Ciliates under the microscope

What kind of microscopic external, unicellular parasites can you find on freshwater fish after a skin scraping and what do they look like under a microscope? In this video we will show you ciliates. In other videos we will look at flagellates, bacteria, spore-forming unicellular parasites, crustaceans, skin flukes and gill flukes.

Trichodina & Trichodinella

Trichodina and Trichodinella belong to the family of Trichodinidae. This family includes over 30 genera and 400 species of similar looking single-celled parasites of fish and amphibians. Under the microscope Trichodina have a disc-shaped cell structure with a row of cilia and internal spicules (denticles), forming the endoskeleton of the cell. Infected fish look stressed, swim erratically or hide in corners and shelters, the body color fades and a whitish slime is visible when they turn and often the fins are pressed against their body.


Chilodonella is one of the three described genera in the family of Chilodonellidae, which includes about 50 species. Chilodonella looks like a baseball cap or a heart. Chilodonella causes loss of color and appetite, accumulation of slime on the body and gills, and irritation of the skin in fish.


The genus Tetrahymena belongs to the family Tetrahymenidae, which includes around 70 species. Tetrahymena looks like a drop of water with a pointy nose and many internal organelles can be seen. Tetrahymena causes changes in fish behavior, for example lethargic swimming, stress and also changes in body color. A lot of times you see big white patches and lifted scales on infected fish.

Ichthyophthirius multifiliis

Ichthyophthirius multifiliis also known as Ich (Ick) or white spot is the only representative of the family Ichthyophthiriidae. Young Ichthyophthirius are small compared to the adults. Adults have a round flexible body shape, and sometimes a large half-moon shaped nucleus is visible. Cells of different sizes and stages of maturity can usually be observed in scrapings. The main symptom of an Ichthyophthirius infection are the white spots on the fish’s skin, that look like small grains. Fish also experience rapid breathing, loss of appetite, lethargy, and start hovering near the surface or bottom of the aquarium.


Epistylis sp. is a genus of colonial ciliates that can infect the skin of fish. Epistylis has an elongated shape with a crown of cilia on top and an attachment foot below. Usually, fish become infected with Epistylis when kept in overcrowded aquariums with poor water quality. They can also appear when skin injuries are present. Epistylis colonies turn up on the fish’s body as white spots of different sizes or as a cluster of mucus coming off the body.

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