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

Long-term continuous light exposure alters phototaxis in the acoel flatworm Praesagittifera naikaiensis | Research Square window.SnipcartSettings = { analytics: { enabled: false } }; (function() { var accessVector = localStorage.getItem('access_vector') || ''; window.dataLayer = window.dataLayer || []; if (accessVector) { window.dataLayer.push({ user: { profile: { profileInfo: { snid: accessVector } } } }); } })(); (function(w,d,s,l,i){w[l]=w[l]||[];w[l].push({'gtm.start':new Date().getTime(),event:'gtm.js'});var f=d.getElementsByTagName(s)[0],j=d.createElement(s),dl=l!='dataLayer'?'&l='+l:'';j.async=true;j.src='https://www.googletagmanager.com/gtm.js?id='+i+dl;f.parentNode.insertBefore(j,f);})(window,document,'script','dataLayer','GTM-K279D39R'); Browse Preprints In Review Journals COVID-19 Preprints AJE Video Bytes Research Tools Research Promotion AJE Professional Editing AJE Rubriq About Preprint Platform In Review Editorial Policies Our Team Advisory Board Help Center Sign In Submit a Preprint Cite Share Download PDF Research Article Long-term continuous light exposure alters phototaxis in the acoel flatworm Praesagittifera naikaiensis Hiroshi Matsui, Yumi Hata This is a preprint; it has not been peer reviewed by a journal. https://doi.org/ 10.21203/rs.3.rs-7802934/v1 This work is licensed under a CC BY 4.0 License Status: Posted Version 1 posted You are reading this latest preprint version Abstract 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. phototaxis acoelomorpha behavioral flexibility avoidance Figures Figure 1 Figure 2 Figure 3 Figure 4 Introduction Behavior is not governed solely by the current environment; it is appropriately adjusted by an organism’s history of interactions and by the physiological states that arise from that history. This is evident in everyday life: what counts as a reward is determined by an individual’s physiological state (e.g., food functions as a reward depending on hunger). Recent formulations of reinforcement learning locate the origin of reward in the monitoring of internal states (Weber, Yee, Small, & Petzschner, 2025 ). In the context of instrumental conditioning, this idea has been interpreted under the principle of reinforcer relativity, according to which what serves as a reinforcer depends on the extent of deprivation relative to baseline (Timberlake & Allison, 1974 ). Indeed, the dependence of behavior on experience and internal state has been demonstrated across numerous behavioral phenomena and taxa. Not only instrumental actions but also behaviors that appear instinctive and mechanical can exhibit fine-grained adjustment. Classically, Charles Darwin reported that the effort earthworms invest in plugging the entrances to their burrows varies with ambient dryness (Darwin, 1892 ). When the air is humid, the plug may be “rough” without risking desiccation, indicating that worms adjust their actions to maintain physiological condition in accordance with environmental circumstances (Reed, 1982 ). In crayfish, the tail-flip escape response is rapid and reflex-like, yet the lateral giant neuron that controls this response is suppressed during feeding, which elevates the response threshold (Krasne & Lee, 1988 ). How widely across evolution are such state-dependent mechanisms of behavioral adjustment distributed? It is important to investigate species across diverse animal lineages that retain relatively basal traits (Lyon & Cheng, 2023 ). Indeed, long before the emergence of full-fledged human cognition, biological functions for interacting with the external world had already evolved, preparing the way for cognition in other lineages. Because the term “cognition” is polysemous and still vague, it is not straightforward to specify what counts as cognitive behavior (Bayne et al., 2019 ). Even so, Charles Darwin’s observations of earthworms indicate that such behavior is not a trivial reflex. The behavioral repertoire sometimes labeled as animals’ “basic behavior” is itself of cognitively interesting (Keijzer, 2001 ; Lyon, 2006 ). This perspective has expanded under the banner of “minimal cognition,” which seeks the minimal requirements for the generation of cognitive phenomena (van Duijn, Keijzer, & Franken, 2006 ; Lyon, 2020 ). We suggest that one productive approach is to examine the plasticity or modifiability of natural behaviors that appear “instinctive” and simple at first glance (cf. Matsui & Hata, 2025a ). Across classical and contemporary accounts, taxis has been regarded as a reflexive form of behavior (Giraldo et al., 2018 ; Parmelee, 1913 ). For example, Pisula ( 2016 ) writes, “Taxis is a stereotyped response to a specific stimulus. The organism acts like an automaton.” Phototaxis, in particular, appears to involve attraction to a clearly defined stimulus (light) and thus to present as a fixed mode of movement. Yet, several studies have reported plastic modulation even in taxis. In Drosophila, the sign of phototaxis changes with flight capability (Gorostiza, Colomb, & Brembs, 2016 ). Planarians that exhibit negative phototaxis show long term habituation to light (Prados, Fisher, Moreno-Fernández, Tazumi, & Urcelay, 2020 ). Taken together, these studies indicate that behaviors that seem mechanical and reflexive can, in fact, be flexibly controlled by internal state and experience. Acoelomorphs occupy an informative position for studying the plasticity of taxis. The clade was delineated relatively recently (Baguñà & Riutort, 2004 ). There is still no consensus on its exact phylogenetic placement: hypotheses variously place it as the sister group to Ambulacraria (Philippe et al., 2011 , 2019 ) or as the basal branch of all other bilaterians (Álvarez-Presas, Ruiz-Trillo, & Paps, 2024 ; Cannon et al., 2016 ; Hejnol et al., 2009 ; Ryan et al., 2013 ). Either way, the group is a useful target for probing the early evolution of behavior in Bilateria. Its vermiform body plan is clearly distinct from the radial symmetry of cnidarians such as jellyfish and sea anemones, exhibiting a definite bilateral organization (Fig. 1 a); it also differs from the pentaradial symmetry of echinoderms (e.g., starfish), which may be close relatives. At the same time, acoelomorphs possess relatively simple nervous systems: depending on the species, neural centralization can be weak, approaching a nerve-net–like organization, whereas other species show aggregation of neurons toward the head region (Gavilán et al., 2016 ; Perea-Atienza et al., 2015 ; Sakagami et al., 2024 ). Flexible behavioral adjustment has begun to be documented even in acoelomorphs. The convolutid Praesagittifera naikaiensis possesses small rhabdomeric eyes and exhibits positive phototaxis in response to light (Yamasu, 1991 ). Because it acquires nutrients via photosynthesis performed by symbiotic algae, this behavior serves a foraging function. Matsui and Hata ( 2025b ) presented P. naikaiensis with a “choice” in a T-maze in which illumination differed between the two arms. As in vertebrates and arthropods, choice proportions tracked the difference in “reward” magnitude, operationalized as light intensity. This matching behavior was disrupted when animals were pre-exposed to light for 24 hours. This pattern may indicate that prior light exposure acted as “satiation,” reducing behavioral sensitivity. Thus, phototaxis is not a purely mechanical reflex, as traditionally conceived, but is executed in a manner that depends on physiological state. In a closely related species, Symsagittifera roscoffensis , spontaneous avoidance of excessively intense light has been reported (Thomas, Tang, & Coates, 2024 ). Because overly strong light can damage the symbiotic algae, intensity-dependent switching in phototactic behavior is likely adaptive. Within Acoelomorpha, these taxa exhibit relatively pronounced anterior neural centralization (Gavilán et al., 2016 ; Perea-Atienza et al., 2015 ; Sakagami et al., 2024 ). They therefore provide informative models for probing the flexibility of behavior in early animal evolution. In this study, we systematically examined how prior light exposure affects phototaxis in P. naikaiensis . In Experiment 1, we examined whether pre-exposure to light for 1 to 5 days attenuates phototaxis, and whether light deprivation enhances it. Experiment 2 served as a control, quantifying spontaneous locomotor activity after five days of light exposure to determine whether the outcome of Experiment 1 could be attributed merely to debilitation caused by prolonged light exposure. In Experiment 3, we constructed a platform with continuously illuminated and shaded regions and allowed P. naikaiensis to move freely between them for five days; using this procedure, we tested whether the animals would come to avoid light spontaneously as a function of their cumulative exposure time. Materials and Method Subjects and housing. We used the acoel flatworm Praesagittifera naikaiensis collected from public beaches on Awaji Island, Hyōgo Prefecture, and Ushimado, Okayama Prefecture (Yamasu, 1982 ; Fig. 1 a). Collection locations were selected with reference to the publicly available map reported by Hikosaka-Katayama, Watanuki, Niiho, and Hikosaka ( 2020 ). Housing method followed Hikosaka ( 2015 ) and was identical to Matsui and Hata (2025); therefore, we describe here only procedures relevant to the present study, primarily light-related parameters. The animals were group housed in an acrylic aquarium (25 cm W × 20 cm D × 20 cm H) that was initially filled with seawater from the collection sites and then gradually replaced with artificial seawater (Delphis, Japan). The light cycle was maintained at 12 h light and 12 h dark. Illuminance ranged from approximately 2,000 to 10,000 lux depending on aquarium location. The aquarium was continuously aerated. No additional food was provided, and the animals were acclimated for at least 14 days before experimentation. After completion of the experiments, animals were returned to holding aquaria and maintained. To prevent reuse of experimental subjects, individuals were