The crayfish plague pathogen (Aphanomyces astaci) causes mass mortalities of European crayfish when transmitted from its original North American crayfish hosts. Little is known, however, about interspecific transmission of the pathogen between different American crayfish species, although evidence from trade of ornamental crayfish suggests this may happen in captivity. We screened signal and virile crayfish for A. astaci at allopatric and sympatric sites in a UK river. Whilst the pathogen was detected in signal crayfish from both sites, infected virile crayfish were only found in sympatry. Genotyping of A. astaci from virile crayfish suggested the presence of a strain related to one infecting British signal crayfish. We conclude that virile crayfish likely contracted A. astaci interspecifically from infected signal crayfish. Interspecific transmission of A. astaci strains differing in virulence between American carrier species may influence the spread of this pathogen in open waters with potential exacerbated effects on native European crayfish.

Centre for Environment Fisheries and Aquaculture Science The Nothe Weymouth Dorset DT4 8UB United Kingdom

Department of Ecology Faculty of Science Charles University Prague Viničná 7 CZ 12844 Prague 2 Czech Republic

School of Biosciences Cardiff University Cardiff CF10 3AX United Kingdom

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Prevalence and diversity of Aphanomyces astaci in cambarid crayfish of Pennsylvania: where native and introduced hosts meet