If you’ve seen the movie “Finding Nemo” or its charming sequel, then you’re probably familiar with the clownfish in the film. In real life, though, Nemo’s counterparts live in coral reefs threatened by climate change, and now researchers have found a new challenge for the little fish to face: light pollution.

An intriguing study from Flinders University and the University of Melbourne in Australia finds that the clownfish fail to reproduce when exposed to artificial light at night (ALAN), as they would be when living in coral reefs near coastal communities. Remove the light, and the fish go back to flourishing.

The problem is, there seems to be more and more of the artificial light. It’s affecting the clownfish, and likely others, the scientists said in a recent paper published in Biology Letters.

“Coastlines near reefs are exposed to artificial light at night by increasing numbers of housing developments, promenades, ports, harbors and dockyards- which use LED lights that penetrate into seawater,” explained the research team, led by Dr. Emily Fobert. “While tourist hotspots include floating accommodation above coral reefs, some of these fancy overwater bungalows even have glass floors with lights shining directly on the reefs below so guests can see the fish at night.”

The scientists – well aware of the disruptions that ALAN is causing to some species on land – wanted to see what was happening when that artificial light penetrates into the undersea world of marine life and disrupts the day-night balance the fish have known across history.

So the Australian teams selected 10 mating pairs of the clownfish, also known as Amphiprion ocellaris, to observe what happens to reproduction when the fish are exposed to artificial light at night. Half of the clownfish were exposed to a 12-hour day of light, followed by dim lighting for the 12-hour night. The other half were left with the natural rhythm of dark nights and bright days.

What they found was that there was no difference in the rates at which the fish spawned, or in their egg fertilization success, but there was an alarming difference in whether or not the eggs hatched.

For the clownfish that were left to the natural light rhythms, the success rate was 86 percent. For the fish that were dealing with the artificial LED light, the success rate was … zero.

No eggs hatched at all.

Yet when they put the fish back into normal conditions in the lab, the capacity for clownfish to breed bounced right back. “There was no difference in the proportion of eggs hatched between ALAN and control groups during the recovery period,” the paper said, “indicating that this negative effect of ALAN on hatch rate disappeared as soon as the impact period ended, and light regimes were restored to ambient conditions.”

The findings likely extend to other reef fish in the same family, and many species are known to share similar reproductive patterns, including the clownfish timing mechanism of hatching the eggs during early evening.

The scientists say more study is needed beyond the lab conditions, and more broadly on the impacts of artificial light in marine ecosystems.

“Artificial light at night is becoming a greater concern among ecologists, as light is spreading globally, and the impacts on organisms can be severe, but very little research has been done around ALAN in the marine environment,” said senior author Dr. Karen Burke DaSilva.

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