Aggression and competition for shelter between a native and an introduced crayfish in Europe

1. The introduced North American crayfish, Pacifastacus leniusculus Dana, i s expanding its range in Europe and locally often replaces two native crayf ish 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. Inters pecific aggressive interactions and competition for shelter were experiment ally studied in the laboratory to assess the potential impact of P. leniusc ulus on A. torrentium. 2. Neither species was inherently dominant in aggressive interactions, but dominance was strongly size-dependent, favouring P. leniusculus. 3. Access to limited shelter was generally determined by aggressive dominan ce, although species-specific preference also influenced the outcome of com petition for shelter. Austropotamobius torrentium had a higher preference f or experimental shelters and often defended these even against larger P. le niusculus. 4. In accordance with theoretical models of animal conflicts, agonistic int eractions between equally sized contestants were more severe than between a nimals 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. T he crayfish transmitted the disease to non-resistant A. torrentium which di ed approximate to 2 weeks after contact with P. leniusculus.