The tail flip of the Norway lobster, Nephrops norvegicus

Summary

• 1. In the Norway lobster, Nephrops norvegicus dynamic righting reactions are produced by tail flips which are executed from tilted body positions. A study of their properties has been made in tethered animals using a two dimensional strain gauge system (Fig. 1).

• 2. Righting forces are produced in both the initial, giant fibre mediated flip which is elicited by abrupt mechanical or electrical stimulation (Fig. 2), and in subsequent swimming flips (Fig. 4). The lateral forces produced are proportional to the degree of tilt (Figs. 3, 4).

• 3. By inactivating the thoraco-abdominal joint and the uropods in various combinations it has been established that both rotation of the abdomen and movements of the uropods make major contributions to the righting forces (Figs. 5, 6).

• 4. Cinematographic and myographic analyses of the uropod movements in tilted tail flips identify flexion of the exopodite blade, produced by the action of the productor and exopodite flexor muscles, as the asymmetric movement which induces righting torque (Figs. 7, 8). However, the uropods remain fully opened throughout the tail flip flexion.

• 5. Removal of the statocysts abolishes dynamic righting reactions in the tail flips (Fig. 9).

• 6. These results are discussed in terms of a prebiassing of the tail flip motor circuitry by the statocyst signal, and in relation to other possible sources of sensory modulation of this ‘stereotyped’ behaviour.