Key to the Neotropical species of Spirotaenia

1. Chloroplast consisting of four spiraling ribbons ............................................................................................................. S tetrahelica

- Chloroplast consisting of a single ribbon ...........................................................................................................................................2

2. Cells narrowly fusiform with acute poles...........................................................................................................................................3

- Cells cylindric with broadly rounded poles........................................................................................................................................4

3. Cells 5–8 times longer than broad, two tiny reddish dots at each end .......................................................................... S. beijerinckii

- Cells 5 times longer than broad, without reddish dots at each end ....................................................................................... S. minuta

4. Cells narrow, 17–31 times longer than broad.................................................................................................................... S. filiformis

- Cells wide, 2–10 times longer than broad ..........................................................................................................................................5

5. Cells curved ..................................................................................................................................................................... S. closteridia

- Cells straight.......................................................................................................................................................................................6

6. Cells less than 30 μm long................................................................................................................................................ S. endospira

- Cells over than 30 μm long.................................................................................................................................................................7

7. Cells less than 80 μm long, chloroplast notably red at the ends.............................................................................. S. erythrocephala

- Cells over than 80 μm long, chloroplast without red cap at the ends............................................................................ S. condensata