JOINTED SETAE - THEIR ROLE IN LOCOMOTION AND GAIT TRANSITIONS IN POLYCHAETE WORMS

Many families of polychaete worms have jointed setae in which the join t is external to the body and is not directly controlled by muscles or nerves. We assessed the role of these specialized structures in the h esionid polychaete, Ophiodromus pugettensis, by examining speed, step length, stride distance, stride frequency and gait transitions in worm s with and without setal joints. Individual worms were videotaped whil e they moved over sandy surfaces at a range of speeds. The worms were then anaesthetized and all their compound setae were trimmed either di stally or proximally to the setal joints. After two days of recovery t he worms were videotaped a second time while they again moved over san dy surfaces at a range of speeds. From the video tapes we analyzed the ir 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, st ride frequency or gait transitions. Animals in which the joint had bee n removed both changed gaits at slower speeds (walking to undulatory w alking and undulatory walking to swimming) and showed a significant de crease in maximum swimming speeds and stride distance. A subset of dat a containing only cases where the worms were moving at the same speed in the same gait before and after setal ablation was analyzed. In thes e instances, after the removal of the joint, the worms had significant ly smaller stride distances and compensated for this by increasing str ide frequency. In O. pugettensis, the undulatory walking gait is analo gous 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 achie ve propulsion, however, unlike quadrupedal mammals this transition tak es place over a wide range of speeds and at different sites on the bod y as speed increases. These experiments indicate that jointed setae ma y be important both in allowing a worm to better control setal contact and traction with the substrate as well as in altering the effectiven ess of its swimming stroke. (C) 1998 Elsevier Science B.V. All rights reserved.