Spirogyra is a filamentous green algae (Chlorophyta) which is common in freshwater habitats. Macroscopically this alga can be seen in lakes or ponds of freshwater as slimy masses of filamentous, floating as scum on the surface. From a distance these slimy tangles look perhaps a bit dirty, but under the microscope the filaments are very beautiful and moreover, they have a spectacular way of reproducing .
Spirogyra lacks a motile variant at all stages of its life history (no motile gametes, no zoospores). Sexual reproduction is by a process called conjugation as shown in the bottom photo [explanation].
Spirogyra owes its name to the helical or spiral arrangement of the chloroplasts, a unique property of this genus which makes it easily to recognize, although there are over 400 species in the world whose identification requires to look for reproducing specimens with spores.
Protoctista - Chlorophyta - Chlorophyceae - Zygnematales - Zygnemataceae - Spirogyra