diam. (Illustrations of The Japanese Fresh-water Algae, 1977).
S. acuminatus f. tortuosus (skuja) Uherkovich: Cells at both end strongly curved; 20-45 μm long, 3-7 μm diam. (Illustrations of The Japanese Fresh-water Algae, 1977).
S. acuminatus var. elongatus G. M. Smith: Only two cells attached at center; both cells curved outside; 40-50 μm long, 2-5 μm diam. (Illustrations of The Japanese Fresh-water Algae, 1977).