Aggressive interactions and competition for shelter between a recently introduced and an established invasive crayfish: Orconectes immunis vs. O. limosus.

Orconectes immunis, a crayfish species from North America, has been recorded first from habitats along the Rhine river (Germany) in the 1990 th . Coincidental with the arrival of O. immunis resident populations of O. limosus, also a non-native crayfish species from North America, declined. We studied in laboratory experiments aggressive interactions and competition for shelter between the two invasive crayfish species as these interactions can result in species replacements. Agonistic behaviour was monitored in heterospecific 1: 1 combinations of both species in 20-L aquaria. Orconectes immunis was strongly aggressively dominant over O. limosus when size-matched form I males or females were combined. Even 4 mm smaller O. immunis (carapace length) were still dominant over larger O. limosus and males of O. limosus were not dominant over similar-sized females of O. immunis. Orconectes immunis was also highly superior in competition for shelter. Shelter occupancy was approximately 6 times higher in O. immunis than in O. limosus. We conclude that one of the reasons for the observed decline of O. limosus coincidental with the arrival of O. immunis in some stretches of the Rhine catchment may be the inferiority of the former in aggressive contests. Inferiority in aggressive interactions may force O. limosus to leave refuges, making them vulnerable to predators.