Long-term continuous light exposure alters phototaxis in the acoel flatworm Praesagittifera naikaiensis

Many animal behaviors are executed with appropriate form, direction, and intensity in accordance with an individual’s physiological state and its history of interactions with the environment. In contrast, taxis has traditionally been regarded as a reflexive and inflexible response, although several studies have reported plasticity. Here, we examined whether phototaxis in the acoel flatworm Praesagittifera naikaiensis undergoes experience-dependent modification. We conducted three experiments. First, we tested whether prior light exposure for 1 to 5 days attenuates phototaxis. A prolonged light exposure weakened phototactic attraction. This attenuation could reflect general debilitation caused by light exposure. We therefore assessed in Experiment 2 whether five days of light exposure reduces spontaneous locomotor activity. No such reduction was detected. In Experiment 3, we placed P. naikaiensis on a platform containing a continuously illuminated region and a shaded region for five days, and asked whether prolonged exposure would induce spontaneous light avoidance. The worms were attracted to light for approximately the first 24 hours, after which they tended to avoid light and moved into the shaded region. These results indicate that phototaxis in P. naikaiensis is robust but not purely mechanical. It is adaptively tuned to the individual’s history of light exposure.