Positional Orientation Determined By the Behavioural Context in Procambarus Clarkii Girard (Decapoda: Macrura)

Abstract

1. The variability in the occurrence probability of the uropod steering response, one of the equilibrium responses, has been analysed quantitatively by a combination of electromyogram techniques and visual observation in the crayfish Procambarus clarkii Girard. 2. Presence of the tilt stimulus alone, whether it might be the body tilt or the substratum tilt, did not necessarily lead to the emergence of uropod steering (occurrence probability = 0.08 and 0.19 in the free condition and in the hanging (fixed) condition respectively). 3. Whenever uropod steering occurred, its direction and magnitude were strictly determined by those of the tilt stimulus. 4. During the abdominal posture movement, the occurrence probability of steering in response to the tilt stimulus was significantly increased (occurrence probability = 0.24 and 0.49). 5. During forward walking in the free-animal observation, the occurrence probability of steering was also significantly higher than during the stance period (occurrence probability = 0.45). 6. During backward walking without the abdominal movement, the occurrence probability of steering showed no significant change. It was increased significantly (0.21), however, when the backward walking was occurring together with the abdominal movement. 7. Turning movements involved the characteristic uropod configuration. The steering response was overridden by turning. 8. The defence reflex also suppressed the steering response. 9. Forward and backward walking, turning, and the defence reflex showed significantly high probabilities of association with the abdominal movement. 10. The uropod steering was preceded by the abdominal movement or the forward walking with a significantly high probability. 11. It was concluded that the association of the uropod steering with the abdominal posture movement or the forward walking was causal: the operation of the abdominal posture system or the forward walking system is the necessary condition for the uropod steering to be released by the tilt stimulus. The uropod steering response thus provides a typical example of postural facilitation. 12. The results of this study clarified the functional organization of the locomotion-related acts including the equilibrium responses in crayfish, showing how they interact with each other to result in a particular behavioural output. The biological significance of such an organization is discussed.