How do electric fish make electricity?

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1:06Caroline Steel:Does he have teeth? Yeah, little ones. He has teeth. Baby ones, yeah. What if he bites me? No, he will not do this. Welcome to CrowdScience on the BBC World Service. I'm Caroline Steele and I'm about to feed an animal that could leave me needing a few days off work. He's called Phoenix. So you can take this. Luckily, I'm with two scientists who have spent their careers studying creatures like him. I'm scared. Hi, Phoenix. And this is the sound of Phoenix's superpower. He can generate electricity.

1:56Caroline Steel:Oh my gosh, he's so powerful.

2:03Caroline Steel:I found myself here thanks to a voice message from one of our listeners. Hi, BBC World Service. My name is Mohamed. I live in Cameroon. I recently learned there is an electric fish, which I found so surprising. The question to cloud science is how and why does this fish produce electricity? I also want to know what happens if this fish touches a cable. Thank you, Mohamed. To me, electric fish are almost magical. They can generate and sense electric fields in water. So why do they do it? How does it help them thrive in their watery world? And perhaps even more intriguingly, how do they do it? The fish I've heard most about, and it's probably the most well-known electric fish, is the electric eel.

3:00Caroline Steel:Now, as far as I know, I've never come across one myself. But we've found someone who's spent more than 25 years studying them.

3:11I have a fascination, like obsession by electric animals and electricity

3:17Caroline Steel:This is Nonato Mendez Jr, an ecologist from MACPA in northern Brazil We have a lake, we have a lot of electric ewes in that lake So since my childhood, fishing with my friends, I see electric ewes in the invasive environment It's not an uncommon animal here Could you describe to me sort of the environment that you find them in? The electroreus occurs mainly in the lakes, floodplains and small streams, but not in the main channel of the rivers. In a more slow flow water, that place has electroreus. Are they hard to spot? You know, if we went out with you into the Amazon, how would you go about finding an eel?

4:00So if you go to the Amazon and go to a lake, maybe you can encounter an ewe. I see a lot of eels bellow, wood, homes we call palafitas here in Amazon. And the people build this above the water.

4:18Caroline Steel:Wow. OK. I guess I tried to share an environment. Both people and the eels are trying to fish from shallower areas. How common is it that people get shocked? Most of the reports are not recorded. It's a common thing. So the accident occurs when the people are fishing in the shallow water with one eel. It's not a group of eels. We know electric eels are mega predators. Nonato has first-hand experience of how powerful an electric discharge or shock can be. So we catch the eel, take off my rubber gloves, and I use only those very, very thin rubber gloves to take samples. Nonato picked up an electric eel wearing only thin gloves.

5:09I take the eels from the floor, I make one step with the eel in my hand, and I feel the discharge.

5:20Caroline Steel:Oh gosh! I feel the discharge passing to my left arm, to my chest. Even with my eye open, the whole vision is black. It went black. Did you faint or anything? Or did you manage to stay standing? So after I recovered my vision, I say to my field assistant, please give me some coffee. In the grand scheme of things, Nonato got off lightly. He only needed a coffee and not medical attention after his shock. Now, fortunately, not all electric fish are quite as formidable as the electric eel.

6:02Caroline Steel:So how many electric fish species are there? Oh. Lots? A lot. Over 300, I guess. This is Susie Proschke, a PhD student in the Department of Animal Physiology at the University of Bayreuth in Germany. We're in a windowless room in a basement and behind us their electric eel Phoenix is swimming around in a big tank. So there are several groups of electric fish, like the weekly electric fish. So they produce a small amount of electricity and of course the big ones, the strong electric fish. like the electric eel. So they use this high voltage. Also in the room here with us is Stefan Schuster, professor of animal physiology.

6:51So the strongly electric fish, they use their strong high voltage discharges to hunt or to defend themselves. And the weakly electric fish, they use it to navigate, to sense the environment in the dark or in turbid waters and to communicate with other members of their species.

7:08Caroline Steel:So what's the difference between weakly electric fish and strongly electric fish? I mean, I'm guessing one is stronger. Yeah, basically the voltage. You don't feel anything if you touch into a tank with them. Yeah, you don't feel anything in the weakly electric fish, but in the strongly, definitely you will. So there are weakly electric fish and strongly electric fish. Weakly electric fish produce lower voltage shocks to navigate and communicate, while strongly electric fish can produce shocks with a higher voltage to help them catch dinner. And the electric eel produces the highest voltage of them all.

7:43So the highest voltage they measured, I guess, was 860 volt.

