A clashing colour combination with deadly consequences

Humans have a powerful ability to illuminate the night sky. We leave streetlights on from dusk until dawn, operate businesses through the night and light our homes well after sunset. In a world where natural light cycles govern the rhythm of life, we have created a major disruptor known as artificial light at night. In some cases, this light pollution we create is considered ‘diffuse’, such that there are many light sources originating from multiple directions and therefore weak shadows are cast in the environment (imagine an illuminated soccer pitch). In other cases, we create ‘direct’ sources of light that cast many dark shadows in the environment (imagine a lighthouse). In a fascinating new study, Kathryn Bullough and her team of researchers from the University of Exeter, UK, investigated the impacts of both diffuse and direct light pollution on the sea roach (Ligia oceanica), a marine crustacean that changes colour to blend in with its surroundings and avoid the prying eyes of predators. The researchers discovered that illuminated nights trigger an intriguing clash between sea roach behaviour and their colour-changing tactics at night.Sea roaches live near the shore, where light from major coastal cities illuminates the underwater world. To understand how brighter nights impact these crustaceans, the team first collected dozens of sea roaches from the rocky shoreline of Swanpool Beach, UK. Afterwards, the team placed the animals into a pitch-black box to activate their colour-changing superpowers, encouraging them to turn as dark as possible. The researchers then transferred the sea roaches into buckets in which half of the bottom was lined with black gravel and half with white gravel. Some buckets were exposed to a direct source of light that projected strong shadows among the textured gravel floor, whereas others were exposed to diffuse light that obliterated any shadows. Over a 15-min observation period, the crustaceans chose to spend more time on black gravel. Moreover, when shadows were available, sea roaches actively chose to be in shadowy regions of the bucket, suggesting they had a clear preference for a dark background. This is where things got interesting. When the sea roaches experienced direct lighting, they generally stayed darker in colouration – sometimes becoming even darker than before to better match their coveted black and shadowy background. In contrast, sea roaches experiencing diffuse lighting became lighter in colour, even when on a dark background. Thus, the diffuse light pollution causes sea roaches to mix up which colour they should become – a mishap that leaves them especially visible and vulnerable to predators.The team decided to take the study one step further and explore the movement of sea roaches in their buckets. The team predicted that sea roaches would move quickly but erratically when they clashed with their background to draw less attention to themselves. Bullough and colleagues hypothesized that if the crustaceans camouflage well with their environment, they would have no reason to move in such a stealthy manner. Indeed, when faced with diffuse lighting – where shadowy hideouts on both white and dark backgrounds were few and far between – the sea roaches were clever enough to make fast and irregular movements on white backgrounds, but if camouflaged, they hardly moved.In the end, this study sheds light on the effects that our artificially brightened nights have on underwater animals. The team showed that artificially brightening our nights leads to colour-changing confusion in sea roaches, which can have major consequences on their ability to survive. Let's all turn down our lights at night and give sea roaches a chance to shine – just not too brightly.

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