REPORT ON A PARASITIC PROTOZOAN OBSERVED ON FISH IN THE AOUARIUM. 



BY CHARLES WARDELL STILES, PH. D., 

 Zoologist, Bureau of Animal Industry. 



INTRODUCTION. 



During the early part of the Columbian Exposition season a lot of young catfish 

 (Ameiurus albidus) were brought to the aquarium from the Potomac River and some 

 time after were discovered to be seriously infested with a protozoan parasite. The 

 general pressure of aquarium operations prevented any special study of the subject 

 until a spread of the parasitic disease to other species adjacent, notably to young 

 trout, compelled attention to it. Some preliminary studies made by Prof. S. A. 

 Forbes, director of the aquarium, resulted in a determination of the parasite as a 

 species of the genus Ichthyophtliirius of Fouchet, and some practical experiments with 

 solutions'of common salt, copperas, carbolic acid, and other materials, showed that 

 the parasite was easily destroyed in a free state by several of these substances. It 

 seemed necessary, however, that solutions should be found capable of destroying the 

 parasite while imbedded in the mucous layer of the shin without injury to the fish 

 infested — an undertaking which required a large amount of continuous work, sys- 

 tematically planned with reference to both scientific and practical ends. Prof. Forbes 

 consequently applied to Secretary Morton, of the U. S. Department of Agriculture, for 

 the temporary transfer of the author of this report from the Agricultural Department 

 to the U. S. Fish Commission, with a view to having such methodical experimental 

 work carried forward until definite results were reached. 



Arriving at the aquarium in July, I found that in some tanks scarcely a catfish 

 or a trout was free from the parasites. The latter were perfectly visible to the naked 

 eye, and were scattered over the entire external surface of the fish. They were numer- 

 ous on the gills and in the mouth, and in the case of the cattish they were also found 

 in the stomach, the latter specimens probably having been swallowed. The protozoa 

 were not entirely superficial, but were imbedded in the epithelial layer of the skin, 

 lying in small round cavities large enough to contain but one or two individuals, or in 

 elongated galleries or pustules (especially in the case of the cattish) containing num- 

 erous specimens of the ciliate. In most cases the fish were covered with a thick slime, 

 which extended also into the mouth and covered the gills more or less completely. 



The temperature of the water was high, running some days up to 74° F. (23 # 3° C), 

 and this factor should be taken into consideration in connection with the high mortality 

 of the infested fish. 



Fish infested with this parasite usually preferred the upper half of the tank. They 

 gasped very rapidly and seemed unable to eat. When they took a morsel of food 

 into their mouths, it was retained but a moment and then expelled. Gradually they 



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