segregated into separate aquarium. Apparatus . For Experiment 1, we used a straight runway; for Experiment 3, we used a platform with illuminated and shaded regions (Fig. 1 b). Both devices were designed with computer-aided design software (Fusion 360, Autodesk, USA) and fabricated with a 3D printer (Bambu Lab A1, Bambu Lab, China). The straight runway was printed in semitransparent PETG, and the platform in opaque PLA. Because light in the runway was delivered from the side, a transmissive material was expected to facilitate attraction of the animals. The runway measured 14.5 cm in length, 3.0 cm in width, and up to 1.5 cm in depth. For Experiment 2, we used a commercially available round dish with a flat bottom, 11.5 cm in diameter (Seria, Japan). To prevent animals from becoming occluded by vertical walls when imaged from above, the platform used in Experiment 3 was shaped as an inverted square frustum with sloped sides. Its maximum width was 15 cm, the bottom measured 9 cm, and the maximum depth was 3 cm. A central light-attenuating wall, 5 mm thick, was installed to create illuminated and shaded regions. Because complete darkness would have made counting P. naikaiensis in the shaded region difficult, the wall thickness was chosen to attenuate but not entirely block light. Openings were present on both sides of the wall, allowing P. naikaiensis to move between regions (up to 3 cm wide at the top and 5 mm at the bottom). Phototaxis measurements and light pre-exposure were conducted using a standard LED light (LTC-LC08U-KN 06-0910, OHM, Japan). Illuminance was 10,000–12,000 lux, which was approximately comparable to the maximum illuminance in the housing aquaria. On the platform used in Experiment 3, the shaded region measured 100–300 lux. Behavior was recorded from above with a 4K-resolution webcam (ELP-USB4KCAM01H-H120 jp, ELP, China). All experiments were performed in a dark room to prevent incidental illumination such as room lights and sunlight. The 3D-printable files for Experiments 1 and 3 are publicly available and may be modified and reused with appropriate citation ( https://github.com/HeathRossie/stlfiles ) Experimental procedure and Analysis Experiment 1. Aggregated P. naikaiensis in the housing aquarium were gently aspirated with a 1 mL pipette and introduced to the end of the straight runway opposite the light source. The animals were then exposed to light continuously for 1 hour from the side, and their movement toward the illuminated end via phototaxis was recorded. Timing began when the last individual had been introduced into the runway. Analyses focused on the 1, 5, 15, 30, and 60 minute time points. Still images automatically captured by the camera were processed in ImageJ (National Institutes of Health, USA) to digitize individual positions and extract x and y coordinates. The number of detected individuals ranged from 40 to 94. Although sample size varied, prior work indicates that such variation does not interfere with phototaxis (Matsui & Hata, 2025b ). In the “satiation” group, animals were pre-exposed to light for 1 to 5 days. For this procedure, individuals were transferred to 200 mL glass cups whose tops were sealed with transparent plastic wrap to prevent evaporation. In this group, we expected phototaxis to attenuate as a function of exposure duration. We also prepared a “deprivation” group in Experiment 1, in which phototaxis was predicted to be enhanced. Animals were placed in the same type of glass cups as in the satiation group, and the containers were covered with aluminum foil to completely block light for 1 to 5 days. In the control group, P. naikaiensis were housed in glass cups under the standard 12 h light and 12 h dark cycle for 1 to 5 days. To eliminate prior runway experience as a potential confounding, we used separate cohorts for each exposure duration (1 to 5 days) and did not reuse individuals across conditions. Analyses used a three-factor analysis of variance (ANOVA) with all factors between subjects. The factors were condition (control, satiation, and deprivation), elapsed time (1, 5, 15, 30, and 60 minutes), and exposure duration (1 to 5 days). Although elapsed time would ordinarily be treated as a within-subject factor, individual identification and continuous tracking were not feasible with the present procedure, so it was analyzed as a between-subjects factor. For follow-up comparisons, we conducted pairwise t tests with p values adjusted by Holm’s method. Experiment 2. This experiment was designed to control for the possibility that the change in phototaxis observed in Experiment 1 reflected debilitation caused by pre-exposure to light. Deprivation unexpectedly reduced phototaxis, and the effect was unstable; therefore, only the satiation and control groups were used from Experiment 2 onward. We compared a group pre-exposed to light for 5 days using the same procedure as in Experiment 1 with a group maintained on the standard light–dark cycle for the same duration. To quantify spontaneous locomotion, we adopted an analogue of the open-field test commonly used in mammals. P. naikaiensis were gently placed into a seawater-filled round dish (diameter 11.5 cm), and their movement was recorded from above for 10 minutes at 3 Hz. Five individuals were introduced simultaneously, and three independent cohorts were tested per condition (total n = 15 per condition). Illumination was provided from above with a stand loupe light (B09FSX3Y3S, OTraki, China) arranged to minimize spatial bias. Locomotion was quantified using UMATracker (Yamanaka & Takeuchi, 2018 ). From the trajectories of x and y coordinates, we computed the total distance traveled. If prolonged light exposure debilitates the animals, spontaneous locomotion should decrease. We therefore compared total distance between the control and light-exposed groups using an independent-samples t test. Experiment 3. Using the platform with illuminated and shaded regions, we tested whether P. naikaiensis would avoid light as a function of prior exposure. In other words, we asked whether animals that ordinarily display positive phototaxis can shift to negative phototaxis in accordance with their physiological state. We conducted two independent 5-day experimental runs. Using a 1 mL pipette, approximately 200 P. naikaiensis were gently introduced onto the platform and distributed as evenly as possible between the illuminated and shaded regions. Timing began when the last individual had entered the platform. Still images were captured from the overhead camera every 10 minutes to record the positions of P. naikaiensis . Salinity within the platform was maintained at 30–35‰, with measurements taken approximately every 24 hours. When salinity needed to be lowered, roughly half of the seawater on the platform was removed with a manual pump and replaced with fresh seawater. To avoid altering the lighting geometry, the platform itself was not moved during this procedure. We continuously monitored the water removal to ensure that P. naikaiensis were not drawn into the pump. Unlike Experiment 1, the number of individuals in Experiment 3 was large, making manual detection impractical, so we performed automated position detection from the images at each time point. First, to generate masks delineating the illuminated and shaded regions of the platform, we applied Gaussian smoothing (σ = 1.2) to create a background image and then performed Otsu thresholding to extract the bright region. We retained only connected components with an area ≥ 15,000 px² to define the platform mask and removed small holes by morphological closing. Using this mask, we classified each P. naikaiensis position as either illuminated or shaded. Individuals were detected with the particle-detection algorithm implemented in OpenCV’s SimpleBlobDetector(). At each 10-minute time point, we computed the occupancy probability of the illuminatedregion for P. naikaiensis . We then tested whether occupancy deviated significantly from chance using exact binomial tests. Because recordings were made every 10 minutes over 5 days (720 time points), we controlled the familywise Type I error rate using Holm’s method. All statistical analyses and data visualizations were conducted in R (version 4.1.0). Image analysis for Experiment 3 was performed in Python (version 3.9.15). All data and code are publicly available at https://github.com/HeathRossie/naikaiensis_phototaxis . Result and Discussion Experiment 1: the prior light exposure attenuates phototaxis At 60 minutes, most individuals in the control group without pre-exposure moved toward the light source (Fig. 2 ). A three-way ANOVA revealed a significant condition × elapsed time × day interaction, F (32, 4,288) = 2.341, p = .00003. Accordingly, we tested differences among conditions at each elapsed-time point and on each day. As a result, animals in the satiation group moved toward the light at approximately the same rate as controls on Day 1, and the final mean position did not differ. By Day 2, however, phototaxis in the satiation group was attenuated, and from Day 3 onward this attenuation stabilized (.00008 < ps < .024). The final mean position in the control group ranged from 10.10 to 11.18 cm, whereas in the satiation group from Day 3 onward it ranged from 6.89 to 7.51 cm. Given that P. naikaiensis has a body length of approximately 1 to 3 mm, a 3 to 4 cm reduction in displacement represents a change of more than ten body lengths. Taken together, these results indicate that satiation gradually weakens phototaxis, reaching a plateau by Day 3. These results suggest that phototaxis is not a mechanically fixed behavior determined by immediate stimuli. As noted, S. roscoffensis shows an avoidance response to excessively strong light (Thomas et al., 2024 ). What differs from their study is that the light intensities we used are ordinarily favorable stimuli that elicit positive phototaxis. At these intensities, nutrition via photosynthesis by the algal symbionts is arguably possible, so the illuminated region is, in effect, a foraging target for P. naikaiensis . That