The tail flip of the Norway lobster, Nephrops norvegicus

Summary

• 1. In response to abrupt mechanical stimuli delivered to the rostrum or telson, the Norway lobster, Nephrops norvegicus, performs short-latency tail flips (Fig. 2) which are preceded by giant fibre activity in the abdominal nerve cord (Fig. 1).

• 2. High speed cinematography of freely moving animals demonstrates that two distinct categories of giant fibre mediated tail flip are elicited by stimuli to the rostrum or telson (Fig. 3), and that these have flat or elevated rearward trajectories, respectively (Fig. 4). By analogy with the crayfish these have been identified as MG and LG flips. A third, more variable form of swimming tail flip follows these initial giant fibre flips.

• 3. Analysis of tail flip flexion in terms of the velocities of abdominal segments relative to the centre of mass (Fig. 5) and of intersegmental angles (Fig. 6) reveals gradients in the order of flexion according to the site of stimulation: caudo-rostrally for the MG flip and rostrocaudally for the LG flip.

• 4. The timings of flexion movements have been studied in tethered animals using a combination of movement monitors, a force transducer, and electromyographical recordings (Figs. 7–10). These confirm that differences exist in both the form of abdominal flexion and the order of activation of the segments in MG and LG flips (Figs. 7, 8). They also demonstrate that, in contrast to the crayfish, all segments flex during the LG flip (Fig. 9). These features are also expressed when the tail flip is induced by direct electrical stimulation of the giant fibres (Fig. 10).

• 5. The differences observed between Nephrops and crayfish in the tail flip trajectories and the contribution of caudal flexion to the LG flip are discussed in terms of the underlying neuronal circuitry.