Abstract

Many crayfish species have been introduced to novel habitats worldwide, often threatening
extinction of native species. Here we investigate competitive interactions and parasite infections in the
native Austropotamobius pallipes and the invasive Pacifastacus leniusculus from single and mixed species
populations in theUK. We found A. pallipes individuals to be significantly smaller in mixed compared to single
species populations; conversely P. leniusculus individuals were larger in mixed than in single species
populations. Our data provide no support for reproductive interference as a mechanism of competitive
displacement and instead suggest competitive exclusion of A. pallipes from refuges by P. leniusculus leading to
differential predation. We screened 52 P. leniusculus and 12 A. pallipes for microsporidian infection using
PCR. We present the first molecular confirmation of Thelohania contejeani in the native A. pallipes; in
addition, we provide the first evidence for T. contejeani in the invasive P. leniusculus. Three novel parasite
sequenceswere also isolated fromP. leniusculus with an overall prevalence of microsporidian infection of 38%
within this species; we discuss the identity of and the similarity between these three novel sequences. We also screened a subset of fifteen P. leniusculus and three A. pallipes for Aphanomyces astaci, the causative agent
of crayfish plague and for the protistan crayfish parasite Psorospermium haeckeli. We found no evidence for
infection by either agent in any of the crayfish screened. The high prevalence of microsporidian parasites and occurrence of shared T. contejeani infection lead us to propose that future studies should consider the impact of
these parasites on native and invasive host fitness and their potential effects upon the dynamics of native-invader
systems.