prolonged light exposure attenuates phototaxis may indicate suppression of the behavior because light is no longer necessary for the animal. In other words, it may point to an interaction between metabolic processes associated with products generated by the symbionts upon light reception and the sensorimotor system comprising the neural pathway from the eyespots to the ciliary motor system. We discuss this point in greater detail in the General Discussion. In contrast to the clear change observed in the satiation group, results for the deprivation group ran counter to prediction: phototaxis did not increase and was instead attenuated overall (.00001 < ps < .044). In the deprivation group, effects were evident from Day 1. On Day 4, however, no difference from the control group was detected, for reasons that remain unclear. Because the deprivation manipulation did not yield a stable, systematic pattern comparable to that of the satiation group, it was not included in subsequent experiments. Experiment 2. Pre-exposure does not reduce spontaneous locomotion A potential limitation of the previous experiment was that the attenuation of phototaxis under the satiation manipulation could not rule out debilitation caused by prolonged illumination. Experiment 2 therefore tested this possibility by recording whether five days of pre-exposure to light altered spontaneous locomotion (Fig. 3 a). It was visually confirmed that none of the individuals introduced into the apparatus were dead. If the reduction in phototaxis was caused by debilitation resulting from prolonged light exposure, spontaneous movement would also be expected to decrease. No significant effect of the satiation manipulation on 10-minute distance traveled was observed, t (25.89) = 1.39, p = .18. Thus, five days of light pre-exposure did not substantially alter spontaneous locomotion. Interindividual variability was large, with total distance ranging from near immobility to nearly 100 cm. Such long-distance movements were observed in both groups, suggesting that prolonged light exposure did not affect locomotor ability. The observed locomotion was perhaps not based on taxis. In this setup, illumination was applied as uniformly as possible from above, and seawater salinity was kept constant, making chemotaxis unlikely. Although P. naikaiensis is known to exhibit positive geotaxis (Sakagami, Watanabe, Ikeda, & Ando, 2021 ), geotactic behavior was unlikely to be predominant in our setup because the dish was positioned as horizontally as possible. No external stimuli were provided except for the transfer of individuals into the dish by the experimenter. Taken together, across the first two experiments, we found that phototaxis is attenuated by light pre-exposure, and that this attenuation is not attributable to debilitation from excessive illumination. Experiment 3. Twenty-four hours of light exposure switches the sign of phototaxis Prolonged illumination in P. naikaiensis may not only induce nutritional satiation but also be harmful, because excessive light can damage the symbiotic algae. In Experiment 1, there was no shaded region, so the animals had no opportunity to avoid light. In Experiment 3, therefore, we used a platform with illuminated and shaded regions and continuously illuminated P. naikaiensis for five days (Fig. 1 b, right). This setup allowed us to assess whether P. naikaiensis spontaneously avoided light depending on the duration and amount of light they had received. As a result, P. naikaiensis was found to switch the sign of phototaxis according to the duration of illumination (Fig. 4 a). Animals that had been introduced approximately evenly into the illuminated and shaded regions moved mostly into the illuminated region within the first hour (Fig. 4 b), consistent with the positive phototaxis observed in Experiment 1. By around 24 hours, phototaxis had reversed: of the roughly 90% of individuals in the illuminated region, about 30% shifted to the shaded region. Thereafter, occupancy of the illuminated region remained between 40% and 60%. Was the 2–5 day trajectory of occupancy in the illuminated and shaded regions driven by the same individuals remaining stationary? Because measurements were taken every 10 minutes, precise individual tracking was not possible in this experiment. However, video inspection confirmed that P. naikaiensis occasionally engaged in spontaneous movements ( https://youtu.be/k6m0-yHovJg ). Therefore, it is more likely that the observed occupancy patterns reflect active repositioning in accordance with the history of sustained illumination rather than the same individuals remaining in place. The observed behavior supports the view that the phototaxis of P. naikaiensis can be modulated by prior experience with light exposure. Specifically, they can reverse their default behavioral mode of positive phototaxis to its opposite. The possible adaptive significance of this flexibility is discussed in the following section. General Discussion Phototaxis has traditionally been regarded as a mechanical and reflexive behavior. This study critically examined this assumption by testing whether phototaxis is adjusted according to a history of light exposure in this species. In P. naikaiensis , which derives nutrition from photosynthesis by symbiotic algae, phototaxis functions as a foraging behavior. Across three experiments, we evaluated experience-dependent modification of phototaxis in P. naikaiensis . Phototaxis was attenuated by prior light exposure. Moreover, when a shaded option was available, the animals reversed the sign of phototaxis and spontaneously avoided illumination. Our results are consistent with Matsui and Hata ( 2025b ), which suggested an interaction between the metabolic system and the sensorimotor circuitry that controls phototaxis. In that study, animals were presented with a two-arm choice, and choice proportions matched light intensity; however, after 24 hours of prior illumination, choices became more at random. In the present work, changes in responsiveness to light appeared by 48 hours in Experiment 1 and by 24 hours in Experiment 3. Although the procedures and apparatus differed, both sets of findings demonstrate plasticity in behavior that appears reflexive at first glance. In acoelomorphs, the symbiotic algae in P. naikaiensis are not intracellular but are embedded in the intercellular spaces of subepidermal tissues (Arboleda et al., 2018 ; Bailly et al., 2014 ). The route by which amino acids and other products generated by photosynthesis enter the body remains unresolved, but it is likely that metabolic processes modulate the sensorimotor system that produces phototaxis. Notably, our Experiment 3 showed that P. naikaiensis actively avoided light under sustained illumination. Why would an animal that derives nutrition from photosynthesis by its symbiotic algae need to avoid light? A plausible reason is that excessive light is likely harmful to the symbionts (Barber & Andersson, 1992 ). Prolonged high-intensity illumination induces photoinhibition, a decline in photosynthetic efficiency. This arises from toxic by-products generated during photochemical reactions, whose removal is required for repair. Under ordinary conditions, repair mechanisms maintain algal viability; however, when strong light coincides with environmental stress, repair cannot keep pace, photosynthetic capacity is lost, and in the worst case, cells die. Accordingly, altering the sign of phototaxis in accordance with cumulative light exposure appears adaptive for safeguarding both the animal and its symbiotic algae. Potentials for behavioral modification through interactions between metabolic and sensory systems offers useful insights into the seemingly endless debate over what counts as cognitive behavior. The attitude that elevates human cognition as the absolute standard has become increasingly untenable (Lyon, 2020 ; Matsui & Hata, 2025a ). Instead, the view that cognition is widespread across the living world even beyond the animal kingdom, including non-neural organisms such as plants and fungi, is gaining currency (Crippen, 2020 ; Garzón & Keijzer, 2009 ; Lyon, Keijzer, Arendt, & Levin, 2021 ). Cognition was originally conceptualized as a computational and representational process detached from the body (Adams & Aizawa, 2010 ; Fodor, 1983 ; Foder & Pylyshin, 1984). However, that conception is no longer sustainable, and a perspective is taking hold that cognition is mostly related to sensorimotor systems, bodily morphology, and modes of contact with the surrounding medium (Barton & Barrett, 2025 ; Segundo-Ortin, 2020 ). Even phototaxis cannot simply be excluded from cognition as an innate stimulus response behavior; it must be understood as a regulatory process carried out in ongoing interplay with experience. This, in turn, may offer a clue to what forms of cognition became possible in animals with the advent of nervous systems. Our study has limitations. In Experiment 1, we found no effect of deprivation; that is, we did not succeed in identifying a motivational manipulation that increased the strength of phototaxis. There is also a discrepancy between Experiments 1 and 3. In Experiment 1, 24 hours of prior illumination did not alter phototaxis, whereas in Experiment 3, within 24 hours approximately 30% of individuals reversed the sign of phototaxis and moved from the illuminated to the shaded region. The present study cannot determine the source of this quantitative difference. Finally, state-dependent adjustment of behavior has been documented across diverse taxa. In humans, hunger, an internal physiological state, modulates cognitive and behavioral responses to current environmental stimuli (Loeber, Grosshans, Herpertz, Kiefer, & Herpertz, 2013 ). In fishes, aggressive interactions alter hormonal dynamics and subsequently influence behavior (Earley, Lu, Lee, Wong, & Hsu, 2013 ). In arthropods, sawflies adjust the trade-off between food discovery and predation risk according to prior starvation experience (Singh, Wolthaus, Schielzeth, & Müller, 2023 ). These flexible adjustments are mediated by interactions between central neural information processing and peripheral bodily states. Our study suggests that P. naikaiensis may possess mechanisms for monitoring its own nutritional status and the health of its algal symbionts, although how this information is conveyed to the sensorimotor system that generates phototaxis remains unknown. Given that Acoelomorpha likely retain a primitive form of neural organization among bilaterians, such mechanisms may have already been present early in bilaterian evolution and could have contributed to the emergence of goal-directed behavior. Declarations Funding This research is not supported by any funding agency. Authorship contribution HM: Conceptualization, Methodology, Software, Formal analysis, Investigation, Writing – review & editing, Writing – original draft. YM: Conceptualization, Methodology, Writing – review & editing. All authors reviewed the manuscript. Declaration of generative AI usage During the preparation of this work the authors used chatGPT 5.0 in order to refine the manuscripts written by non-native English speakers. After using this tool/service, the authors reviewed and edited the content as needed and takes full responsibility for the content of the publication. Data Availability All data and code are publicly available at https://github.com/HeathRossie/naikaiensis_phototaxis. References Adams F, Aizawa K (2010) Defending the bounds of cognition. In: Menary R (ed) The extended mind. 