Abstract

Populations of the crayfish Austropotamobius pallipes (Lereb.) were examined in 1979 and 1980 for evidence of parasitic disease. Only 0.7% of the White Lake crayfish and 1.2% of Brittas River specimens had porcelain disease caused by the microsporidian Thelohania, a parasite also noted in three other major rivers systems. Microscopic examination revealed that a 1.3% rate of infection went undiscovered in macroscopic screening. The disease was only seen in crayfish over three years old, and it is suggested that feeding behaviour of different life stages may be responsible for the restriction of the disease to larger forms. Diseased crayfish may live in deeper, less favourable lake habitats. Methods of capture of crayfish may also affect the occurrence of infected crayfish in a sample. 'Burn-spot' fungal disease was noted on 3% of lake crayfish, but was not seen elsewhere. Smaller lesions, chiefly on the claws of adult males, were probably the scars of agonistic contact. No evidence was found for fungal plague (Aphanomyces) in Ireland.

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