Morphallaxis in an aquatic oligochaete, Lumbriculus variegatus: reorganization of escape reflexes in regenerating body fragments.

We describe functional and anatomical correlates of the reorganization of giant nerve fiber-mediated escape reflexes in body fragments of an aquatic oligochaete, Lumbriculus variegatus, a species that reproduces asexually by fragmentation. Since fragments from any axial position always regenerate short heads (seven or eight segments long) and much longer tail sections, segments originating from posterior fragments become transposed along the longitudinal axis and acquire, by morphallaxis, features of escape reflex organization that conform to their new anterior position. Using noninvasive electrophysiological recordings we have quantified, on a day-to-day and a segment-by-segment basis, the reorganization that occurs in sensory field arrangements of the medial (MGF) and lateral (LGF) giant nerve fibers, as well as changes in giant fiber conduction velocity and morphometry. Our results show that (1) posterior fragments, originally subserved by the LGF sensory field gradually become subserved by the MGF sensory field; (2) appropriate increases in the ratio of MGF:LGF cross-sectional area, perimeter, and conduction velocity accompany the reorganization in giant fiber sensory fields; and (3) sensory field reorganization can be repeatedly reversed by additional amputations. These results demonstrate that the functional organization of escape reflexes is highly plastic and that morphallaxis may result from the counterbalance of morphogenic influences localized within the anterior and posterior ends of regenerating body fragments.