Backward scrunching: planarians escape from noxious stimuli, cannibalism, and predation.

Animals have evolved escape behaviors to survive. Freshwater planarians have many predators, including conspecifics in cannibalistic species such as Dugesia japonica. When agitated, planarians escape via a locomotory behavior called "scrunching", whereby the planarian periodically elongates and contracts its whole body. Current understanding is that planarians move forward when they scrunch. However, when transversely cutting D. japonica, we observed that while head pieces scrunched forward, tail pieces scrunched backward instead. Previous work has shown that forward scrunching is a conserved planarian escape behavior in response to transection and adverse stimuli such as low pH, noxious heat, hydrogen peroxide, and allyl isothiocyanate. Testing these stimuli, we found that backward scrunching is more difficult to induce than forward scrunching in both intact planarians and pieces. Using electrostimulation, we show that behavior progresses with increasing voltage from turning to backward scrunching to flailing when stimulated anteriorly, demonstrating that backward scrunching is a response to moderately strong stimuli. Moreover, we show that tail pieces - originating from transection or asexual reproduction (fission) - exhibit backward scrunching when attacked by intact planarians. Backward scrunching and backward steps of tail pieces were also observed when worms were bitten into pieces by dragonfly or damselfly nymphs, natural predators of planarians. The survival of tail pieces after fission is necessary for the expansion of asexual planarian populations. Thus, we propose that backward scrunching serves as an effective escape behavior to protect planarian tails from being eaten.