Stentor amethystinus is a trumpet-shaped ciliate with a violet to purple appearance. The ciliate is almost 1 mm long and common in lake and pond plankton. Inside the ciliate unicellular algae (the globular green cells) live in symbiosis with S. amethystinus. Stentor is the Unicellular Eukaryote of the Year 2014. This population was sampled by Samuel Vorhauser from Puntleidersee in South-Tyrol (Italy) and photographed by Barbara Kammerlander.
Due to so-called cortical granules S. amethystinus appears violet to purple.
Algal symbionts live in symbiosis with S. amethystinus.