Dominance hierarchy-dependent behavioural plasticity of crayfish avoidance reactions.

Crayfish showed avoidance reactions when mechanical stimulation was applied to their tailfan. The response pattern of the avoidance reaction was dependent on crayfish size. Small crayfish showed an escape-like dart response while larger crayfish displayed a defensive-like turn response. We show that the response pattern to the same sensory stimulus changed with social status in the small crayfish. Crayfish are territorial animals and a dominant-subordinate relationship was determined when two previously unacquainted animals were paired. This winner-loser hierarchy was observed in pairs of small crayfish. Before fighting, all crayfish showed a dart response following mechanical stimulation of the tailfan. However, the probability of occurrence of a turn response increased significantly in the crayfish that won a fight. This behavioural transition from dart to turn response occurred immediately and was maintained for at least 3 days even if a dominant crayfish was isolated again with no opponent.