Cothurnia is a genus of freshwater and marine peritrichs in the family Vaginicolidae. It is characterised by living in a transparent tubular lorica. During the feeding or vegetative phase of its life cycle, Cothurnia attaches to submerged surfaces through a short stalk — mostly on the surfaces of fishes, crustaceans and aquatic plants. It is commonly studied for its epibiotic relationship with the host that it is attached to.

The etymology of the genus name Cothurnia derives from the ancient greek word κόθορνος (kóthornos), and from the Latin word cothurnus, meaning "buskin, or high boot".

Cothurnia has been noted for its correlation with water quality (especially in water treatment plants). It has been observed a decrease in the prevalence of Cothurnia on prawns as the water quality deteriorates, making it a good indicator of the quality of water in the environment.

Cothurnia is often confused with Vaginicola due to their similar morphologies.

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