Artificial lighting in harbours is attracting sea creatures that damage ships and boats, a study suggests.
Scientists believe that the night-time illumination is altering the behaviour of some animals that attach to vessels' hulls. Keel worms, for example, are lured in by the lights.
Other shade-loving animals were deterred by the brightness, seeking darker waters elsewhere.
"The presence of lighting at night can change the composition of these marine communities," said Dr Thomas Davies, an ecologist from the University of Exeter, UK.
"There is also what we call an 'ecosystem disservice'. The presence of artificial lighting might actually increase 'fouling' species that can damage boats."
Scientists estimate that just under a quarter of the world's coastal regions, excluding Antarctica, experience artificial lighting at night. Harbours, marinas, oil rigs and fisheries contribute to the glow.
To study the impact, researchers looked at sessile creatures: small animals that live in sediment or fix to hard surfaces. The group includes mussels, barnacles, sponges and sea squirts.
When these invertebrates are at the larval stage of their life cycle, they use light to seek out the best place to cling to, where they will then spend their adult years.
Dr Davies said: "That settlement process - that process of finding the right habitat - is guided in many species by the intensity and spectral quality of lighting."
To investigate, the team built artificial rafts and exposed them to different lighting conditions.
They were placed in the Menai Strait, which lies between the island of Anglesey and the northwest coast of Wales.
They found that some species, including a small bristle-like creature called Plumularia setacea,and a colonial sea squirt, preferred darkness; and few settled on the well-lit rafts.
Others, such as keel worms, were drawn by the light, which is unfortunate for the shipping industry - an influx of these animals can slow down boats and they are expensive to remove.
The scientists now want to look at how lighting affects a wider range of marine organisms, and to quantify exactly how much disruption is it causing.
"If it is causing a significant problem in harbours and marinas, there are certain things that can be done," explained Dr Davies.
"Avoiding using artificial light is one of them, but, if necessary, we need to conduct studies to find what types of artificial lighting are likely to be causing these effects.
"In terrestrial environments, some species' responses are dependent on the spectra of light you are using - so shorter wavelengths tend to be more attractive to moths than longer wavelengths, for example. And we may find something similar in marine environments."