Red light mitigates ALAN-induced memory impairment in the clownfish Amphiprion ocellaris

Abstract Artificial light at night (ALAN) is an increasingly pervasive anthropogenic pollutant in coastal ecosystems. However, its effects on brain function and behaviour in reef fishes remain poorly understood. Here, we investigated the impact of ALAN on learning and memory in the clownfish, Amphiprion ocellaris , a diurnal reef fish with well-characterised photoreceptive systems. Cognitive performance was assessed using a colour discrimination task following exposure to ALAN of varying intensities and spectral compositions. ALAN impaired learning and memory after only two consecutive nights of exposure, with effects emerging at intensities as low as 0.5 lux. At higher intensities (5–50 lux), memory was consistently disrupted under white-cool, white-warm, blue, and green LED light, whereas red light had no detectable effect. Together, these findings demonstrate that ALAN rapidly alters cognitive performance in clownfish and that these effects are strongly wavelength-dependent. Our results highlight the vulnerability of diurnal reef-associated fishes to nocturnal light pollution and underscore the need for ecologically informed lighting strategies that use longer-wavelength red-emitting light sources to reduce both the intensity and spectral impact of ALAN in coastal environments.