Stentor polymorphus. Light microscope footage of several trumpet-like green stentor (Stentor polymorphus) ciliate protozoa. This species of freshwater protozoan feeds mainly on bacteria. Their green colour is caused by the presence of numerous individual Zoochlorella sp. algae, with which they have a symbiotic relationship. The algae photosynthesise inside the protozoan, providing it with sugars and oxygen, while deriving protection from their host. Filmed with differential interference contrast illumination.