Head Removal Enhances Planarian Electrotaxis

Abstract

Sensing electric fields is an ability that certain animal species utilize for communication, hunting, and spatial orientation. Freshwater planarians move toward the cathode in a static electric field (cathodic electrotaxis). First described by Raymond Pearl more than a century ago, planarian electrotaxis has received little attention and the underlying mechanisms and evolutionary significance remain unknown. We developed an apparatus and scoring metrics for automated quantitative and mechanistic studies of planarian behavior upon exposure to a static electric field. Using this automated setup, we characterized electrotaxis in the planarian Dugesia japonica and found that this species responds to voltage instead of to current, in contrast to results from previous studies using other species. Because longer planarians exhibited more robust electrotaxis than shorter planarians, we hypothesized that signals from the head impede cathodic electrotaxis. To test this hypothesis, we took advantage of the regenerative abilities of planarians and compared electrotaxis in head and tail fragments of various lengths. We found that tail and trunk fragments electrotaxed while head fragments did not, regardless of size. However, we could restore cathodic electrotaxis in head fragments via decapitation, demonstrating that the presence of the head impaired cathodic electrotaxis. This result is in stark contrast to other stimulated behaviors such as phototaxis, thermotaxis or chemotaxis, which are weaker or absent in headless fragments. Thus, electrotaxis may be an important ability of headless planarian fragments to support survival prior to head regeneration. Summary statementWe present a new method for quantitative studies of planarian electrotaxis and show that Dugesia japonica move toward the cathode. This behavior is enhanced by removal of the head.