7:49Caroline Steel:That's a lot. That's more than mains electricity, right? So maybe you can imagine the household socket. So in Germany, we've got 230 volt. So, yeah, three times higher. So you could charge my phone really quickly. Yeah. While weekly electric fish can produce shocks of mere millivolts. And in the room next door, Susie has a weekly electric fish to introduce us to. What species is this? So this is Gymnotus carappo. Wonderful. Where are they found? They are found also in South America and they are related to the electric eel. Are they? They are knife fish. Knife fish? Yeah. Susie is luring what essentially looks like two long metal sticks into a tank containing a small black fish shaped like the blade of a knife.

8:46Caroline Steel:OK, so you've put something in the tank. What have you put in the tank? We've put some electrodes in the tank to record these EODs. What's an EOD? It's an electric organ discharge. But first you have to put it on the amplifier. The electrodes are wired up to a plastic box outside the tank. It's an amplifier with a speaker so we can hear the electrical discharges. And now we can put on the speaker. OK. And then you can hear it.

9:23Caroline Steel:So is it pulse, pulse, pulse, pulse, pulse, pulse, pulse, pulse? Oh, wow.

9:32Caroline Steel:Oh, when you knock on the tank, you can hear it. Yeah, yeah. So it's moving then. You might be able to hear that the knife fish is producing a nearly constant stream of electrical pulses. It's using electricity to paint a picture of the environment around it. And when Susie changes that environment by tapping on the tank glass, the stream of pulses becomes more frequent.

10:00Caroline Steel:wow okay so it's it it's almost hard to hear the individual pulses but you can just because they're going so quick wow that feels like that must be a lot of effort do these fish need to eat more than non-electric fish i know it seems like hard work i don't know it's right yeah yeah it's got a lot of energy but no they they can they eat normal but one thing that you probably have noticed in the recording when suzy knocked on the wall yeah then they increase the discharge rate they produce more discharges per time which gives them more information per time about the environment because something interesting happens they they're so cool it's called a novelty response and can be used to determine how good they hear how good they see how good they sense because the fish tells you something if he notices something then he tells it so is it a bit like if you're If you're taking a video and you want a clearer picture, you'll take more images and you'll get higher resolution.

11:03Caroline Steel:If something interesting is happening in the knife fish's environment, like another fish swims past or Susie taps on the glass, it sends out more frequent pulses so it can get more information about what's going on. Because the commotion could be the difference between finding dinner and becoming dinner. Speaking of dinner, it's time to feed our electric eel, Phoenix. Now, Phoenix isn't pretty. He's one and a half metres long, red and black, with two tiny beady eyes and a mouth which looks a little bit like a human trying to force a smile. No offence, Phoenix. So first we have to put the electrodes inside the tank.

11:49So did you hear the one pulse?

11:52Caroline Steel:On the go. Is that him electro sensing? That's a sim, yeah. Oh, he's, as he's swimming up to the electrode, he's... Now he recognized the environment. So needs less information. Yeah, but this is low voltage, so it's not high voltage. Like our knife fish, Phoenix is using low-voltage pulses to build a picture of the tank around him. We've got here some deep frozen fish and I put it on the hot water. It smells. Yeah, but he loves it. He loves it. But you need some gloves. Gloves, yes please. Susie hands me some huge rubber gloves that go up to my elbow and a small silver fish to feed to Phoenix OK, so...

12:44Caroline Steel:I'm scared! Hi Phoenix! Come on!

12:57Caroline Steel:When I dip my hand plus fish into the tank Phoenix rushes towards me And it's fair to say, I jump out of my skin. That's OK. That's OK. That's OK. So was he trying to shock the fish? That was the sound of him giving out a strong electric pulse. Yeah. As I'm feeding Phoenix, I'm triggering his hunting response. He's sending out huge electrical pulses to stun his prey, which today is a total waste of energy as I'm feeding him a dead fish. But still, an impressive effort, Phoenix.

13:33Caroline Steel:Now, how exactly does he do this? That's what we'll be finding out next.

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14:46Caroline Steel:You're listening to CrowdScience from the BBC World Service, the show powered by your science questions. Later in the year, it's our 10th anniversary. I genuinely can't believe time has gone this quickly. and as the show is entirely driven by you listeners we also would love you to help us celebrate so we want to know what's your favourite ever episode of CrowdScience and why? did you have a question answered? what did that mean to you? did an episode of CrowdScience change the way you thought about something? or maybe inspire you in some way? we want to hear all of your CrowdScience stories email them to crowdscience at bbc.co.uk or whatsapp us a message or voice note to PLOS44 8000 314 773.

