Jointed setae – their role in locomotion and gait transitions in polychaete worms

Abstract

Many families of polychaete worms have jointed setae in which the joint is external to the body and is not directly controlled by muscles or nerves. We assessed the role of these specialized structures in the hesionid polychaete, Ophiodromus pugettensis, by examining speed, step length, stride distance, stride frequency and gait transitions in worms with and without setal joints. Individual worms were videotaped while they moved over sandy surfaces at a range of speeds. The worms were then anaesthetized and all their compound setae were trimmed either distally or proximally to the setal joints. After two days of recovery the worms were videotaped a second time while they again moved over sandy surfaces at a range of speeds. From the video tapes we analyzed their locomotory performance before and after setal ablation. Animals in which the setae were shortened but in which the joint was left intact showed no consistent change in speed, step length, stride distance, stride frequency or gait transitions. Animals in which the joint had been removed both changed gaits at slower speeds (walking to undulatory walking and undulatory walking to swimming) and showed a significant decrease in maximum swimming speeds and stride distance. A subset of data containing only cases where the worms were moving at the same speed in the same gait before and after setal ablation was analyzed. In these instances, after the removal of the joint, the worms had significantly smaller stride distances and compensated for this by increasing stride frequency. In O. pugettensis, the undulatory walking gait is analogous to the trot–gallop transition in quadrupedal mammals because the animal switches from moving the appendages on a relatively rigid body to using a combination of body flexion and appendage movement to achieve propulsion, however, unlike quadrupedal mammals this transition takes place over a wide range of speeds and at different sites on the body as speed increases. These experiments indicate that jointed setae may be important both in allowing a worm to better control setal contact and traction with the substrate as well as in altering the effectiveness of its swimming stroke.