Female crabs have displayed an attraction towards the electromagnetic fields emitted by underwater power cables, a reaction that scientists suggest could disrupt both migration patterns and population levels among crustaceans.

This new research – produced by scientists at the University of Portsmouth in the UK – shows that when it comes to crabs, the females are displaying a ‘significantly greater sensitivity’ to the electromagnetic fields (EMFs) produced by underwater power cables than males. 

The most critical finding was that these female crabs were affected by EMFs from cables already found in the ocean environment, and not just artificially high levels produced in laboratories. This could have a knock-on effect on plans for subsea power cables running from offshore installations, including those from wind farms.

It matters because millions of crabs migrate along coastlines each year to reproduce. Disrupting these patterns could affect marine food chains.

The study – conducted by PhD student, Elizabeth James from the University of Portsmouth’s Institute of Marine Sciences – exposed 120 juvenile common shore crabs (Carcinus maenas) to electromagnetic fields found near underwater power cables. The results showed significant behavioural differences between male and female responses.

James found that female crabs were – on average – almost twice as likely to stay near areas with electromagnetic fields compared to those without them (with results indicating somewhere between 87 and 131 per cent more time). Male crabs were not as affected by the electromagnetic fields, showing no consistent spatial preference across different strengths.

“This is the first study to demonstrate sex-specific responses to submarine power cable electromagnetic fields in crabs. The fact we’re seeing such clear behavioural differences between males and females, even at relatively low, as well as high electromagnetic field strengths – suggests we need to think much more carefully about how offshore energy infrastructure might be affecting marine ecosystems differently than we previously thought,” said James.

To reach their conclusions, the research team used a controlled laboratory setup with Helmholtz coils – a specialised equipment that produces magnetic fields – to generate precise electromagnetic fields ranging up to 3,200 micro teslas (levels which can be emitted by submarine power cables).

Over a sequence of ten-minute trials, researchers tracked crab behaviour including distances moved, zone preferences, and mobility levels using advanced video tracking software. 

The amount female crabs moved was also reduced by more than a third (38%) at moderate field strengths (1,000 micro teslas). There were even noticeable effects at low EMF levels below 250 micro teslas.

The findings have since been published in Environmental Science and Technology Letters and suggest that as offshore wind farms expand globally, the sex-specific impacts on crustacea could have significant ecological consequences.

If female crabs are drawn to stay near underwater cables instead of continuing their natural migrations, this could affect where they lay their eggs and ultimately impact crab populations along entire coastlines.

While underwater cables will cover less than 0.1% of the ocean floor by 2050, they could still cause major problems if they’re placed in the wrong spots – such as along routes where crabs migrate to breed. The researchers suggest studies now need to be conducted into how these cables affect male and female marine life separately to avoid the assumption that they respond in the same way.

Alex Ford from the University of Portsmouth’s School of the Environment and Life Sciences, and co-author on the paper, said: “The sex of an animal often gets overlooked when we look into the impact of pollution on aquatic life. With chemical pollutants, sometimes the males are more sensitive because females are able to excrete their contaminants with the eggs of offspring.

“In this instance, the female crabs were more sensitive which might be down to a unique ability to detect electromagnetic fields which we are exploring further.

“This research builds on our international efforts to incorporate the effects on behaviour from many types of pollution, when assessing human impacts on our ecosystems. As we rapidly expand offshore renewable energy to meet climate targets, we need to ensure we’re not inadvertently creating new environmental problems while solving others.”

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