Undersea cables are the big arteries to our energy grid. But apparently, they can also affect marine life. According to a new study, the cables act as an irresistible lure for female shore crabs, while the males remain completely indifferent.
This draw could derail their ancient reproductive migrations, creating an unforeseen ecological trap.
Shore crabs (Carcinus maenas) are one of the most common species of crabs. They’re native to the Atlantic Ocean and Baltic Sea but have expanded to become one of the most common invasive species in the world. But in their natural environment, they play a key role.
They’re tough, common, and ecologically vital, but they are also known to be sensitive to environmental stress, making them an excellent bioindicator — a kind of canary in the aquatic coal mine.
Females can produce up to 185,000 eggs, holding them on their bellies. The larvae can disperse over great distances, carried by both ocean currents and ship hulls. But where the females go with their eggs matters a lot for their species.
Crabs and Coils
Elizabeth James and colleagues from the University of Portsmouth brought 120 crabs into the lab for testing. They got 60 males and 60 females, and built a crab-sized testing arena: a long, rectangular acrylic tank filled with seawater kept at the precise temperature and salinity of the crabs’ home harbor. At one end of this tank, they placed a powerful set of Helmholtz coils that mimicked the electromagnetic field produced by undersea cables at different strengths, from a moderate 500 microteslas (µT) to a powerful 3200 µT, a level realistic for areas right near unburied or shallowly buried cables.
The tank was effectively divided into three invisible zones. The “COIL Zone” at one end had the highest EMF, the “FAR Zone” at the other end had almost none, and a “MID Zone” served as a transition area. During a 10-minute trial, cameras equipped with tracking software watched every move the crabs made. The researchers gathered data on how long the crabs moved and where they stayed.
One by one, each crab was placed in the center of the tank and set free to explore its new environment, unaware it was being presented with a choice between business-as-usual and a strange, invisible lure.
When the coils were off, the crabs wandered around the tank more or less randomly. There didn’t seem to be any preference of any sort. But the moment the power was switched on, a dramatic split in behavior emerged. The male crabs didn’t seem to notice. They continued their aimless meandering, showing no real preference for one part of the tank over another.
But the females did change.
A Broken Compass
The female crabs started to make a beeline for the COIL Zone, the area with the strongest electromagnetic field, and they stayed there. At the tested EMF levels, females spent around two times more time in the high-EMF zone compared to the control group.
The attraction was clear. The female crabs actively sought out the field and lingered within its influence. They spent less time in the FAR Zone, preferring the area with an electromagnetic field.
“This is the first study to demonstrate sex-specific responses to submarine power cable electromagnetic fields in crabs. The fact that we’re seeing such clear behavioural differences between males and females, even at relatively low, as well as high electromagnetic field strengths, suggests that we need to think much more carefully about how offshore energy infrastructure might be affecting marine ecosystems differently than we previously thought,” James said in a press release.
So, Why Does This Matter?
According to the researchers, even small behavioral changes could have massive, cascading consequences for the entire coastal ecosystem.
Female shore crabs undertake critical seasonal migrations.
They move from intertidal zones to deeper, more stable subtidal waters to spawn and release their larvae. The timing and destination of this journey are essential for the survival of the next generation. Now, imagine a string of power cables lying directly across this ancient migratory path. This new research suggests that instead of completing their journey, the females might be drawn to the cables, where they could aggregate, disoriented and delayed.
This disruption could throw the entire reproductive cycle into chaos. A delayed migration could mean larvae are released at the wrong time or in the wrong place, drastically reducing their chances of survival. If large numbers of females are effectively “trapped” along the cable routes, it could also skew sex ratios at the vital spawning grounds, further hampering reproductive success.
Carcinus maenas is also a keystone species. Crabs are voracious predators and scavengers, keeping other populations in check and mixing up sediment on the seafloor. They are also a vital food source for fish and seabirds. A significant decline in the crab population would send shockwaves up and down the food chain.
Why Only Females?
The question, of course, is why. Why are females susceptible why the males are not? The study didn’t attempt to answer this, but researchers have some theories.
The answer may lie in the fundamental biological differences linked to their reproductive roles. In many migratory species, from sea turtles to spider crabs, females often show stronger navigational instincts and site fidelity, particularly when it comes to nesting or spawning. Their internal compass may be more finely tuned, making it more easily scrambled by artificial magnetic noise. Males, whose reproductive strategy is often less tied to a specific location, may simply have a less sensitive system.
Still, regardless of why this is, it’s an important alarm call.
The methods we currently use for environmental risk assessment are often a one-size-fits-all model, disregarding things like sex differences. As this research shows, that is a dangerous oversimplification. A test that only used male crabs, or that averaged the results of both sexes, would have concluded that EMFs have a negligible effect. The authors argue that sex must be considered a key variable in future ecological assessments for any offshore infrastructure. Other species could also be similarly affected.
There are currently 1.4 million km (870,000 miles) of telecommunication cables on the seafloor. That’s a very small part of the regions inhabited by shore crabs, but this impact has not yet been thoroughly assessed.
The findings are, however, important to consider for offshore wind farms, which can add new cables to the seafloor. This should be considered for offshore wind projects, the researchers conclude.
The study was published in Environmental Science & Technology Letters.