15:35Caroline Steel:This week, we're answering a question from listener Mohamed in Cameroon. He wants to know why and how electric fish generate electricity. So far, we've discovered why fish have this superpower. It helps them work out what's going on around them. And for some fish, like our electric eel Phoenix, it can be used to stun or even kill prey. But that leaves the other half of Mohammed's question. How on earth does a fish produce electricity in the first place? Let's go back to Susie and her electric eel Phoenix again. So what is going on inside electric fish? How do they produce electricity? So they've got an electric organ.

16:22Caroline Steel:An electric organ? Yes. No, not that sort of organ. How big is his electric organ, do you think? So almost 20%, everything is in front of his head. So every organ, the heart, the brain, everything is made up 20 % of his body. And the 80%, everything is electric organ. So he's 80 % electric organ. So all of his other organs are squashed into his head area. And then beyond that, he's just one giant electric organ. So he's actually kind of like a small fish with an electric organ hanging on the back. Yeah, you can say that, yeah. OK, and what does the electric organ look like? Is it like a liver?

17:08Caroline Steel:Now, don't worry. We're not going to cut Phoenix open to have a look at his electric organ. Instead, Susie has a photo of a slice through electric eel. See the swim bladder? Oh, OK. So there's a sort of right in the middle of the slice of the electric eel. So the kind of the top half is muscles to get him around, to make him move. Yeah. And beyond there are these are the organs, the electric organs. So this is the main organ. Most of the round cross section is taken up by a white fleshy mass, the electric organ. So it's sort of the electric organ looks to me almost like like when you cut through a grapefruit.

17:49Caroline Steel:Yeah. It has segments. Each of those segments is packed full of what looks like stacks of coins, but they're actually electric cells. Harold Zacon is a neuroscientist and evolutionary biologist at the University of Austin in Texas, and he spent decades studying these electric cells, or as he sees them, tiny batteries. The cell is called an electrocyte. An electrocyte, OK. Each of the electrocytes is a battery. It's functioning like a battery. Living organisms generate electricity. We generate electricity. And in fact, all of our excitable tissues, neurons, muscle, those are essentially little batteries.

18:39Caroline Steel:Like electrocytes, muscle cells also produce tiny electrical pulses. Each of our muscles is composed of many what are called muscle cells or muscle fibers. Each one of those generate a small amount of electricity to trigger the contraction that a muscle makes. Electric organs generally evolved from muscle. And so muscle is already making electricity. It was just a matter during evolution of doing a few things. One is making the muscle not contract anymore. Otherwise, every time the fish discharged, it would be twitching. Then there are anatomical changes to the basic muscle to make it insulated.

19:34So the electricity will all flow in one direction. and to have the timing from the nervous system such that they all go off at once and they are activated at the same time.

19:49Caroline Steel:So billions of years of evolution have transformed twitching muscle cells into electric cells or electrocytes, which stack on top of one another to produce electricity. In the strong electric fish, the cells were flattened like pancakes and stacked up So more and more cells could add up and give a strong discharge. That's really the main difference between a strong and weak electric fish is the number of cells. If you think of a flashlight with, let's say, 9-volt batteries in it, the more batteries you put, you know, if you have two 9-volt batteries in series, you have 18 volts. If you have three, you have 27 volts.

20:38So the more of these batteries you have, the more voltage it can generate. So the electric organ has cells that are each generating as much voltage as they can, which is still only a few thousandths of a volt.

20:56Caroline Steel:In a flashlight or torch, the more batteries you add, the brighter the light. The same is true with electrolytes, which combine their voltage to produce a larger shock. Having many cells in a line with each other allows the current to flow along the whole length of the electric organ. And there's connective tissue around the electric organ to keep all the current inside until it reaches across the skin and the current can go into the water.

21:35Caroline Steel:So if an electric eel wants to work out what the watery world around it looks like, it might deploy 100 electric cells to create a small electrical pulse. But if it wants to stun a tasty-looking fish, it might use hundreds or thousands of electrocytes. So we now know how and why electric fish generate electricity. But Mohammed had one other question for us. What happens if an electric fish comes into contact with human-made electricity? And actually, a very similar question of why an electric fish doesn't shock itself was first asked thousands of years ago by the ancient Egyptians. Who knew, of course, the electric catfish from the Nile and wondered themselves why they don't shock themselves.

22:24The question is very old, but so far the answer to it has never been really satisfactorily given.