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Adapt Behav 14(2):157–170. https://doi.org/10.1177/10597123060140020 Weber LA, Yee DM, Small DM, Petzschner FH (2025) The interoceptive origin of reinforcement learning. Trends Cogn Sci 9:840–854. https://doi.org/10.1016/j.tics.2025.05.008 Yamanaka O, Takeuchi R (2018) UMATracker: an intuitive image-based tracking platform. J Exp Biol 221(16):jeb182469. https://doi.org/10.1242/jeb.182469 Yamasu T (1982) Five new species of acoel flat worms from Japan. Galaxea 1:2943 Yamasu T (1991) Fine structure and function of ocelli and sagittocysts of acoel flatworms. Hydrobiologia 227(1):273–282. https://doi.org/10.1007/BF00027612 Additional Declarations No competing interests reported. Cite Share Download PDF Status: Posted Version 1 posted You are reading this latest preprint version Research Square lets you share your work early, gain feedback from the community, and start making changes to your manuscript prior to peer review in a journal. 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05:53:49","extension":"xml","order_by":11,"title":"","display":"","copyAsset":false,"role":"acdc-reference","size":112918,"visible":true,"origin":"","legend":"","description":"","filename":"de77d2f663b6406cbe7b4805458893021structuring.xml","url":"https://assets-eu.researchsquare.com/files/rs-7802934/v1/fd5c9f02439cb794657ce665.xml"},{"id":93103106,"identity":"28230b5a-ef8a-4540-91a2-4c206e75b984","added_by":"auto","created_at":"2025-10-09 05:53:49","extension":"html","order_by":12,"title":"","display":"","copyAsset":false,"role":"acdc-reference","size":120563,"visible":true,"origin":"","legend":"","description":"","filename":"earlyproof.html","url":"https://assets-eu.researchsquare.com/files/rs-7802934/v1/f1d079b4828ace4dd8917048.html"},{"id":93103103,"identity":"66b4b960-53f8-4c0f-986d-b89b0d17fbd6","added_by":"auto","created_at":"2025-10-09 05:53:49","extension":"png","order_by":1,"title":"Figure 1","display":"","copyAsset":false,"role":"figure","size":536875,"visible":true,"origin":"","legend":"\u003cp\u003e(a) Studied species \u003cem\u003ePraesagittifera naikaiensis\u003c/em\u003e. The right-side photo shows individuals of \u003cem\u003eP. naikaiensis\u003c/em\u003e discovered in the tide pool at the collection site. (b) Experimental apparatus. Ellipse-like dots represent each \u003cem\u003eP. naikaiensis\u003c/em\u003e individual.\u003c/p\u003e","description":"","filename":"1.png","url":"https://assets-eu.researchsquare.com/files/rs-7802934/v1/c75d0d8f914332c75205e636.png"},{"id":93103115,"identity":"b6036a36-623f-4349-9651-0b50cc3d2e0e","added_by":"auto","created_at":"2025-10-09 05:53:50","extension":"png","order_by":2,"title":"Figure 2","display":"","copyAsset":false,"role":"figure","size":164655,"visible":true,"origin":"","legend":"\u003cp\u003eTime course of phototaxis on a straight runway. The origin of the y-axis represents the end opposite to the light source. Small dots indicate individual data points, and large dots indicate the mean (± SEM). The orange asterisks indicate differences between the control and satiation groups, while the blue asterisks indicate differences between the control and deprivation groups.\u003c/p\u003e","description":"","filename":"2.png","url":"https://assets-eu.researchsquare.com/files/rs-7802934/v1/655d6aa7bac3849aafe14770.png"},{"id":93103107,"identity":"bea290ee-bb7a-4008-bf49-9287cadb2a23","added_by":"auto","created_at":"2025-10-09 05:53:49","extension":"png","order_by":3,"title":"Figure 3","display":"","copyAsset":false,"role":"figure","size":283519,"visible":true,"origin":"","legend":"\u003cp\u003e(a) Example of movement trajectories of the animals. Each color represents an individual. (b) Comparison of total travel distance over 10 minutes.\u003c/p\u003e","description":"","filename":"3.png","url":"https://assets-eu.researchsquare.com/files/rs-7802934/v1/5b726c8fac9f8ec79fb39df9.png"},{"id":93103082,"identity":"19f885c1-173f-4df1-81bd-ca9d6c3bee5f","added_by":"auto","created_at":"2025-10-09 05:53:47","extension":"png","order_by":4,"title":"Figure 4","display":"","copyAsset":false,"role":"figure","size":79551,"visible":true,"origin":"","legend":"\u003cp\u003e(a) Variation over 5 days in the probability that \u003cem\u003eP. naikaiensis\u003c/em\u003e individuals occupy the illuminated region. Abrupt changes likely reflected animals being displaced by the pump stream during water exchanges. The black and gray lines indicate datasets collected on different days. Pink horizontal lines at the bottom mark the time points at which occupancy significantly deviated from 50% for the first and second datasets, respectively. (b) Close-up of the first 5 hours on Day 1.\u003c/p\u003e","description":"","filename":"4.png","url":"https://assets-eu.researchsquare.com/files/rs-7802934/v1/eaa96d1e967eeee5605bd4a8.png"},{"id":98430613,"identity":"0aafe15e-8fc3-4acb-8f84-c44f46fbfb0f","added_by":"auto","created_at":"2025-12-17 16:45:54","extension":"pdf","order_by":0,"title":"","display":"","copyAsset":false,"role":"manuscript-pdf","size":1680980,"visible":true,"origin":"","legend":"","description":"","filename":"manuscript.pdf","url":"https://assets-eu.researchsquare.com/files/rs-7802934/v1/2e70002c-a5a8-4523-9167-20492adb4f48.pdf"}],"financialInterests":"No competing interests reported.","formattedTitle":"Long-term continuous light exposure alters phototaxis in the acoel flatworm Praesagittifera naikaiensis","fulltext":[{"header":"Introduction","content":"\u003cp\u003eBehavior is not governed solely by the current environment; it is appropriately adjusted by an organism\u0026rsquo;s history of interactions and by the physiological states that arise from that history. This is evident in everyday life: what counts as a reward is determined by an individual\u0026rsquo;s physiological state (e.g., food functions as a reward depending on hunger). Recent formulations of reinforcement learning locate the origin of reward in the monitoring of internal states (Weber, Yee, Small, \u0026amp; Petzschner, \u003cspan citationid=\"CR46\" class=\"CitationRef\"\u003e2025\u003c/span\u003e). In the context of instrumental conditioning, this idea has been interpreted under the principle of reinforcer relativity, according to which what serves as a reinforcer depends on the extent of deprivation relative to baseline (Timberlake \u0026amp; Allison, \u003cspan citationid=\"CR44\" class=\"CitationRef\"\u003e1974\u003c/span\u003e). Indeed, the dependence of behavior on experience and internal state has been demonstrated across numerous behavioral phenomena and taxa.\u003c/p\u003e\u003cp\u003eNot only instrumental actions but also behaviors that appear instinctive and mechanical can exhibit fine-grained adjustment. Classically, Charles Darwin reported that the effort earthworms invest in plugging the entrances to their burrows varies with ambient dryness (Darwin, \u003cspan citationid=\"CR11\" class=\"CitationRef\"\u003e1892\u003c/span\u003e). When the air is humid, the plug may be \u0026ldquo;rough\u0026rdquo; without risking desiccation, indicating that worms adjust their actions to maintain physiological condition in accordance with environmental circumstances (Reed, \u003cspan citationid=\"CR37\" class=\"CitationRef\"\u003e1982\u003c/span\u003e). In crayfish, the tail-flip escape response is rapid and reflex-like, yet the lateral giant neuron that controls this response is suppressed during feeding, which elevates the response threshold (Krasne \u0026amp; Lee, \u003cspan citationid=\"CR22\" class=\"CitationRef\"\u003e1988\u003c/span\u003e).\u003c/p\u003e\u003cp\u003eHow widely across evolution are such state-dependent mechanisms of behavioral adjustment distributed? It is important to investigate species across diverse animal lineages that retain relatively basal traits (Lyon \u0026amp; Cheng, \u003cspan citationid=\"CR26\" class=\"CitationRef\"\u003e2023\u003c/span\u003e). Indeed, long before the emergence of full-fledged human cognition, biological functions for interacting with the external world had already evolved, preparing the way for cognition in other lineages. Because the term \u0026ldquo;cognition\u0026rdquo; is polysemous and still vague, it is not straightforward to specify what counts as cognitive behavior (Bayne et al., \u003cspan citationid=\"CR8\" class=\"CitationRef\"\u003e2019\u003c/span\u003e). Even so, Charles Darwin\u0026rsquo;s observations of earthworms indicate that such behavior is not a trivial reflex. The behavioral repertoire sometimes labeled as animals\u0026rsquo; \u0026ldquo;basic behavior\u0026rdquo; is itself of cognitively interesting (Keijzer, \u003cspan citationid=\"CR21\" class=\"CitationRef\"\u003e2001\u003c/span\u003e; Lyon, \u003cspan citationid=\"CR24\" class=\"CitationRef\"\u003e2006\u003c/span\u003e). This perspective has expanded under the banner of \u0026ldquo;minimal cognition,\u0026rdquo; which seeks the minimal requirements for the generation of cognitive phenomena (van Duijn, Keijzer, \u0026amp; Franken, \u003cspan citationid=\"CR45\" class=\"CitationRef\"\u003e2006\u003c/span\u003e; Lyon, \u003cspan citationid=\"CR25\" class=\"CitationRef\"\u003e2020\u003c/span\u003e).