What’s the deal with eels? – Lucy Porter, David Righton and Caroline Durif

Introduction

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Host Introduction

2:26Hello, I'm Robin Ince. And I'm Brian Cox. And this is the Infinite Mustard Seed Cage. The Infinite Mustard Seed Cage. Yep, because we followed exactly what you said you wanted to do this week. And I was as surprised as the producer was when you said you would love to do a show on the science of elves. Why elves are so mysterious. And how you'd only ever caught and eaten one elf. Elver. Yeah, elf. Baby eel. Well, that's not the questions we've got for you today. This is a lot more of a D&D related show.

2:56So, I'm afraid to tell you. Well, mustard seeds are fairy, not an elephant. Yeah, see, you have got a little bit of knowledge there. So, if we just drop the round about Legolas, Elrond and Hawk the Slayer, which is a pity. Because that means we won't have all the questions about Bernard Breslau. Who was an actor, younger people? You should know. Anyway, so... He's very odd, isn't he? Yeah. You're odd. Careful, I'm not averse to using a gullship spell. No one here knows about the gullship spell. I cannot believe. Totally the wrong one is.

3:27I do. Do you? I don't think you can do it because you'll get the verbal and somatic components required to make a water vessel fly. Oh, I'm going to choose my own adventure. Anyway, so today, what are we discussing, Brian?

Eel Discussion

3:38Today, we're discussing eels. Why is their life cycle so complex? How do they navigate on their long migrations? And where do they sit on the tree of life? And why are jellied eels not as popular as they used to be? To help us untangle the complexity of the eel, we are joined by a fish behaviourist, a fish ecologist, and taking to the surface of the river, someone who is the regular hook-a-duck lady at her school fates. Cheers. And they are. I'm David Wrighton, and I'm a scientist at the UK Centre for Environment, Fisheries

4:10and Aquaculture Science. I've been studying European eels for nearly 20 years, including tracking them across the Atlantic to the Azores and beyond. And one of my most memorable journeys was going for a short walk on a misty Welsh mountainside with my children, and turning that into quite a long and lost walk, emerging from the mist a couple of hours later than originally intended, and I've never been so glad to see a pay and

4:40display car park. See, Brian, that's like the kind of adventure Legolas would have.

4:47I'm Caroline DeReef, and I work at the Institute of Marine Research in Norway, but I grew up in France and studied eels in France, and I'm interested in fish migration, especially my favourite fish, that's eels, and not electric eels. Those are very different. And my weirdest voyage was when I was travelling with my family in Myanmar, and I lost my family and was trying to catch up with them and asked a kid.

5:20We were in a village, and I asked a kid along the side of the road if he had seen a sort of European-looking family. And he took me to his home and showed me his family, which was very sweet, but very awkward.

5:40Adorable. I'm Lucy Porter, I am a comedian, and I was trying to think of a long journey that I've done related to water, given that we're talking about eels and stuff. So the longest water journey I ever did was on the River Thames, and it was when I was in my 20s, and my friend was marrying an American woman, and we got on a boat at the Thames Embankment at midday, and they set sail. The couple were going to get married on the boat, and we were going to sail up and down

6:10until about 7pm. And they started handing out the champagne, and we all went, oh, this tastes a bit funny. We're all British people in our 20s. And they said, oh, yeah, because the Americans had organised it, and they said, oh, yeah, yeah, yeah, no, we're just having elderflower champagne for now, because we thought we'll wait to have alcohol till we get back. Now, if you know British people, in their 20s, spent seven hours, people, one person attempted to jump off when they spotted a pub. And we, I mean, I'd be fine with it now, but at that age, yeah, we all crawled off like

6:46we were sort of returning from an Arctic adventure, and it felt like a very long time. And this is our panel.

7:00I love that story. It reminds me of, I went to a Jewish wedding in Philadelphia, and the waiters had all been told, once all the toasts have been done, we don't need any more wine. Apart from that table, they're all from Europe.

