Helical Swimming in a Freshwater Oligochaete

A novel pattern of undulatory swimming is described in the freshwater oligochaete, Dero digitata (Family Naididae). Movements are rhythmic (6-12 cycles/s), each cycle consisting of a single, helical body wave that passes from the worm's anterior to posterior end, thus propelling the worm forward. Successive cycles of these waves alternate between right-handed and left-handed helical orientations. Helical swimming in response to posterior tactile stimulation was commonly expressed in all stages of asexual reproduction, including non-fissioning "normal" worms, late prefission worms, and separated anterior or posterior zooids. Swimming also occurred in amputated anterior and posterior body fragments. Reynolds numbers ranged from approximately 50 (in the shortest and slowest worms) to 300 (in the longest and fastest worms). During slow swimming, wave velocities and forward velocities were 37 mm/s and 10 mm/s, respectively; during fast swimming these values were 55 mm/s and 25 mm/s, respectively. Thus, corresponding values of overall "slippage" were 73 and 55%, respectively. Central conduction of swim wave excitation likely involves intersegmental, non-giant fiber pathways that are (1) functionally coupled between anterior and posterior zooids of prefission worms and (2) readily activated by posterior mechanosensory inputs in whole worms and isolated zooids.