Falmouth promenade at night with a clear sky, with streetlights creating direct light that spills onto the rockpools below (right)
News — Artificial night-time light confuses a colour-changing coastal woodlouse, new research shows.
The sea slater is an inch-long woodlouse that lives around the high-tide line and is common in the UK and Europe.
Sea slaters forage at night and can change colour to blend in and conceal themselves from predators.
The recent research, conducted by the University of Exeter, examined the impacts of a singular light source (which creates distinct shadows) and "scattered" light (resembling the "urban glow" prevalent in urban areas).
Although the solitary light did not disrupt the camouflage of sea slaters, the dispersed light resulted in their paler appearance when concealed against a dark backdrop, rendering them more noticeable.
"As nocturnal skies become increasingly illuminated across the globe, comprehending the implications for the natural ecosystem becomes crucial," emphasized Kathryn Bullough, the study's leader during her master's program at the Centre for Ecology and Conservation on Exeter's Penryn Campus in Cornwall.
"While we are aware that artificial light has various detrimental effects on animals and plants, our findings demonstrate that light sources casting shadows can have distinctly different consequences compared to diffuse skyglow, even if their overall brightness is similar.
“Under point-source light, the sea slaters turned darker and sought out refuge on dark stones and shadows when available.
“But diffuse light caused them to become lighter, even though this would make them easier for predators to spot.”
In the study, sea slaters were placed in a circular chamber with white pebbles covering half of the floor and black pebbles on the other half.
They were then exposed either to point-source or diffuse light.
Regardless of the type of light, sea slaters exhibited a higher duration of stay on the black pebbles, as these closely resembled their original coloration and thereby offered enhanced camouflage.
Under point-source light, they also turned darker – better matching the black pebbles.
But under diffuse light, they became on average 27% lighter.
"We cannot pinpoint the exact cause, but it is evident that diffuse light disrupts the sea slaters' color-matching mechanism," noted Dr. Jolyon Troscianko, who is also affiliated with the Centre for Ecology and Conservation.
“One possibility is that they respond to this light as if dawn was approaching.
“We know that nocturnal animals have very sensitive light vision, so skyglow will appear very bright to them.”
With human populations clustered in coastal areas, shorelines are disproportionately affected by light pollution.
While sea slaters are not a threatened species, they are commonly eaten by shore birds such as oystercatchers and gulls.
"Given the significant decline of numerous shorebird species, it is vital for us to comprehend how human activities impact their prey within the context of a complex ecosystem," emphasized Dr. Troscianko.
The research received partial funding from the Natural Environment Research Council (NERC).
The paper titled "Artificial light at night causes conflicting behavioural and morphological defense responses in a marine isopod" has been published in the journal Proceedings of the Royal Society B.
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