7:16Carol, because I realised we actually weren't going to deal with it in the show, but because you've mentioned it, I want to know the difference between an electric eel and the eels that we're

Eel Definition

7:24going to be talking about. So what is that, apart from obviously the charge, what is the difference? They're completely different species, families, so they have nothing to do actually with anguillid eels, which is what we're going to talk about. And those are also called freshwater eels, and they live in both habitats, so in freshwater and saltwater, whereas electric eels, it's, yeah, it's actually not really an eel.

7:55Just completely different. The question has to be, then, what is an eel? We might as well start with a definition. So you mentioned they're fish already, but what actually specifically is it? Are they related to a salmon or something? Where do you go? Where's the common ancestor? Why do you always ask that? Every show you do on Nature, because you're a physicist, that one we did about chimpanzees, you said, how are they related to salmon? The salmon seems to be, it's whatever the last sandwiches you've had. What I mean is it's a non-fish-looking fish. Yeah, well, the name of the eels is anguilliforms, which is not really very helpful because it

8:30means something in the shape of an eel.

8:36That's probably a bad place to start. But there are hundreds of species of eels, many of which remain within the marine environment all the time. You'd think of things like moray eels or conga eels, or perhaps if any of you scuba dived, you might have seen some things called garden eels and that sort of thing. But the anguillid eels, which we're talking about today, are those that are also called freshwater eels, and there are about 20 species of those. If you were to try and go back and find a common ancestor with us, you'd need to go back about 500 million years, something like that, to what would be a jawless fish, a very primitive

9:11fish, similar to the hagfish. So that would be our common ancestor from all those years ago. But the anguillid eels that we're talking about are relatively recent in that context. They emerged only about 50 to 70 million years ago. And the European eel, which Caroline and I both study, emerged even more recently than that, probably about three to five million years ago. I love that. What is an eel? Something that looks like an eel. It gives us an overview of them.

9:42What's the smallest one? What's the biggest one? Well, the biggest anguillid eel is the mottled eel, which can reach about 200 centimetres in length, two metres, and weigh up to about eight kilograms in weight. So that's quite a sizable individual. It's taller than me, but it weighs less. Yeah. I'll just put that out there. But I would try and fight one. I like the fact that this is what we're going to use now for every other show we do about any form of living creature.

10:13We shall use the measurement of Lucy Porter. Now, of course, this is merely half a Lucy Porter, but full grown. It can be three Lucy Porters.

10:24Yeah. So the smallest, it's difficult to say. Probably, Caroline, you might know this one, but one of the tropical eels, I would imagine, that the temperate eels that we work on tend to be relatively large compared to some of the tropical species. Yeah. So you would think the maximum of the smallest eels, which is the maximum length, would be one metre. So, I mean, because the smallest would be the juveniles. So it's sort of hard. That's true. It's a silly question. It's sort of hard. It's one that's just been born. Yeah. It's the smallest one. Yeah. But the maximum size of, like, the eels we study, it would be one metre, and you rarely

11:01encounter those. So they're, like, usually 60 centimetres. This might sound like a silly question, but what's the defining characteristic? I mean, is it just like it's this thing that's basically a fish with no fins? Is that... What's the specification? Well, they do have fins. They have a long fin all along the top of their body, which joins the tail and then comes around under the belly. So they have this fin all the way along the top and along the bottom, and that actually enables them to swim backwards as well as forwards. They just change the wave of swimming that they're undertaking to do that.

11:34So they swim like a snake? Yes. Yes. It's a particular form of swimming called anguiliform swimming. LAUGHTER Which... Sounds very much like an eel. Yes. Yes. But they also have sort of pectoral fins. Those are the fins at the side of the body, which are used for changing their sort of angle to move upwards or move downwards or break or whatever. So they're not finless. They have very, very tiny scales, and they do produce a lot of mucus to cover those scales,

12:05which means that they have this sort of characteristic slipperiness. I read that some of them use the mucus to sort of protect themselves from scratches. So I've tried that tonight. LAUGHTER I apologise to whoever gets this chair afterwards, but is it a sort of protective layer or what's the function of the mucus? One very special characteristic about eels is that they can live out of the water for many hours. Is Michael Gove one?

12:36LAUGHTER I just always have my suspicions, but that's amazing. So they... So, yeah, and the mucus helps so that they don't dry up, and also it allows them to breathe, through their skin, somehow, to at least, yeah, to get some oxygen. So how long could they... How long can they be out of the water for? Well, if it's cold, if it's not, you know, in the sun, right in the sun, then they could stay. I mean, I've sometimes... Because they also have this ability to escape, which is crazy.

