Lobster righting responses and their neural control

Abstract

This paper reports a behavioural analysis of the movements and righting responses of the abdominal pleopods (swimmerets) and uropods (tail fans) of the lobster Homarus americanus. The movements of individual swimmerets, studied from high-speed motion pictures, consist of a basic power stroke-return stroke cycle, upon which other cyclic power and control movements are superimposed. Tilting a lobster around the long axis of its body initiates bilaterally asymmetrical swimmeret beating. The swimmerets on the side tilted upward beat out toward the side, while those on the side tilted downward beat either straight to the rear, or not at all. These bilaterally asymmetrical swimmeret movements produce a cyclic righting torque around the long axis of the lobster's body. The peak-to-peak amplitude of this torque is directly proportional to the frequency of swimmeret beating. The characteristics of the righting torque were related to the movements of individual swimmerets. Tilting a lobster around its long axis also evokes bilaterally asymmetrical uropod movements. The uropod on the side tilted downward is spread open, while the uropod on the side tilted upward is closed. It is argued that these movements also contribute to the production of righting torque. The righting responses of both the swimmerets and the uropods are controlled exclusively by the statocyst receptors. The position and acceleration receptors of the statocysts probably both participate in the control of the responses. Destroying one statocyst shifts the position of symmetry for the swimmerets and uropods from upright to 20 degrees toward the injured side. Either the right or left statocyst can alone control the righting responses of the uropod and swimmerets of either side, however, even though the afferent responses of the two statocysts to roll in one direction are opposite. Neural models based on the bilaterally reciprocal organization of statocyst influences are proposed to explain these findings.