Reorganization of escape reflexes during asexual fission in an aquatic oligochaete, {{\textless}I{\textgreater}Dero} {digitata{\textless}/I{\textgreater}}

Asexual reproduction in the aquatic oligochaete Dero digitata involves 1) formation of a mid-body fission zone and transverse fission plane separating the worm into anterior and posterior zooids; 2) development of four new head segments in the posterior zooid, just behind the fission plane; and 3) development of new tail segments in the anterior zooid, just anterior to the fission plane. Electrophysiological recordings from normal (i.e., non-fissioning) and prefission worms indicate similar proportioning of giant nerve fiber sensory fields; the medial {(MGF)} and lateral {(LGF)} giant fiber fields encompass the anterior-most 30% and posterior-most 70%, respectively, of the worm’s length. Throughout prefission stages both {MGF} and {LGF} spikes were reliably through-conducted, though at reduced velocity, across the fission zone. This agrees with anatomical results showing continuity and reduced diameter of {MGF} and {LGF} in the fission zone. Within seconds after fissioning, sensory fields in the rostral segments of posterior zooids (= 18% of zooid length) switch from {LGF} to {MGF} sensitivity. Further changes in giant fiber sensory fields and conduction velocities in the posterior zooid, during the next 7 days, lead to a functional organization of escape reflex pathways comparable to that in normal worms. Our results suggest that an escape reflex switching mechanism, along with strong modulatory influences, ensures appropriate and adaptively significant escape reflex functioning in posterior zooids both before and after fissioning.