\u003c/p\u003e\u003cp\u003eWe suggest that one productive approach is to examine the plasticity or modifiability of natural behaviors that appear \u0026ldquo;instinctive\u0026rdquo; and simple at first glance (cf. Matsui \u0026amp; Hata, \u003cspan citationid=\"CR28\" class=\"CitationRef\"\u003e2025a\u003c/span\u003e). Across classical and contemporary accounts, taxis has been regarded as a reflexive form of behavior (Giraldo et al., \u003cspan citationid=\"CR16\" class=\"CitationRef\"\u003e2018\u003c/span\u003e; Parmelee, \u003cspan citationid=\"CR30\" class=\"CitationRef\"\u003e1913\u003c/span\u003e). For example, Pisula (\u003cspan citationid=\"CR34\" class=\"CitationRef\"\u003e2016\u003c/span\u003e) writes, \u0026ldquo;Taxis is a stereotyped response to a specific stimulus. The organism acts like an automaton.\u0026rdquo; Phototaxis, in particular, appears to involve attraction to a clearly defined stimulus (light) and thus to present as a fixed mode of movement. Yet, several studies have reported plastic modulation even in taxis. In Drosophila, the sign of phototaxis changes with flight capability (Gorostiza, Colomb, \u0026amp; Brembs, \u003cspan citationid=\"CR17\" class=\"CitationRef\"\u003e2016\u003c/span\u003e). Planarians that exhibit negative phototaxis show long term habituation to light (Prados, Fisher, Moreno-Fern\u0026aacute;ndez, Tazumi, \u0026amp; Urcelay, \u003cspan citationid=\"CR35\" class=\"CitationRef\"\u003e2020\u003c/span\u003e). Taken together, these studies indicate that behaviors that seem mechanical and reflexive can, in fact, be flexibly controlled by internal state and experience.\u003c/p\u003e\u003cp\u003eAcoelomorphs occupy an informative position for studying the plasticity of taxis. The clade was delineated relatively recently (Bagu\u0026ntilde;\u0026agrave; \u0026amp; Riutort, \u003cspan citationid=\"CR4\" class=\"CitationRef\"\u003e2004\u003c/span\u003e). There is still no consensus on its exact phylogenetic placement: hypotheses variously place it as the sister group to Ambulacraria (Philippe et al., \u003cspan citationid=\"CR32\" class=\"CitationRef\"\u003e2011\u003c/span\u003e, \u003cspan citationid=\"CR33\" class=\"CitationRef\"\u003e2019\u003c/span\u003e) or as the basal branch of all other bilaterians (\u0026Aacute;lvarez-Presas, Ruiz-Trillo, \u0026amp; Paps, \u003cspan citationid=\"CR2\" class=\"CitationRef\"\u003e2024\u003c/span\u003e; Cannon et al., \u003cspan citationid=\"CR9\" class=\"CitationRef\"\u003e2016\u003c/span\u003e; Hejnol et al., \u003cspan citationid=\"CR18\" class=\"CitationRef\"\u003e2009\u003c/span\u003e; Ryan et al., \u003cspan citationid=\"CR38\" class=\"CitationRef\"\u003e2013\u003c/span\u003e). Either way, the group is a useful target for probing the early evolution of behavior in Bilateria. Its vermiform body plan is clearly distinct from the radial symmetry of cnidarians such as jellyfish and sea anemones, exhibiting a definite bilateral organization (Fig.\u0026nbsp;\u003cspan refid=\"Fig1\" class=\"InternalRef\"\u003e1\u003c/span\u003ea); it also differs from the pentaradial symmetry of echinoderms (e.g., starfish), which may be close relatives. At the same time, acoelomorphs possess relatively simple nervous systems: depending on the species, neural centralization can be weak, approaching a nerve-net\u0026ndash;like organization, whereas other species show aggregation of neurons toward the head region (Gavil\u0026aacute;n et al., \u003cspan citationid=\"CR14\" class=\"CitationRef\"\u003e2016\u003c/span\u003e; Perea-Atienza et al., \u003cspan citationid=\"CR31\" class=\"CitationRef\"\u003e2015\u003c/span\u003e; Sakagami et al., \u003cspan citationid=\"CR40\" class=\"CitationRef\"\u003e2024\u003c/span\u003e).\u003c/p\u003e\u003cp\u003e\u003c/p\u003e\u003cp\u003eFlexible behavioral adjustment has begun to be documented even in acoelomorphs. The convolutid \u003cem\u003ePraesagittifera naikaiensis\u003c/em\u003e possesses small rhabdomeric eyes and exhibits positive phototaxis in response to light (Yamasu, \u003cspan citationid=\"CR49\" class=\"CitationRef\"\u003e1991\u003c/span\u003e). Because it acquires nutrients via photosynthesis performed by symbiotic algae, this behavior serves a foraging function. Matsui and Hata (\u003cspan citationid=\"CR29\" class=\"CitationRef\"\u003e2025b\u003c/span\u003e) presented \u003cem\u003eP. naikaiensis\u003c/em\u003e with a \u0026ldquo;choice\u0026rdquo; in a T-maze in which illumination differed between the two arms. As in vertebrates and arthropods, choice proportions tracked the difference in \u0026ldquo;reward\u0026rdquo; magnitude, operationalized as light intensity. This matching behavior was disrupted when animals were pre-exposed to light for 24 hours. This pattern may indicate that prior light exposure acted as \u0026ldquo;satiation,\u0026rdquo; reducing behavioral sensitivity. Thus, phototaxis is not a purely mechanical reflex, as traditionally conceived, but is executed in a manner that depends on physiological state. In a closely related species, \u003cem\u003eSymsagittifera roscoffensis\u003c/em\u003e, spontaneous avoidance of excessively intense light has been reported (Thomas, Tang, \u0026amp; Coates, \u003cspan citationid=\"CR43\" class=\"CitationRef\"\u003e2024\u003c/span\u003e). Because overly strong light can damage the symbiotic algae, intensity-dependent switching in phototactic behavior is likely adaptive. Within Acoelomorpha, these taxa exhibit relatively pronounced anterior neural centralization (Gavil\u0026aacute;n et al., \u003cspan citationid=\"CR14\" class=\"CitationRef\"\u003e2016\u003c/span\u003e; Perea-Atienza et al., \u003cspan citationid=\"CR31\" class=\"CitationRef\"\u003e2015\u003c/span\u003e; Sakagami et al., \u003cspan citationid=\"CR40\" class=\"CitationRef\"\u003e2024\u003c/span\u003e). They therefore provide informative models for probing the flexibility of behavior in early animal evolution.\u003c/p\u003e\u003cp\u003eIn this study, we systematically examined how prior light exposure affects phototaxis in \u003cem\u003eP. naikaiensis\u003c/em\u003e. In Experiment 1, we examined whether pre-exposure to light for 1 to 5 days attenuates phototaxis, and whether light deprivation enhances it. Experiment 2 served as a control, quantifying spontaneous locomotor activity after five days of light exposure to determine whether the outcome of Experiment 1 could be attributed merely to debilitation caused by prolonged light exposure. In Experiment 3, we constructed a platform with continuously illuminated and shaded regions and allowed \u003cem\u003eP. naikaiensis\u003c/em\u003e to move freely between them for five days; using this procedure, we tested whether the animals would come to avoid light spontaneously as a function of their cumulative exposure time.\u003c/p\u003e"},{"header":"Materials and Method","content":"\u003cp\u003e\u003cb\u003eSubjects and housing.\u003c/b\u003e We used the acoel flatworm \u003cem\u003ePraesagittifera naikaiensis\u003c/em\u003e collected from public beaches on Awaji Island, Hyōgo Prefecture, and Ushimado, Okayama Prefecture (Yamasu, \u003cspan citationid=\"CR48\" class=\"CitationRef\"\u003e1982\u003c/span\u003e; Fig.\u0026nbsp;\u003cspan refid=\"Fig1\" class=\"InternalRef\"\u003e1\u003c/span\u003ea). Collection locations were selected with reference to the publicly available map reported by Hikosaka-Katayama, Watanuki, Niiho, and Hikosaka (\u003cspan citationid=\"CR20\" class=\"CitationRef\"\u003e2020\u003c/span\u003e). Housing method followed Hikosaka (\u003cspan citationid=\"CR19\" class=\"CitationRef\"\u003e2015\u003c/span\u003e) and was identical to Matsui and Hata (2025); therefore, we describe here only procedures relevant to the present study, primarily light-related parameters. The animals were group housed in an acrylic aquarium (25 cm W \u0026times; 20 cm D \u0026times; 20 cm H) that was initially filled with seawater from the collection sites and then gradually replaced with artificial seawater (Delphis, Japan). The light cycle was maintained at 12 h light and 12 h dark. Illuminance ranged from approximately 2,000 to 10,000 lux depending on aquarium location. The aquarium was continuously aerated. No additional food was provided, and the animals were acclimated for at least 14 days before experimentation. After completion of the experiments, animals were returned to holding aquaria and maintained. To prevent reuse of experimental subjects, individuals were segregated into separate aquarium.\u003c/p\u003e\u003cp\u003e\u003cb\u003eApparatus\u003c/b\u003e. For Experiment 1, we used a straight runway; for Experiment 3, we used a platform with illuminated and shaded regions (Fig.\u0026nbsp;\u003cspan refid=\"Fig1\" class=\"InternalRef\"\u003e1\u003c/span\u003eb). Both devices were designed with computer-aided design software (Fusion 360, Autodesk, USA) and fabricated with a 3D printer (Bambu Lab A1, Bambu Lab, China). The straight runway was printed in semitransparent PETG, and the platform in opaque PLA. Because light in the runway was delivered from the side, a transmissive material was expected to facilitate attraction of the animals. The runway measured 14.5 cm in length, 3.0 cm in width, and up to 1.5 cm in depth. For Experiment 2, we used a commercially available round dish with a flat bottom, 11.5 cm in diameter (Seria, Japan).