ABSTRACT
Steering movements of the abdomen and the terminal appendages, the uropods, of the rock lobster Jasus lalandii have been examined during slow postural responses and during tail-flips using a combination of movement and force measurements and by monitoring muscle activity. Gentle mechanical stimulation on one side of the animal elicits a postural steering response of the uropods. The activities of the individual uropod muscles that underlie these movements have been determined.
Progressively intense mechanical stimuli initiate tail-flips, which may incorporate steering responses dependent upon the position of the stimulus. Symmetrical stimuli cause both uropods to open and close symmetrically and only a longitudinally directed force to be produced. However, stimuli delivered to only one side elicit asymmetrical movements of the uropods and rotation of the abdomen to the stimulated side during the preflexion phase of the tail-flip. The pattern of activity in the uropod muscles during this preflexion phase is the same as that in the postural steering response. During flexion itself, the uropods open to their full extent while the abdomen rotates further to the stimulated side. As a result of these movements, appropriate steering forces are produced to take the animal away from the point of stimulation. During re-extension, the uropods close and the abdomen re-extends.
These results are discussed in terms of the relationship between postural reactions and the preflexion phase of the tail-flip, the relative contribution of the different abdominal motor systems to tail-flip steering and the occurrence of steering in different forms of the tail-flip.
