Behavioral transition of crayfish avoidance reaction in response to uropod stimulation.

Unilateral mechanical stimulation of a uropod elicited an avoidance reaction in the crayfish Procambarus clarkii. Depending upon the animal's size, either one of two alternative behavioral acts emerged as the avoidance reaction to similar stimuli. A stationary resting posture of the crayfish and a passive extension of the abdomen were prerequisite conditions for inducing the avoidance reaction. In small animals (less than 10 cm in body length), the "dart" response was mainly elicited. Animals responded with a sudden closing of both uropods and rapid forward walking to scuttle away from the stimulus source. This act was regarded as one form of escape behavior. With increasing animal size, the "turn" response became more probable than the "dart" response. During the "turn" response, the uropod on the stimulated side opened and that on the contralateral side closed. This act was frequently followed by a rapid turning movement towards the stimulus source with an extension or flexion of the abdomen. This act was also regarded as a variant form of defensive behavior. In large animals with autotomized chelipeds, the response to the uropod stimulation reverted to that of the "dart" response after 2 to 3 days.