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Losing Nemo? Nighttime Light Pollution Can Stop Clownfish From Hatching, Study Shows

Oceans
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Light pollution is increasing near coral reefs as coastal developments expand. Some bungalows are even built above the reef with clear floors, so that tourists can watch the fish at night. But that means the fish can see the light from the bungalows, too.


A group of Australian researchers set out to investigate how that light might impact one of reef's most iconic residents: the clownfish. Their results were dramatic: When clownfish eggs were exposed to artificial light at night (ALAN), not a single one hatched.

"The overwhelming finding is that artificial light pollution can have a devastating effect on reproductive success of coral reef fish," lead study author and Flinders University Research Associate Dr. Emily Fobert said in an email to EcoWatch.

The results, published in Biology Letters Wednesday, show that the fish are very sensitive to even low levels of artificial light.

"When ALAN is present, no eggs hatched but when the light was removed during the recovery period, eggs from the ALAN exposure hatched like normal, so the presence of light is clearly interfering with an environmental cue that initiates hatching in clownfish," Fobert said.

To achieve their results, Fobert and her team examined 10 pairs of common clownfish (Amphiprion ocellaris) in a lab, as National Geographic explained. Each pair had its own tank. Half of the tanks were lit like a natural coral reef: light for 12 hours and dark for 12 hours. The other half were exposed to nighttime light levels similar to those a typical coastal town would emit. The researchers then studied the fish for 60 days to determine how the differing light levels impacted spawning, egg fertilization and egg hatching. They found no change to spawning or fertilization, but a surprisingly extreme change to hatching.

"I wasn't expecting the result [in the paper] to be that nothing hatched," Thomas Davies, a Bangor University in Wales conservation ecologist, told National Geographic. "It's quite worrying … a really big result that speaks to how light pollution can have a really big impact on marine species."

Clownfish are so sensitive to light because they spawn around the full moon and their eggs typically hatch after sunset, according to a Flinders University press release. The nighttime hatching likely keeps the new babies safe from predators, National Geographic explained.

"These findings likely extend to other reef fish as many share similar reproductive behaviors, including the timing of hatching during early evening," Fobert said in the release.

Davies thought the study's implications for clownfish could be severe.

"Zero percent hatching is essentially no recruiting to the next generation and could cause extinction in a species. It's quite profound," he told National Geographic.

The paper authors wrote that more work needed to be done to see how light levels might impact clownfish in the wild, but ALAN is a growing concern. A 2017 study found that the earth area lit by artificial light at night grew by around 2 percent per year from 2012 to 2016.

Senior author, founder of the conservation group Saving Nemo and Flinders Prof. Karen Burke da Silva explained that the clownfish study began to fill an important knowledge gap.

"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," she said in the press release.

Correction: A previous version of this article said that 10 clownfish were used in the experiment. It has been corrected to reflect the fact that 10 pairs were used.

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