The host-parasite relationship between freshwater crayfish and the crayfish disease fungus Aphanomyces astaci: responses to infection by a susceptible and a resistant species.

SUMMARY: In studies of the responses to infection by the crayfish disease fungus Aphanomyces astaci of the susceptible European crayfish Astacus astacus and the resistant Western American species Pacifastacus leniusculus, the latter was consistently more resistant to infection by injection or via aquarium water; the difference in resistance was more striking in the second method. The American crayfish was only slightly less sensitive than the European crayfish to the toxic action of large numbers of dead spores of A. astaci and to infection by living spores of another phycomycete, Entomophthora apiculata, which is not a natural parasite of crayfish. A. astacus acquired a significantly higher degree of resistance to infection by injection or via the ambient water as a result of two previous exposures to sublethal numbers of spores of the pathogenic fungus. The two strains of Aphanomyces astaci tested behaved identically within the cuticular layer of the exoskeleton of both crayfish species, showing strain-dependent degrees of ability to penetrate and grow within the cuticle. The epicuticle offered a major barrier to penetration in both hosts. The fungus was unable to establish growth within the musculature and developed only sparsely along the ventral nerve cord of living animals of both species; fungal growth was luxurious shortly before and after the death of both crayfish. Aggregation of injected spores and their encirclement by haemocytes occurred rapidly in living Astacus astacus as well as in Pacifastacus leniusculus, but melanization of the spore aggregates and hyphal elements was considerably more pronounced in the resistant crayfish, P. leniusculus. The observations suggest the participation of at least some active processes of native and acquired resistance of crayfish to the crayfish disease fungus, particularly in the internal tissues and the epicuticle.