Agonistic behaviours and movement in the signal crayfish, Pacifastacus leniusculus: can dominance interactions influence crayfish size-class distributions in streams?

Size-structured population distributions can result from stage-specific differences in habitat preference or from intraspecific dominance interactions between large and small individuals. The relative importance of these two factors in structuring population distributions was examined in the signal crayfish (Pacifastacus leniusculus (Dana, 1852)), an organism whose populations are distinctly spatially size-sorted in streams, with adults (A) occupying deeper pools and juveniles (J) inhabiting riffles. In the first of two experiments, groups of crayfish (2A, 2A+2J, or 4J) were released in a constructed riffle–pool–riffle sequence in an experimental channel and individual behaviours and rates of movement were recorded at 2 min intervals. There were differences in habitat preference between the two size classes: adults showed a distinct preference for pools, whereas juveniles did not. In the second experiment, pairs of individuals (A+A, A+J, J+J) were placed in experimental arenas. Analysis of recorded intera...