PREVIEW

BY MARK WORGAN

Artificial light from coastal developments is keeping coral reef fish and may be affecting their brains, potentially putting these valuable ecosystems at risk, according to new research.

Scientists from Bar-Ilan University in Israel found that even relatively low levels of artificial light at night can significantly alter the behaviour and physiology of fish living on coral reefs.

The study, published in Current Biology, examined the effects of light pollution from sources such as cities, ports, roads and hotels on the blue-green damselfish (Chromis viridis), a common species found on coral reefs.

Researchers found that fish exposed to artificial light slept less, experienced more fragmented sleep, became more aggressive and fed at unusual times, effectively behaving as though night had become day.

"Artificial light at night is rapidly expanding across coastal environments worldwide," said Prof. Oren Levy of Bar-Ilan University's Faculty of Life Sciences and the H. Steinitz Marine Biology Laboratory in Eilat, who co-led the study with Prof. Lior Appelbaum and doctoral student Shachaf Ben-Ezra, from the Bar-Ilan Faculty of Life Sciences. "We found that even relatively low levels of illumination can disrupt natural sleep patterns and are associated with changes in markers of neuronal health."

The research team studied blue-green damselfish in both laboratory conditions and on coral reefs in the Gulf of Aqaba/Eilat in Israel.

Using infrared video cameras, machine-learning tracking systems and field observations, the scientists first established that the fish display clear sleep-like states, including inactivity, distinctive resting postures and reduced responsiveness to their surroundings.

However, when exposed to levels of artificial light commonly found in coastal environments, those patterns changed dramatically.

The fish no longer remained within their usual sheltering areas inside corals during the night. Instead, they expanded their range of activity, fed at unusual hours, interacted more aggressively, and spent significantly less time sleeping.

To investigate the biological impact of these behavioural changes, researchers examined neurons in a part of the brain associated with sleep-related functions.

Fish exposed to artificial light at night showed increased levels of markers linked to DNA damage when compared with fish living under natural dark conditions.

While the study does not prove that artificial light directly damages DNA, the researchers say the findings suggest that disrupted sleep may interfere with the brain's normal repair and maintenance processes.

The changes were detected after only a few nights of exposure and continued throughout a five-month field experiment conducted on a coral reef, indicating that long-term exposure to light pollution could have lasting effects.

"Sleep is a critical period for biological repair," said Prof. Appelbaum. "Our findings suggest that disrupting sleep with artificial light may have measurable consequences even in wild marine animals."

The researchers noted that approximately 22% of the world's coastal regions and 35% of marine protected areas are already affected by artificial light at night.

In the Gulf of Eilat, where the study was carried out, light levels near developed coastal areas can be up to 60 times brighter than natural starlight because of urban growth and port activity.

Previous research by Prof. Levy's team found that artificial light can affect coral health, disrupt the relationship between corals and algae and interfere with the timing of coral spawning.

The latest findings suggest that fish living within those ecosystems are also being affected, raising concerns about wider consequences for reef environments.

"Coral reefs depend on tightly connected biological interactions," Levy said. "If artificial light is affecting both corals and the fish that depend on them, the consequences could ripple throughout the reef ecosystem."

The levels of artificial light used during the study were comparable to those recorded near developed coastlines, highlighting the relevance of the findings to real-world conditions.

Researchers are calling for improved coastal lighting management, including reducing unnecessary nighttime illumination, directing light away from shorelines and coastal waters, using smart lighting technologies and developing guidance on lighting wavelengths that minimise ecological disruption.

Future studies will investigate whether the effects can be reversed and how widespread they may be among other marine species.