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Protozoa: Chilodonella salvelinus

3, Issue 2
March 2001
Updated November 2002

Phylum: Protozoa
Subphylum: Ciliophora
Class: Cyrtophora
Subclass: Phyllopharyngea
Order: Cyrtophorida
Family: Chiliodonelllidae
Genus: Chilodonella

Chilodonella salvelinus

Protozoa are ubiquitous; they are present in an active state in any aquatic or moist environment, and cysts are present everywhere in the biosphere ready to give rise to active populations. Protozoa are unicellular or colony eukaryotes. The phylum Protozoa contains many very different organisms. It is a grouping of convenience more than scientific likeness. As more is learned about individual protozoans, they are being regrouped.

This is a fascinating protozoan. In the Spring of 2001 this Chilodonella species was isolated from Lake trout fry Salvelinus namaycush fry, Splake fry Salvelinus hybrid, and Brook trout fry Salvelinus fontinalis at a hatchery in Augusta, Maine. The organism was living and reproducing on the gills and gill arches of these small fish. Although primarily a free-living freshwater organism, these Chilodonella were living on mucous and secretions from the fish. The warm water temperatures of the hatchery’s well (9°C) was the perfect conditions for this epizootic. There were thousands of organisms on each fish. They were so numerous that they were killing fish. Microscopic examination of the fish’s gills showed these tiny Chilodonella sweeping along the gills like the “scrubbing bubbles” animated television commercial for Tub & Tile Cleaner. A 40 second digital movie of the organism moving on the fish’s gills was made. It may be available on the web version of this document.

Chilodonella sp. have been reported as responsible for fish kills since 1902. They are opportunistic pathogens, taking advantage of otherwise compromised fish. This epizootic may have been triggered by overcrowding, environmental gill diseases, and/or microscopic mineral crystals injuring the fry’s gills. Reports of fish health professionals treating fish indicate that formalin is an effective treatment. In this epizootic, formalin was not effective at treating these Chilodonella.

The epizootic continued for about 6 weeks until the fish were treated with 100 ppm hydrogen peroxide for 30 minutes every other day for 3 treatments.

Note: This may be Chilodonella cyprinii or another closely related species. The organism responsible for this epizootic had 12 kinetics on the left and right side of the macronucleus. It also had a very small posterior indentation, a prominent oral opening.

Special points of interest:


Chilodonella are free-living organisms.
Chilodonella are easily stained with crystal violet and can be seen unstained with phase contrast microscopy.
For more information read: Thorp & Covich. 1991. Ecology and Classification of North American Freshwater Invertebrates.

Images were made possible by a grant from the Maine Outdoor Heritage Fund.