\u003c/p\u003e\u003cp\u003eTo prevent animals from becoming occluded by vertical walls when imaged from above, the platform used in Experiment 3 was shaped as an inverted square frustum with sloped sides. Its maximum width was 15 cm, the bottom measured 9 cm, and the maximum depth was 3 cm. A central light-attenuating wall, 5 mm thick, was installed to create illuminated and shaded regions. Because complete darkness would have made counting \u003cem\u003eP. naikaiensis\u003c/em\u003e in the shaded region difficult, the wall thickness was chosen to attenuate but not entirely block light. Openings were present on both sides of the wall, allowing \u003cem\u003eP. naikaiensis\u003c/em\u003e to move between regions (up to 3 cm wide at the top and 5 mm at the bottom).\u003c/p\u003e\u003cp\u003ePhototaxis measurements and light pre-exposure were conducted using a standard LED light (LTC-LC08U-KN 06-0910, OHM, Japan). Illuminance was 10,000\u0026ndash;12,000 lux, which was approximately comparable to the maximum illuminance in the housing aquaria. On the platform used in Experiment 3, the shaded region measured 100\u0026ndash;300 lux. Behavior was recorded from above with a 4K-resolution webcam (ELP-USB4KCAM01H-H120 jp, ELP, China). All experiments were performed in a dark room to prevent incidental illumination such as room lights and sunlight. The 3D-printable files for Experiments 1 and 3 are publicly available and may be modified and reused with appropriate citation (\u003cspan class=\"ExternalRef\"\u003e\u003cspan class=\"RefSource\"\u003ehttps://github.com/HeathRossie/stlfiles\u003c/span\u003e\u003cspan address=\"https://github.com/HeathRossie/stlfiles\" targettype=\"URL\" class=\"RefTarget\"\u003e\u003c/span\u003e\u003c/span\u003e)\u003c/p\u003e\u003cdiv id=\"Sec3\" class=\"Section2\"\u003e\u003ch2\u003eExperimental procedure and Analysis\u003c/h2\u003e\u003cp\u003e\u003cb\u003eExperiment 1.\u003c/b\u003e Aggregated \u003cem\u003eP. naikaiensis\u003c/em\u003e in the housing aquarium were gently aspirated with a 1 mL pipette and introduced to the end of the straight runway opposite the light source. The animals were then exposed to light continuously for 1 hour from the side, and their movement toward the illuminated end via phototaxis was recorded. Timing began when the last individual had been introduced into the runway. Analyses focused on the 1, 5, 15, 30, and 60 minute time points. Still images automatically captured by the camera were processed in ImageJ (National Institutes of Health, USA) to digitize individual positions and extract x and y coordinates. The number of detected individuals ranged from 40 to 94. Although sample size varied, prior work indicates that such variation does not interfere with phototaxis (Matsui \u0026amp; Hata, \u003cspan citationid=\"CR29\" class=\"CitationRef\"\u003e2025b\u003c/span\u003e).\u003c/p\u003e\u003cp\u003eIn the \u0026ldquo;satiation\u0026rdquo; group, animals were pre-exposed to light for 1 to 5 days. For this procedure, individuals were transferred to 200 mL glass cups whose tops were sealed with transparent plastic wrap to prevent evaporation. In this group, we expected phototaxis to attenuate as a function of exposure duration. We also prepared a \u0026ldquo;deprivation\u0026rdquo; group in Experiment 1, in which phototaxis was predicted to be enhanced. Animals were placed in the same type of glass cups as in the satiation group, and the containers were covered with aluminum foil to completely block light for 1 to 5 days. In the control group, \u003cem\u003eP. naikaiensis\u003c/em\u003e were housed in glass cups under the standard 12 h light and 12 h dark cycle for 1 to 5 days. To eliminate prior runway experience as a potential confounding, we used separate cohorts for each exposure duration (1 to 5 days) and did not reuse individuals across conditions.\u003c/p\u003e\u003cp\u003eAnalyses used a three-factor analysis of variance (ANOVA) with all factors between subjects. The factors were condition (control, satiation, and deprivation), elapsed time (1, 5, 15, 30, and 60 minutes), and exposure duration (1 to 5 days). Although elapsed time would ordinarily be treated as a within-subject factor, individual identification and continuous tracking were not feasible with the present procedure, so it was analyzed as a between-subjects factor. For follow-up comparisons, we conducted pairwise \u003cem\u003et\u003c/em\u003e tests with \u003cem\u003ep\u003c/em\u003e values adjusted by Holm\u0026rsquo;s method.\u003c/p\u003e\u003cp\u003e\u003cb\u003eExperiment 2.\u003c/b\u003e This experiment was designed to control for the possibility that the change in phototaxis observed in Experiment 1 reflected debilitation caused by pre-exposure to light. Deprivation unexpectedly reduced phototaxis, and the effect was unstable; therefore, only the satiation and control groups were used from Experiment 2 onward. We compared a group pre-exposed to light for 5 days using the same procedure as in Experiment 1 with a group maintained on the standard light\u0026ndash;dark cycle for the same duration. To quantify spontaneous locomotion, we adopted an analogue of the open-field test commonly used in mammals. \u003cem\u003eP. naikaiensis\u003c/em\u003e were gently placed into a seawater-filled round dish (diameter 11.5 cm), and their movement was recorded from above for 10 minutes at 3 Hz. Five individuals were introduced simultaneously, and three independent cohorts were tested per condition (total \u003cem\u003en\u003c/em\u003e\u0026thinsp;=\u0026thinsp;15 per condition). Illumination was provided from above with a stand loupe light (B09FSX3Y3S, OTraki, China) arranged to minimize spatial bias.\u003c/p\u003e\u003cp\u003eLocomotion was quantified using UMATracker (Yamanaka \u0026amp; Takeuchi, \u003cspan citationid=\"CR47\" class=\"CitationRef\"\u003e2018\u003c/span\u003e). From the trajectories of x and y coordinates, we computed the total distance traveled. If prolonged light exposure debilitates the animals, spontaneous locomotion should decrease. We therefore compared total distance between the control and light-exposed groups using an independent-samples \u003cem\u003et\u003c/em\u003e test.\u003c/p\u003e\u003cp\u003e\u003cb\u003eExperiment 3.\u003c/b\u003e Using the platform with illuminated and shaded regions, we tested whether \u003cem\u003eP. naikaiensis\u003c/em\u003e would avoid light as a function of prior exposure. In other words, we asked whether animals that ordinarily display positive phototaxis can shift to negative phototaxis in accordance with their physiological state.\u003c/p\u003e\u003cp\u003eWe conducted two independent 5-day experimental runs. Using a 1 mL pipette, approximately 200 \u003cem\u003eP. naikaiensis\u003c/em\u003e were gently introduced onto the platform and distributed as evenly as possible between the illuminated and shaded regions. Timing began when the last individual had entered the platform. Still images were captured from the overhead camera every 10 minutes to record the positions of \u003cem\u003eP. naikaiensis\u003c/em\u003e. Salinity within the platform was maintained at 30\u0026ndash;35\u0026permil;, with measurements taken approximately every 24 hours. When salinity needed to be lowered, roughly half of the seawater on the platform was removed with a manual pump and replaced with fresh seawater. To avoid altering the lighting geometry, the platform itself was not moved during this procedure. We continuously monitored the water removal to ensure that \u003cem\u003eP. naikaiensis\u003c/em\u003e were not drawn into the pump.\u003c/p\u003e\u003cp\u003eUnlike Experiment 1, the number of individuals in Experiment 3 was large, making manual detection impractical, so we performed automated position detection from the images at each time point. First, to generate masks delineating the illuminated and shaded regions of the platform, we applied Gaussian smoothing (σ\u0026thinsp;=\u0026thinsp;1.2) to create a background image and then performed Otsu thresholding to extract the bright region. We retained only connected components with an area\u0026thinsp;\u0026ge;\u0026thinsp;15,000 px\u0026sup2; to define the platform mask and removed small holes by morphological closing. Using this mask, we classified each \u003cem\u003eP. naikaiensis\u003c/em\u003e position as either illuminated or shaded. Individuals were detected with the particle-detection algorithm implemented in OpenCV\u0026rsquo;s SimpleBlobDetector().\u003c/p\u003e\u003cp\u003eAt each 10-minute time point, we computed the occupancy probability of the illuminatedregion for \u003cem\u003eP. naikaiensis\u003c/em\u003e. We then tested whether occupancy deviated significantly from chance using exact binomial tests. Because recordings were made every 10 minutes over 5 days (720 time points), we controlled the familywise Type I error rate using Holm\u0026rsquo;s method.\u003c/p\u003e\u003cp\u003eAll statistical analyses and data visualizations were conducted in R (version 4.1.0). Image analysis for Experiment 3 was performed in Python (version 3.9.15). All data and code are publicly available at \u003cspan class=\"ExternalRef\"\u003e\u003cspan class=\"RefSource\"\u003ehttps://github.com/HeathRossie/naikaiensis_phototaxis\u003c/span\u003e\u003cspan address=\"https://github.com/HeathRossie/naikaiensis_phototaxis\" targettype=\"URL\" class=\"RefTarget\"\u003e\u003c/span\u003e\u003c/span\u003e.\u003c/p\u003e\u003c/div\u003e"},{"header":"Result and Discussion","content":"\u003cdiv id=\"Sec5\" class=\"Section2\"\u003e\u003ch2\u003eExperiment 1: the prior light exposure attenuates phototaxis\u003c/h2\u003e\u003cp\u003eAt 60 minutes, most individuals in the control group without pre-exposure moved toward the light source (Fig.\u0026nbsp;\u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e2\u003c/span\u003e). A three-way ANOVA revealed a significant condition \u0026times; elapsed time \u0026times; day interaction, \u003cem\u003eF\u003c/em\u003e(32, 4,288)\u0026thinsp;=\u0026thinsp;2.341, \u003cem\u003ep\u003c/em\u003e\u0026thinsp;=\u0026thinsp;.00003. Accordingly, we tested differences among conditions at each elapsed-time point and on each day.