Crayfish plague agent detected in populations of the invasive North American crayfish Orconectes immunis (Hagen, 1870) in the Rhine River, Germany

Crayfish plague, caused by the parasitic oomycete Aphanomyces astaci, has driven indigenous European crayfish species to regional extinction in many parts of Europe and is among the leading threats to the remaining populations. A. astaci is known to be carried by long-established invasive crayfish species of North American origin, which are also the main vectors of the plague pathogen. In this study, we examined whether a new invasive crayfish of North American origin, the calico crayfish (Orconectes immunis), also carries A. astaci. Orconectes immunis is a recent invader of the Upper Rhine plain, where it seems to displace its invasive predecessor Orconectes limosus, which is a known carrier of the agent of the crayfish plague. Using real-time PCR, we identified the calico crayfish as the fourth invasive crayfish species to be a carrier of the crayfish plague pathogen in Europe and we confirmed the infection with A. astaci in O. limosus. These findings support the concern that all North American crayfish species in European waters are carriers of the crayfish plague pathogen. Such knowledge should prove useful for conservation efforts, management, legislation, and public education about the spread of crayfish plague and non-indigenous crayfish species.