13:08If you have a tank, you really have to block all of the little holes. Otherwise, they'll find the tiniest one and then just, like, climb up the wall of the tank, and then the next day, you just find them on the floor. And I've found eels on the floor up to 24 hours, I guess, after I've left them. So... And that also... There are many stories when they go to reproduce at the time of their migration. Then they will... Sometimes, if they're in a landlocked lake, like...

13:42And then they're able to cross over land, if in wet grass, to reach the other... The stream or to reach the sea, actually. It's like British people going to Magaluf. That we like to reproduce, by the way. This is why they're so fascinating as well, because people have found them in the grass, trying to reach another water body. And maybe that's why also they're kind of scary and remind people of snakes, in addition to their anguillid form.

14:13Yeah, it's one of the many unique characteristics of eel. But coming back to this form, actually, I mean, one of the things that's worth saying is that eels exist in different forms. So when they are in their larval phase, they don't even look like eels at all. They're transparent. They have very tiny heads with very pointy teeth. They have a tiny little tail. It is Michael Goat. They drift through the ocean on ocean currents. They're sort of transparent as a predator avoidance mechanism.

14:43But this difference between the larval eel and the juvenile and the more adult forms is one of the reasons why it took such a long time. And to address the eel question as to where eels came from, because no one could connect where the larvae were found in the ocean and with the existence of the juvenile glass eels and then the yellow eels in rivers and streams and so on.

Eel Life Cycle

15:05What is the life cycle? I suppose there are so many species. There's a typical life cycle. But in terms of how long are they in the larval form, typically, when do they mature? Well, in European eel, the larval form might last up to two or three years. So it's another sort of unique feature, another exceptional sort of aspect to them. So when they hatch as a larva, they're probably about six millimetres long, and they might grow up to about 100 millimetres long in the course of that three-year journey across the Atlantic. And then they metamorphose into what are called glass eels.

15:35So that's a tiny replica of an eel, but transparent. And they migrate in their hundreds and thousands and millions up to the coast and then populate coastlines, estuaries, and streams and rivers. By transform, how abrupt is that transformation? How does it happen? It's a matter of days, isn't it, I think, Caroline? It was an accidental discovery. So the connection David is talking about between, there were these larvae that are called leptocephalus. Lepto means leaf, and cephalus means head.

16:08So they look like leaf. They're transparent, very fragile. And they were thought to be another species. And then it was in the 1800s, two Italian scientists caught some leptocephalus larvae and put them in an aquarium. And they saw this metamorphosis into little glass eels. And so that was, for them, it was a complete surprise. And they made the connection that this wasn't another species, but it was the eel larvae.

16:38Because before that, and this is like a 2,000-year-old mystery, Aristotle thought that eels were the example of spontaneous generation, meaning that they did not reproduce, but just that glass eels would come out of, he called it putrefaction of seaweed. Mice were meant to form in sawdust, weren't they? That was the thing, because if you had sawdust at the bottom of a barrel, you'd go, oh, there's mice there now. They must be made out of sawdust.

17:09Can I just talk about genitals for a minute? Yeah. I've been so good. Do you know what? Every show you've ever been on, you have eventually. It's just a bit earlier tonight. It's just to be Aristotle's or the eel's. Just everybody's. If you're going with eel genitalia, I'm happy. But did he not check and couldn't find any genitals and then kind of went, oh, well, then that's... Yes. That was the other part of the story, is that he never found any eggs in eels like you would find in cod or salmon.

17:41And the reason is that when they go to reproduce, so they spawn in the Sargasso Sea, so they migrate down the rivers and then disappear into the ocean, but they're still pre-adolescent, so they're not sexually mature, so they hardly have any developed gonads. So Aristotle never found any genitalia.

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22:31It's only the little larva that come back. Right, they die, probably. Most likely because they do not eat during that long journey, which can be up to 8,000 kilometers. So they have sex and then die. So they better not have a headache. Yes, that's it. It's this incredible level of just going, this is the only purpose, is to make sure more of you come forward to then only have this purpose. That is biology. Yeah, it's biology, yeah.