\u003c/p\u003e\u003cp\u003e\u003c/p\u003e\u003cp\u003eAs a result, animals in the satiation group moved toward the light at approximately the same rate as controls on Day 1, and the final mean position did not differ. By Day 2, however, phototaxis in the satiation group was attenuated, and from Day 3 onward this attenuation stabilized (.00008\u0026thinsp;\u0026lt;\u0026thinsp;ps\u0026thinsp;\u0026lt;\u0026thinsp;.024). The final mean position in the control group ranged from 10.10 to 11.18 cm, whereas in the satiation group from Day 3 onward it ranged from 6.89 to 7.51 cm. Given that \u003cem\u003eP. naikaiensis\u003c/em\u003e has a body length of approximately 1 to 3 mm, a 3 to 4 cm reduction in displacement represents a change of more than ten body lengths. Taken together, these results indicate that satiation gradually weakens phototaxis, reaching a plateau by Day 3.\u003c/p\u003e\u003cp\u003eThese results suggest that phototaxis is not a mechanically fixed behavior determined by immediate stimuli. As noted, \u003cem\u003eS. roscoffensis\u003c/em\u003e shows an avoidance response to excessively strong light (Thomas et al., \u003cspan citationid=\"CR43\" class=\"CitationRef\"\u003e2024\u003c/span\u003e). What differs from their study is that the light intensities we used are ordinarily favorable stimuli that elicit positive phototaxis. At these intensities, nutrition via photosynthesis by the algal symbionts is arguably possible, so the illuminated region is, in effect, a foraging target for \u003cem\u003eP. naikaiensis\u003c/em\u003e. That prolonged light exposure attenuates phototaxis may indicate suppression of the behavior because light is no longer necessary for the animal. In other words, it may point to an interaction between metabolic processes associated with products generated by the symbionts upon light reception and the sensorimotor system comprising the neural pathway from the eyespots to the ciliary motor system. We discuss this point in greater detail in the General Discussion.\u003c/p\u003e\u003cp\u003eIn contrast to the clear change observed in the satiation group, results for the deprivation group ran counter to prediction: phototaxis did not increase and was instead attenuated overall (.00001\u0026thinsp;\u0026lt;\u0026thinsp;\u003cem\u003eps\u003c/em\u003e\u0026thinsp;\u0026lt;\u0026thinsp;.044). In the deprivation group, effects were evident from Day 1. On Day 4, however, no difference from the control group was detected, for reasons that remain unclear. Because the deprivation manipulation did not yield a stable, systematic pattern comparable to that of the satiation group, it was not included in subsequent experiments.\u003c/p\u003e\u003c/div\u003e\n\u003ch3\u003eExperiment 2. Pre-exposure does not reduce spontaneous locomotion\u003c/h3\u003e\n\u003cp\u003eA potential limitation of the previous experiment was that the attenuation of phototaxis under the satiation manipulation could not rule out debilitation caused by prolonged illumination. Experiment 2 therefore tested this possibility by recording whether five days of pre-exposure to light altered spontaneous locomotion (Fig.\u0026nbsp;\u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e3\u003c/span\u003ea). It was visually confirmed that none of the individuals introduced into the apparatus were dead. If the reduction in phototaxis was caused by debilitation resulting from prolonged light exposure, spontaneous movement would also be expected to decrease.\u003c/p\u003e\u003cp\u003e\u003c/p\u003e\u003cp\u003eNo significant effect of the satiation manipulation on 10-minute distance traveled was observed, \u003cem\u003et\u003c/em\u003e(25.89)\u0026thinsp;=\u0026thinsp;1.39, \u003cem\u003ep\u003c/em\u003e\u0026thinsp;=\u0026thinsp;.18. Thus, five days of light pre-exposure did not substantially alter spontaneous locomotion. Interindividual variability was large, with total distance ranging from near immobility to nearly 100 cm. Such long-distance movements were observed in both groups, suggesting that prolonged light exposure did not affect locomotor ability.\u003c/p\u003e\u003cp\u003eThe observed locomotion was perhaps not based on taxis. In this setup, illumination was applied as uniformly as possible from above, and seawater salinity was kept constant, making chemotaxis unlikely. Although \u003cem\u003eP. naikaiensis\u003c/em\u003e is known to exhibit positive geotaxis (Sakagami, Watanabe, Ikeda, \u0026amp; Ando, \u003cspan citationid=\"CR39\" class=\"CitationRef\"\u003e2021\u003c/span\u003e), geotactic behavior was unlikely to be predominant in our setup because the dish was positioned as horizontally as possible. No external stimuli were provided except for the transfer of individuals into the dish by the experimenter.\u003c/p\u003e\u003cp\u003eTaken together, across the first two experiments, we found that phototaxis is attenuated by light pre-exposure, and that this attenuation is not attributable to debilitation from excessive illumination.\u003c/p\u003e\n\u003ch3\u003eExperiment 3. Twenty-four hours of light exposure switches the sign of phototaxis\u003c/h3\u003e\n\u003cp\u003eProlonged illumination in \u003cem\u003eP. naikaiensis\u003c/em\u003e may not only induce nutritional satiation but also be harmful, because excessive light can damage the symbiotic algae. In Experiment 1, there was no shaded region, so the animals had no opportunity to avoid light. In Experiment 3, therefore, we used a platform with illuminated and shaded regions and continuously illuminated \u003cem\u003eP. naikaiensis\u003c/em\u003e for five days (Fig.\u0026nbsp;\u003cspan refid=\"Fig1\" class=\"InternalRef\"\u003e1\u003c/span\u003eb, right). This setup allowed us to assess whether P. naikaiensis spontaneously avoided light depending on the duration and amount of light they had received.\u003c/p\u003e\u003cp\u003eAs a result, \u003cem\u003eP. naikaiensis\u003c/em\u003e was found to switch the sign of phototaxis according to the duration of illumination (Fig.\u0026nbsp;\u003cspan refid=\"Fig4\" class=\"InternalRef\"\u003e4\u003c/span\u003ea). Animals that had been introduced approximately evenly into the illuminated and shaded regions moved mostly into the illuminated region within the first hour (Fig.\u0026nbsp;\u003cspan refid=\"Fig4\" class=\"InternalRef\"\u003e4\u003c/span\u003eb), consistent with the positive phototaxis observed in Experiment 1. By around 24 hours, phototaxis had reversed: of the roughly 90% of individuals in the illuminated region, about 30% shifted to the shaded region. Thereafter, occupancy of the illuminated region remained between 40% and 60%.\u003c/p\u003e\u003cp\u003e\u003c/p\u003e\u003cp\u003eWas the 2\u0026ndash;5 day trajectory of occupancy in the illuminated and shaded regions driven by the same individuals remaining stationary? Because measurements were taken every 10 minutes, precise individual tracking was not possible in this experiment. However, video inspection confirmed that \u003cem\u003eP. naikaiensis\u003c/em\u003e occasionally engaged in spontaneous movements (\u003cspan class=\"ExternalRef\"\u003e\u003cspan class=\"RefSource\"\u003ehttps://youtu.be/k6m0-yHovJg\u003c/span\u003e\u003cspan address=\"https://youtu.be/k6m0-yHovJg\" targettype=\"URL\" class=\"RefTarget\"\u003e\u003c/span\u003e\u003c/span\u003e). Therefore, it is more likely that the observed occupancy patterns reflect active repositioning in accordance with the history of sustained illumination rather than the same individuals remaining in place.\u003c/p\u003e\u003cp\u003eThe observed behavior supports the view that the phototaxis of \u003cem\u003eP. naikaiensis\u003c/em\u003e can be modulated by prior experience with light exposure. Specifically, they can reverse their default behavioral mode of positive phototaxis to its opposite. The possible adaptive significance of this flexibility is discussed in the following section.\u003c/p\u003e\u003cdiv id=\"Sec8\" class=\"Section2\"\u003e\u003ch2\u003eGeneral Discussion\u003c/h2\u003e\u003cp\u003ePhototaxis has traditionally been regarded as a mechanical and reflexive behavior. This study critically examined this assumption by testing whether phototaxis is adjusted according to a history of light exposure in this species. In \u003cem\u003eP. naikaiensis\u003c/em\u003e, which derives nutrition from photosynthesis by symbiotic algae, phototaxis functions as a foraging behavior. Across three experiments, we evaluated experience-dependent modification of phototaxis in \u003cem\u003eP. naikaiensis\u003c/em\u003e. Phototaxis was attenuated by prior light exposure. Moreover, when a shaded option was available, the animals reversed the sign of phototaxis and spontaneously avoided illumination.\u003c/p\u003e\u003cp\u003eOur results are consistent with Matsui and Hata (\u003cspan citationid=\"CR29\" class=\"CitationRef\"\u003e2025b\u003c/span\u003e), which suggested an interaction between the metabolic system and the sensorimotor circuitry that controls phototaxis. In that study, animals were presented with a two-arm choice, and choice proportions matched light intensity; however, after 24 hours of prior illumination, choices became more at random. In the present work, changes in responsiveness to light appeared by 48 hours in Experiment 1 and by 24 hours in Experiment 3. Although the procedures and apparatus differed, both sets of findings demonstrate plasticity in behavior that appears reflexive at first glance. In acoelomorphs, the symbiotic algae in \u003cem\u003eP. naikaiensis\u003c/em\u003e are not intracellular but are embedded in the intercellular spaces of subepidermal tissues (Arboleda et al., \u003cspan citationid=\"CR3\" class=\"CitationRef\"\u003e2018\u003c/span\u003e; Bailly et al., \u003cspan citationid=\"CR5\" class=\"CitationRef\"\u003e2014\u003c/span\u003e). The route by which amino acids and other products generated by photosynthesis enter the body remains unresolved, but it is likely that metabolic processes modulate the sensorimotor system that produces phototaxis.