1. The introduced North American crayfish, Pacifastacus leniusculus Dana, is expanding its range in Europe and locally often replaces two native crayfish species, Astacus astacus L. and Austropotamobius pallipes Lereboullet. Pacifastacus leniusculus is also expected to invade the habitat of a third native crayfish, the endangered Austropotamobius torrentium Schrank. Interspecific aggressive interactions and competition for shelter were experimentally studied in the laboratory to assess the potential impact of P. leniusculus on A. torrentium.
2. Neither species was inherently dominant in aggressive interactions, but dominance was strongly size-dependent, favouring the larger and faster growing species, P. leniusculus.
3. Access to limited shelter was generally determined by aggressive dominance, although species-specific preference also influenced the outcome of competition for shelter. Austropotamobius torrentium had a higher preference for experimental shelters and often defended these even against larger P. leniusculus.
4. In accordance with theoretical models of animal conflicts, agonistic interactions between equally sized contestants were more severe than between animals of different size.
5. The P. leniusculus used in the present experiment were infected with the crayfish plague, Aphanomyces astaci, to which the animals are resistant. The crayfish transmitted the disease to non-resistant A. torrentium which died ≈ 2 weeks after contact with P. leniusculus.