An automated high-throughput platform reveals chromatic phototactic disruption in Daphnia magna exposed to the UV filters benzophenone-3 and octocrylene

Modern imaging enables real-time observation of behavior under stress. Light is a major stressor for fish larvae and aquatic invertebrates, yet the effects of different wavelengths and intensities on behavior remain poorly understood. The freshwater zooplankton Daphnia magna, a key model species, typically displays negative vertical phototaxis to avoid predation. This study applied a high-throughput video-tracking system to assess chromatic phototactic responses in D. magna exposed to two common UV filters, octocrylene (OC) and benzophenone-3 (BP-3), at concentrations from 0.1 to 1000 μg/L. A custom chamber was designed with two experimental setups: (i) a horizontal rack of six 30 mL arenas to examine vertical phototaxis across wavelengths, and (ii) a 200 mL arena to test color preference. Illumination was provided by five LEDs (red, green, blue, UV-A, white) with infrared backlighting, and tracking performed using an infrared GigE camera with Python-based analysis. Control assays showed adult D. magna preferred blue and white light, followed by green, red, and UV. The strongest negative phototaxis occurred under UV-A, the weakest under red. Locomotor activity was greatest under white and lowest under red. Chemical exposure altered these patterns: OC at 10 μg/L increased negative phototaxis and reduced movement, whereas BP-3 at 100-1000 μg/L enhanced both. UV filters also shifted color preference, increasing selection of UV over other wavelengths, blue over white, and green over blue. Such behavioral disruptions could influence diel vertical migration, foraging, and trophic interactions, potentially increasing zooplankton predation and reducing algae grazing. This platform provides a robust high-throughput approach to evaluate locomotor responses of aquatic organisms under varying light conditions, offering new opportunities to uncover how chemical pollutants disrupt visual perception and behavior in aquatic ecosystems.