Blue Light at Night Disrupts Fish Behavior and Transgenerational Offspring Traits
Artificial light at night (ALAN), particularly blue light, disrupts fish behavior after only a few nights. After just five days of blue light exposure, zebrafish displayed anxiety-like behaviors, and their offspring exhibited decreased activity despite never being exposed themselves. This research highlights how light pollution disrupts biological rhythms in animals and may have lasting ecological effects.
Behavioral Changes in Exposed Fish
The study examined female zebrafish exposed to all-night light at 10 light regimes: nine separate wavelengths across the visible spectrum as well as white light. Lights were set at 20 lux, approximately the intensity of streetlights seen at a distance, and what animals would be exposed to in outdoor environments. After eight nights of exposure, all wavelengths caused fish to swim less, stick closer together, and spend more time near the aquarium's wall — a behavior known as thigmotaxis or wall-hugging, which is an indicator of animal anxiety.
However, the effect of blue light could be seen sooner, after only five days of ALAN exposure, with light at 470 nm having the strongest effect of all. As co-author Aneesh Bose notes, this is consistent with what is known in humans: exposure to the blue light of electronic displays has the biggest effect on sleep and possibly other physiological cycles. The researchers speculate that sleep deprivation could underlie the behavioral changes they observed, noting that fish "could pull a few all-nighters, but after too many nights of disrupted sleep it eventually caught up to them."
Transgenerational Effects on Offspring
The study also revealed that the impacts of light pollution did not end with the individual fish but were passed down to offspring. After exposure to ALAN, the study's female zebrafish were allowed to breed, and the team raised their offspring under natural light conditions. After 15 days, the researchers tested the swimming behaviors of larvae using specialized automated tracking software designed to quantify activity levels of the tiny fish. Offspring of exposed mothers showed decreased daytime movement despite themselves never being exposed to lights at night.
"We found that light pollution disrupted the natural behavior of fish, and this disruption may have fitness and performance consequences," says Ming Duan, the study's final author from the Institute of Hydrobiology Chinese Academy of Sciences. The offspring of exposed mothers displayed less frequent movement and shorter movement distances, regardless of the spectral treatment. Researchers speculate that sleep deprivation may underlie the patterns in their data, though the experiment was not designed to uncover a mechanism.
Study Methods and Wavelength Effects
The research team, led by scientists from the Institute of Hydrobiology Chinese Academy of Sciences and the Max Planck Institute of Animal Behavior, exposed female zebrafish to all-night light at 10 light regimes: nine separate wavelengths across the visible spectrum as well as white light. The study examined both direct effects on the exposed fish and transgenerational effects on their offspring.
The team found the strongest impact at short wavelengths (365 to 470 nm), with individuals and groups of zebrafish showing more anxiety-like behavior after fewer nights of ALAN exposure relative to the other wavelengths. Blue light at 470 nm caused the fastest and strongest behavioral changes, with effects visible after only five days of exposure. All wavelengths eventually caused fish to swim less, stick closer together, and spend more time near the wall after eight nights.
Mitigation and Ecological Implications
The dangers of blue light are well-documented, and this study extends those concerns to aquatic ecosystems. Many of the places we light up at night are close to animal habitats. The best thing we can do is to minimize the use of blue wavelength light sources where animals are trying to sleep. Reducing blue wavelength light in animal habitats can mitigate these effects, the authors say.
Artificial light at night pollutes the environment by adding luminescence to places that would otherwise be dark at nighttime. ALAN exists outdoors through the lights that brighten streets, buildings, and industrial areas all night, and ALAN exists indoors through the devices that hold our attention into the evening. ALAN is known to impact most organisms by disrupting the natural rhythms of biological processes, which are coordinated by cycles of light and dark.
"Sleep is one of the main processes of animals that is disrupted by ALAN, so we were curious to know what that means for their ability to navigate their lives," says Wei Wei Li, the study's first author who did the work as a doctoral student in MPI-AB. "The light levels that we used in our study match what is already shining into the homes of animals at night through the many sources we place outdoors. And we found extremely strong and clear negative effects on the behavior of fish and their offspring after only a few bright nights."
The study was published in Science of the Total Environment and is open access. The original research, titled "Behavioural and transgenerational effects of artificial light at night (ALAN) of varying spectral compositions in zebrafish (Danio rerio)" by Wei Wei Li et al., examined the direct and transgenerational effects of 10 different wavelength treatments of ALAN on behavior in zebrafish, a diurnally active model organism.
Conclusion
After just a few nights of blue light exposure, zebrafish displayed anxiety-like behaviors, and their offspring exhibited decreased activity despite never being exposed themselves. The research highlights how ALAN disrupts biological rhythms in animals and may have lasting ecological effects. These findings emphasize the need to reduce blue light pollution near animal habitats. As the authors conclude, special attention needs to be paid to what light is emitted by human sources, particularly blue wavelength light, near animal sleeping areas.
The takeaway is straightforward: minimize blue light sources near where animals sleep. The study found extremely strong and clear negative effects on fish behavior and their offspring after only a few bright nights. For ecosystems and animal welfare, reducing ALAN — especially short-wavelength blue light — seems like a sensible step.
