On the Adaptation of Aphanomyces astaci as a Parasite

The crayfish plague parasite Aphanomyces astaci was compared with some other species of Aphanomyces in several physiological respects to detect signs of specialization in the plague fungus which could be connected with its life as a parasite. Thus, A. astaci showed good growth in crayfish serum in contrast to the other species. Zoospore germination showed a similar hut not so distinct pattern. Chemo-taxis in zoospores was weak and restricted in its substrate range in A. astaci but was stronger and less restricted in the saprophytic species. Electrotaxis was quite apparent in A. astaci. Virulence in A. astacin was correlated to linear growth rate of individual hyphae in crayfish serum but neither to spore germination percentage in serum of resistant or susceptible crayfishes nor to motility of the zoospores. The saprophytic Aphanomyces strains did not penetrate the soft cuticle of crayfish in vitro but A. astaci strains did, regardless of virulence. Zoospore production in A. astaci was restricted to lower concentrations of mineral salts Mum in the other fungi. No sexual or other resting structures were found in infected crayfish but thick walled as well as gemmae-like hyphal portions were formed in some media in vitro. Infection of crayfish can take place between 2 and 25°C and zoospores are able in survive at least a week in an aquarium of redistilled water at 14°C and still infect the animals, but in highly bacterium-contaminated water they do not cause infection. The results were discussed with respect to the origin of the “abnormal” parasite. Together with those of earlier experiments they suggest Unit A. astaci is physiologically adapted as a parasite on fresh water arthropods. This investigation was supported by a grant from the Swedish Natural Science Research Council and by funds paid to the Swedish state by the Swedish water power industry. The author is deeply indebted to the head of this institute Professor Nils Fries, for his encouragement. advice, and support, let Mr Andrew Watson for help in carrying out the electrotaxis experiment, to Docent Karin Aschan-Aberg for criticizing the manuscript, to Mrs. Linda Fryktund for revising the English text, and to Mr. Can Wihma for skilful technical assistance.