Clownfish may look like coral reefs' natural entertainers, but they don't like being in the spotlight. It appears their shy nature extends to their hatching, as they won’t even leave their eggs if the lights are too bright. This means coastal developments, often accompanied by a lot of light spilling into coastal waters, could be a menace to clownfish, and quite likely most other forms of reef fish as well.

It’s well known that coral reefs are under threat from rising temperatures, ocean acidification, overfishing, and runoff from the shores. Dr Emily Fobert of Flinders University wondered if there might be an additional danger we've been missing – light pollution, which she calls ALAN, or Artificial Light At Night.

“There is a lot of research on the effects of ALAN on terrestrial organisms,” Fobert told IFLScience, “but very little looking at the impact on the marine environment. I found that surprising because fish are not immune to exposure to light.” Fobert is aware of two studies on the effects of ALAN on fish, but both are on behavior such as foraging, not reproduction.

Fobert decided to test clownfish because “they are site-specific. If they have settled in one location and they are exposed to light, they will not move away.” 

When Fobert shone 21 lux onto the surface of the waters beneath which clownfish were living, she found none of their eggs hatched. However, a batch in the same conditions, light exposure aside, hatched normally, she reported in Biology Letters. Fobert told IFLScience it is common for lights on piers to shine 200 lux of light onto the waters below them and 21 is a conservative estimate of the exposure reefs close to tourist establishments can be exposed to.

With sufficient awareness of the problem, this may be a much easier problem to solve than many of the other threats coral reefs face. Fobert’s work was done with relatively blueish LED lights, which penetrate the water more than longer wavelengths. She’s currently doing a follow-up study using warmer-colored sources, but does not yet have the results to see how much it helps. However, she noted that on land, the effects on many species can be minimized by changing wavelengths.

Other alternatives include switching off coastal lights for a few nights a month or several hours a night. Fobert is hopeful on the latter, noting that many reef fish hatch a few hours after sunset and therefore might not need a whole night of darkness to leave their shell.

One way or another, we’ll probably need to make some changes if we want to keep finding Nemo close to holiday sites.