Crabs mesmerised by electric cables

Concern for crabs who interrupt their normal behaviour in response to underwater electromagnetic fields...
19 October 2021

Interview with 

Alastair Lyndon, Heriot Watt University

Crab in rocks

crab in rocks


As we race to replace natural gas, people are increasingly looking to parts of our coastline as places where the wind blows relatively reliably, meaning that it’s a good venue for wind turbines. But, oddly enough, the biggest fans of these big fans turn out to be...crabs! Scientists have noticed that the cables running along the seabed to transport the electricity from offshore to the mainland can affect the behaviour of local crabs, as Harry Lewis heard from Heriot Watt University’s Alastair Lyndon...


Alastair - They're mesmerized, as far as we can tell at the moment, by electromagnetic fields, which we've simulated in the lab using magnets and using a kind of a coil, which produces a magnetic field in the water.

Harry - I mean, this seems like a strange investigation off the top of my head because there aren't that many cables in the sea, are there Alastair?

Alastair - There are a few at the moment, but there's likely to be a lot more soon because of the growth, particularly around the UK of offshore power generation. So that is sort of renewable energy either from offshore wind farms or tidal turbines and these sort of things, and they need cables. And so we anticipate there's going to be larger numbers of cables being deployed into the future.

Harry - Ok, and how do you go about looking into this?

Alastair - We did two types of experiments. One was just with electromagnets under the tanks, which could be switched on or off. And then we also have, as I say, these large coils of wire, which we can produce around bigger tanks of maybe hundreds of liters, which we can put larger crabs into.

Harry - So am I right in thinking that you have your tanks of crab, you got to cable running through them and then above it, you're taking pictures, aerial photographs, or film of the crabs and seeing how they react to you turning on this magnetic field.

Alastair - Yeah, that's correct. We would normally run the magnetic field over a 24 hour period, and we would then look to see what the crabs activity was over that period, compared to a control where the magnets are switched off. When the magnets is switched on the crabs, move about much less than we would normally expect them to do. They generally move around more at night than in the daytime anyway, but when the magnets are on, they move around a lot less at night than they normally would do.

Harry - So we know that they have a behavioral response to electricity running along cable. Do we know why they're attracted to that magnetic field?

Alastair - We don't really know, but there's quite a bit of evidence from a variety of different animal groups that many creatures can detect the Earth's magnetic field and use it for navigational purposes, brown crabs move over quite long distances. Certainly the male crabs, we know, move over tens to hundreds of kilometers in relatively short periods of time. So it seems a reasonable surmise that they perhaps use the Earth's magnetic field in these sort of movements. And that the magnetic fields around cables are actually detected by that mechanism and perhaps more attractive than the, the background magnetic field.

Harry - That's interesting because if they're traveling so far, these brown crab, there's actually quite a, I would assume, a high likelihood that they're going to come into contact with cables of offshore wind farms.

Alastair - That's the concern really. I mean, interestingly until very recently, we weren't really aware that crabs moved around much. And I think the assumption was that they were perhaps quite territorial in this stage, quite close to home. One would anticipate it's something to do with reproduction and finding females, I guess, in a biological sense, it probably helps to avoid inbreeding. And we find that, of course, again, a lot of animals, you get dispersal of one or other sex as they mature. And so it seems likely that that's what's going on here, that the males are dispersing and looking for females with which to mate, helping to mix the gene pool, if you like.

Harry - Why is it important? What effects is it going to have out in the world, out in the ocean?

Alastair - There are various possibilities. And as I said, we don't really know because we haven't looked, but we imagine that there could be issues of accumulations of crabs in the same area. It might result in unintended consequences of, for example, attracting fishing activity into the vicinity of cables, which might not be good for the cables themselves, it might also result in higher catch rates of the crabs. So that could potentially be negative in the longer term as well. And the movement of males, if it is important in terms of the reproductive biology of the crops may disrupt reproduction in some populations where the supply of male crabs is being reduced for whatever reason.


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