No night lights for baby anemonefish

If you have ever looked up at the night sky and thought, ‘that's not very dark at all’, you have experienced the phenomenon known as artificial light at night, also known as light pollution. From streetlights to headlights, this human-made illumination can have profound impacts on living things. Just as scrolling on your smartphone before bed can disrupt your sleep, artificial light can disrupt the biology of the natural world. From metabolism to reproduction, animals’ biology is hardwired to follow cues from the natural light–dark cycle. With light pollution increasing globally by nearly 10% in the last decade, the problems it causes for wildlife and ecosystems are a growing concern. Coastal areas are especially vulnerable as they are often in close proximity to densely populated shoreline cities, and coral reefs may be among the most affected as many reef-dwelling species are highly sensitive to – and reliant on – a daily light–dark cycle. With the consequences of night-light on coral reefs in mind, Thibault Roost from the University of Melbourne, Australia, along with an international team of researchers, set out to understand what light pollution does to one of the reef's more vulnerable residents: baby orange-fin anemonefish (Amphiprion chrysopterus).To find out how artificial light impacts baby orange-fin anemonefish, the researchers SCUBA-dived into a lagoon on the French Polynesian island of Mo'orea every 1–2 days over the course of a month to monitor 19 sets of fish parents and locate their nests. When the team identified the nests, they deployed underwater LED lights near the nests, such that the eggs (and the baby fish inside) would experience artificial light at night at an intensity that mimicked the light pollution that reefs experience from coastal cities. They shone the light on the babies every night for the duration of their time in the egg (from fertilization to hatching). The researchers photographed the eggs and used computer software to measure the size of the egg and the amount of yolk inside from the images. They found that night-light decreased the size of the egg, indicating that light pollution limits how large the babies can grow. Light pollution also decreased the size of the yolk sac, which stores nutrients to feed the fish throughout its time in the egg. A smaller yolk sac suggests that the young fish will burn through its nutrients very quickly, potentially leaving it undernourished before hatching.The team decided to take the study one step further to look behind the scenes at why light pollution makes babies smaller and hungrier. To investigate, the researchers collected 30 eggs from the hundreds of eggs in each nest and brought them back to the lab, where they video recorded the heartbeat of the baby fish within the egg to calculate the heart rate. They found that light pollution sped up the fish's heart rate by nearly 10 times, indicating that light pollution kicks the body into a state of stress. The scientists suggest that this may be the culprit causing the embryos to stay smaller and burn through their nutrient reserves more quickly.Roost and the team made the fascinating discovery that light pollution changes the biology of orange-fin anemonefish babies, ultimately stressing them out and limiting their growth. This study sheds light on how our illumination of the night sky impacts the natural world around us – even underwater animals. So, let's help brighten the future for our wildlife by keeping the nights dark and remembering to turn off the lights.

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