\u003c/p\u003e\u003cp\u003eNotably, our Experiment 3 showed that \u003cem\u003eP. naikaiensis\u003c/em\u003e actively avoided light under sustained illumination. Why would an animal that derives nutrition from photosynthesis by its symbiotic algae need to avoid light? A plausible reason is that excessive light is likely harmful to the symbionts (Barber \u0026amp; Andersson, \u003cspan citationid=\"CR6\" class=\"CitationRef\"\u003e1992\u003c/span\u003e). Prolonged high-intensity illumination induces photoinhibition, a decline in photosynthetic efficiency. This arises from toxic by-products generated during photochemical reactions, whose removal is required for repair. Under ordinary conditions, repair mechanisms maintain algal viability; however, when strong light coincides with environmental stress, repair cannot keep pace, photosynthetic capacity is lost, and in the worst case, cells die. Accordingly, altering the sign of phototaxis in accordance with cumulative light exposure appears adaptive for safeguarding both the animal and its symbiotic algae.\u003c/p\u003e\u003cp\u003ePotentials for behavioral modification through interactions between metabolic and sensory systems offers useful insights into the seemingly endless debate over what counts as cognitive behavior. The attitude that elevates human cognition as the absolute standard has become increasingly untenable (Lyon, \u003cspan citationid=\"CR25\" class=\"CitationRef\"\u003e2020\u003c/span\u003e; Matsui \u0026amp; Hata, \u003cspan citationid=\"CR28\" class=\"CitationRef\"\u003e2025a\u003c/span\u003e). Instead, the view that cognition is widespread across the living world even beyond the animal kingdom, including non-neural organisms such as plants and fungi, is gaining currency (Crippen, \u003cspan citationid=\"CR10\" class=\"CitationRef\"\u003e2020\u003c/span\u003e; Garz\u0026oacute;n \u0026amp; Keijzer, \u003cspan citationid=\"CR15\" class=\"CitationRef\"\u003e2009\u003c/span\u003e; Lyon, Keijzer, Arendt, \u0026amp; Levin, \u003cspan citationid=\"CR27\" class=\"CitationRef\"\u003e2021\u003c/span\u003e). Cognition was originally conceptualized as a computational and representational process detached from the body (Adams \u0026amp; Aizawa, \u003cspan citationid=\"CR1\" class=\"CitationRef\"\u003e2010\u003c/span\u003e; Fodor, \u003cspan citationid=\"CR13\" class=\"CitationRef\"\u003e1983\u003c/span\u003e; Foder \u0026amp; Pylyshin, 1984). However, that conception is no longer sustainable, and a perspective is taking hold that cognition is mostly related to sensorimotor systems, bodily morphology, and modes of contact with the surrounding medium (Barton \u0026amp; Barrett, \u003cspan citationid=\"CR7\" class=\"CitationRef\"\u003e2025\u003c/span\u003e; Segundo-Ortin, \u003cspan citationid=\"CR41\" class=\"CitationRef\"\u003e2020\u003c/span\u003e). Even phototaxis cannot simply be excluded from cognition as an innate stimulus response behavior; it must be understood as a regulatory process carried out in ongoing interplay with experience. This, in turn, may offer a clue to what forms of cognition became possible in animals with the advent of nervous systems.\u003c/p\u003e\u003cp\u003eOur study has limitations. In Experiment 1, we found no effect of deprivation; that is, we did not succeed in identifying a motivational manipulation that increased the strength of phototaxis. There is also a discrepancy between Experiments 1 and 3. In Experiment 1, 24 hours of prior illumination did not alter phototaxis, whereas in Experiment 3, within 24 hours approximately 30% of individuals reversed the sign of phototaxis and moved from the illuminated to the shaded region. The present study cannot determine the source of this quantitative difference.\u003c/p\u003e\u003cp\u003eFinally, state-dependent adjustment of behavior has been documented across diverse taxa. In humans, hunger, an internal physiological state, modulates cognitive and behavioral responses to current environmental stimuli (Loeber, Grosshans, Herpertz, Kiefer, \u0026amp; Herpertz, \u003cspan citationid=\"CR23\" class=\"CitationRef\"\u003e2013\u003c/span\u003e). In fishes, aggressive interactions alter hormonal dynamics and subsequently influence behavior (Earley, Lu, Lee, Wong, \u0026amp; Hsu, \u003cspan citationid=\"CR12\" class=\"CitationRef\"\u003e2013\u003c/span\u003e). In arthropods, sawflies adjust the trade-off between food discovery and predation risk according to prior starvation experience (Singh, Wolthaus, Schielzeth, \u0026amp; M\u0026uuml;ller, \u003cspan citationid=\"CR42\" class=\"CitationRef\"\u003e2023\u003c/span\u003e). These flexible adjustments are mediated by interactions between central neural information processing and peripheral bodily states. Our study suggests that \u003cem\u003eP. naikaiensis\u003c/em\u003e may possess mechanisms for monitoring its own nutritional status and the health of its algal symbionts, although how this information is conveyed to the sensorimotor system that generates phototaxis remains unknown. Given that Acoelomorpha likely retain a primitive form of neural organization among bilaterians, such mechanisms may have already been present early in bilaterian evolution and could have contributed to the emergence of goal-directed behavior.\u003c/p\u003e\u003c/div\u003e"},{"header":"Declarations","content":"\u003cp\u003e\u003cstrong\u003eFunding\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eThis research is not supported by any funding agency.\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eAuthorship contribution\u0026nbsp;\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eHM: Conceptualization, Methodology, Software, Formal analysis, Investigation, Writing – review \u0026amp; editing, Writing – original draft. YM: Conceptualization, Methodology, Writing – review \u0026amp; editing. All authors reviewed the manuscript.\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eDeclaration of generative AI usage\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eDuring the preparation of this work the authors used chatGPT 5.0 in order to refine the manuscripts written by non-native English speakers. After using this tool/service, the authors reviewed and edited the content as needed and takes full responsibility for the content of the publication.\u003c/p\u003e\u003ch2\u003eData Availability\u003c/h2\u003e\u003cp\u003eAll data and code are publicly available at https://github.com/HeathRossie/naikaiensis_phototaxis.\u003c/p\u003e"},{"header":"References","content":"\u003col\u003e\u003cli\u003e\u003cspan\u003eAdams F, Aizawa K (2010) Defending the bounds of cognition. In: Menary R (ed) The extended mind. 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Hydrobiologia 227(1):273\u0026ndash;282. \u003cspan class=\"ExternalRef\"\u003e\u003cspan class=\"RefSource\"\u003ehttps://doi.org/10.1007/BF00027612\u003c/span\u003e\u003cspan address=\"10.1007/BF00027612\" targettype=\"DOI\" class=\"RefTarget\"\u003e\u003c/span\u003e\u003c/span\u003e\u003c/span\u003e\u003c/li\u003e\u003c/ol\u003e"}],"fulltextSource":"","fullText":"","funders":[],"hasAdminPriorityOnWorkflow":false,"hasManuscriptDocX":false,"hasOptedInToPreprint":true,"hasPassedJournalQc":"","hasAnyPriority":true,"hideJournal":true,"highlight":"","institution":"","isAcceptedByJournal":false,"isAuthorSuppliedPdf":false,"isDeskRejected":"","isHiddenFromSearch":false,"isInQc":false,"isInWorkflow":false,"isPdf":false,"isPdfUpToDate":true,"isWithdrawnOrRetracted":false,"journal":{"display":true,"email":"[email protected]","identity":"researchsquare","isNatureJournal":false,"hasQc":true,"allowDirectSubmit":true,"externalIdentity":"","sideBox":"","snPcode":"","submissionUrl":"/submission","title":"Research Square","twitterHandle":"researchsquare","acdcEnabled":true,"dfaEnabled":false,"editorialSystem":"","reportingPortfolio":"","inReviewEnabled":false,"inReviewRevisionsEnabled":true},"keywords":"phototaxis, acoelomorpha, behavioral flexibility, avoidance","lastPublishedDoi":"10.21203/rs.3.rs-7802934/v1","lastPublishedDoiUrl":"https://doi.org/10.21203/rs.3.rs-7802934/v1","license":{"name":"CC BY 4.0","url":"https://creativecommons.org/licenses/by/4.0/"},"manuscriptAbstract":"\u003cp\u003e Many animal behaviors are executed with appropriate form, direction, and intensity in accordance with an individual\u0026rsquo;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 \u003cem\u003ePraesagittifera naikaiensis\u003c/em\u003e 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 \u003cem\u003eP. naikaiensis\u003c/em\u003e 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 \u003cem\u003eP. naikaiensis\u003c/em\u003e is robust but not purely mechanical. It is adaptively tuned to the individual\u0026rsquo;s history of light exposure.\u003c/p\u003e","manuscriptTitle":"Long-term continuous light exposure alters phototaxis in the acoel flatworm Praesagittifera naikaiensis","msid":"","msnumber":"","nonDraftVersions":[{"code":1,"date":"2025-10-09 05:53:17","doi":"10.21203/rs.3.rs-7802934/v1","editorialEvents":[{"type":"communityComments","content":0}],"status":"published","journal":{"display":true,"email":"[email protected]","identity":"researchsquare","isNatureJournal":false,"hasQc":true,"allowDirectSubmit":true,"externalIdentity":"","sideBox":"","snPcode":"","submissionUrl":"/submission","title":"Research Square","twitterHandle":"researchsquare","acdcEnabled":true,"dfaEnabled":false,"editorialSystem":"","reportingPortfolio":"","inReviewEnabled":false,"inReviewRevisionsEnabled":true}}],"origin":"","ownerIdentity":"7856678d-1345-4e25-b0a4-05a7a25fbcc4","owner":[],"postedDate":"October 9th, 2025","published":true,"recentEditorialEvents":[],"rejectedJournal":[],"revision":"","amendment":"","status":"posted","subjectAreas":[],"tags":[],"updatedAt":"2026-02-19T09:10:09+00:00","versionOfRecord":[],"versionCreatedAt":"2025-10-09 05:53:17","video":"","vorDoi":"","vorDoiUrl":"","workflowStages":[]},"version":"v1","identity":"rs-7802934","journalConfig":"researchsquare"},"__N_SSP":true},"page":"/article/[identity]/[[...version]]","query":{"redirect":"/article/rs-7802934","identity":"rs-7802934","version":["v1"]},"buildId":"8U1c8b4HqxoKbykW_rLl7","isFallback":false,"isExperimentalCompile":false,"dynamicIds":[84888],"gssp":true,"scriptLoader":[]}