Historical Exposure to Artificial Light at Night Shapes <i>Daphnia</i> Responses: An Experiment Across an Urban–Rural Gradient

Artificial light at night (ALAN) is an increasingly pervasive environmental stressor, yet its long-term impacts on organisms are poorly understood. We used <i>Daphnia pulicaria</i> to investigate how historical exposure to ALAN influences life-history and morphological responses when exposed to ALAN under laboratory conditions. The experiment spanned three generations and included clonal populations from historically exposed and unexposed lakes. Our results show that historically exposed populations have longer lifespans and clutch intervals than unexposed populations, irrespective of the presence or absence of laboratory ALAN. In the presence of laboratory ALAN, individuals from historically exposed populations mature earlier and produce more offspring, whereas individuals from unexposed populations show no changes in maturation or offspring production but have reduced spine length. Under laboratory ALAN, all populations have shorter clutch intervals and lifespans, reduced reproductive success, and smaller eyes and body sizes. Interestingly, the negative effect of laboratory ALAN on reproductive success intensifies across generations for historically unexposed populations, whereas the effect weakens for exposed populations. These findings indicate that historically exposed populations have partially adapted to ALAN whereas unexposed populations respond maladaptively and suffer reduced reproductive success across generations. This emphasizes the need to explore the evolutionary impacts of light pollution, its ecological consequences, and the potential of organisms to adapt to the stressor.