Anatomical and physiological organization of the swimmeret system of the spiny lobsterJasus lalandii as adaptive components of the tail flick

Summary

• 1. The swimmerets ofJasus lalandii, in contrast to those well known in the nephropid lobsters (e.g.Homarus) and astacurans (crayfish), do not display spontaneous antero-posterior beating, but are either apposed actively to the ventral surface of the abdomen, or rotated outward (Fig. 2). These movements are imposed by the geometrical arrangement of the bicondylar joints at the base of the swimmeret (Fig. 3), and involve contraction of either the remotor muscle, or the promotor-rotator muscles (Figs. 2, 3). Each swimmeret includes a short, thick blade-like exopodite that contains two antagonistic muscles, a large curler and a small adductor muscle (Fig. 3). Each swimmeret is innervated by 80 motor neurons (MNs) which are disposed in two clusters in the ganglion.

• 2. The modulation of the tonic discharge of the muscles which maintain the swimmeret position at rest (remotor and curler) has been studied in two situations: body rolling (Fig. 4) and walking activity (Fig. 5). In the female, in which the most anterior pair of swimmerets are biramous, both endopodite and exopodite curler muscles display the same responses to body rolling (Fig. 4). In all these situations no overt swimmeret movement occurs.

• 3. Nevertheless, rhythmicity exists inJasus, but it is limited to the gravid female when the swimmerets bear the eggs (Fig. 6). In contrast to other decapod Crustacea, this swimmeret beating is not metachronous (Fig. 6).

• 4. Movement monitoring (Fig. 7) and EMG recordings (Figs. 9, 10) have demonstrated the involvement of the swimmerets in the three phases of the tail flick response (preparation, flexion, extension). During the preparatory phase, in response to mechanical stimulation of the legs, the swimmerets open on the stimulated side (on both sides in the case of a symmetrical stimulation) (Fig. 7). During the rapid abdominal flexion of the tail flick all swimmerets open fully regardless of the stimulus (Figs. 7, 8). Two different units in the rotator muscle EMG are responsible for swimmeret opening during the preparatory and the flexion phases of the tail flick (Figs. 9, 10).

• 5. The curler muscle of the endopodite in the female displays antagonistic activities to that of the exopodite during tail flicks (Fig. 10).

• 6. Selective swimmeret blockage demonstrates that they contribute to the thrust efficacy in tail flicks. In particular they are responsible for the variation of the maximal force produced at its onset. This effect could be interpreted as a consequence of force redistribution by the swimmerets acting on water flow (produced by the tail fan). This mechanism implies a functional role for the swimmerets in righting and steering responses (Fig. 11).