Species Displacement Between an Introduced and A ‘vulnerable’ Crayfish: The Role of Aggressive Interactions and Shelter Competition

Abstract

In eastern Hokkaido, Japan, the native crayfish Cambaroides japonicus de Haan has been declining rapidly while the exotic crayfish Pacifastacus leniusculus Dana has been expanding its range. We speculated that the observed displacement of Cambaroides by Pacifastacus is partly due to an interference interaction for shelter between the two species. We studied the impacts of Pacifastacus on Cambaroides by testing experimentally whether intensity of interference, the outcome of dominance order and shelter occupancy differed between single- and mixed-species combinations of the two species. Pacifastacus showed less defensive behaviours towards Cambaroides than to conspecifics. In contrast, Cambaroides exhibited more defensive behaviours towards Pacifastacus than to conspecifics. Pairwise comparison of agonistic behaviours in mixed species groups revealed that Pacifastacus exhibiting frequently more aggressive attacks while Camaboides showing more defensive behaviours to their respective counterparts. Dominance in aggressive encounter generally dictated shelter occupancy in mixed-species and Pacifastacus pairs but not in Cambaroides pairs. Because shelter utilization, when these are not limited in supply, was much more frequent in Cambaroides than Pacifastacus, Cambaroides suffered interference from both conspecifics and heterospecifics when they competed for a single shelter. Pacifastacus, however, did not show any change in shelter occupancy in the presence or absence of conspecics or heterospecifics. Inferiority in aggressive interactions and shelter occupancy can therefore be a critical disadvantage for Cambaroides if shelters are limited in natural situations.