23:02It's a whole of biology. Exactly. And it can seem from a human perspective, you know, not having a hobby or, you know, all those other things. It can feel very unfair as far as I'm concerned. It raises so many questions, Dave, because that migration for a start, I mean, that's a monarch butterfly style migration.

Eel Migration

23:22So what do you say, it's 8,000 kilometers? Yeah, at its maximum it would be sort of 8,000 kilometers or so from the easternmost point in which Eels are found. The obvious question is how? Well, yeah, that's one of the things that people have been trying to work out for a long time. Ever since the Danish scientist Johan Schmidt discovered that the spawning area was in the Sargasso Sea and that journey across the Atlantic Ocean is something that's motivated, you know, generations of scientists, possibly since Aristotle, you know, including myself and Caroline, to try and understand a little bit more about that and how they do it.

23:57Because they must transform into this final version of an eel in this part of their life cycle. They start as a leptocephalus. They turn into a glass eel. They then become elvers and then yellow eels, which is where they spend the majority of their lives, which can be decades long in the case of the European eel. The oldest ever recorded was about 90 years old. Wow. But the sort of average age of eels in the UK is probably sort of 15 or so, or perhaps a little bit even higher than that. But silver eels are the migratory form, so-called because of their silvery belly, but they also have a dark upper side, the dorsal side.

24:33And they have special adaptations for migrating across those ocean distances. So when they, an individual sort of chooses, let's say it's the wrong word, but when it begins its migration, so that varies. It's not just when they are 10 years old or when they are 15 years old. It can be when they are 30 years old. Yes, yes, or even older. And this is one of the challenges in understanding eel population dynamics, because the relationship between the age of the eels leaving and the larvae that then come back is very, very difficult to untangle, what we call the stock recruitment relationship.

25:08So how do they migrate? You mean how do they navigate? Yeah. It's a huge challenge when you think about, they are distributed all over Europe and North Africa. So eels from northern Norway will have to congregate with eels swimming from Morocco, for example, and find a common place. And because we see that the genetic structure is just a big mix. So this is what happens. It's a big orgy in the Sargasso Sea.

25:39They all meet. And so they have to meet each other at the right time, at the right location, at the right depth. And it's just amazing. And so there's a series of cues that they have, both in time. So generally, they migrate during the fall. But that will be different according to where they are, because the Spanish eels will take a shorter time. And magically, they leave later.

26:11Yes. And then the Norwegian ones leave earlier in the season. And it's regulated by, we think, by photoperiod. So there's more light in Spain versus Norway. And so that kind of triggers the timing of their migration. And after that, to find their way to the spawning area, then there aren't many signposts. They do have huge, what we call olfactory bulbs. So they have a tiny brain.

26:41But the part of the brain that is responsible for smell is huge. So that probably plays roles. And it increases also at the reproductive stage. But what we think is the major signpost or cue for them to navigate is the Earth's magnetic field. And that's what I've been working on for 20 years. Wow. Partly because we know that many other organisms, like turtles, birds, butterflies, use the Earth's magnetic field.

27:14In the magnetic field, you have three characteristics that you can use. So you have the direction of the field, which is magnetic north. So that's your basic compass. But you also have the intensity, the pole of the magnet, which changes between the pole and the equator. So it's very strong at the poles. And it's weak at the equator. So if you can sense the intensity of the magnetic field, then you know where you are more or less on a latitude.

27:45And if you also can sense the magnetic north, then you can more or less navigate. So this is probably the only model that makes sense with regards to finding the sargassism. It's amazing how mysterious it all is, isn't it? So what, for you, was that first story that you thought, this is something I have to unravel, this is something I have to know more about?

Eel Mystery

28:10Well, for one, we've never found the spawning area. I mean, literally, we... That was going to be my question. How accurate are they? Yeah, the only reason we know is that because this Danish oceanographer trolled the Atlantic Ocean in the 19-something. And he measured Leptocephalus larvae for 20 years, and he saw that the smallest ones were in the Sargasso Sea. But up to very recently, that was all we had. And then now we're able to track eels, and this is where David's studies come in.

28:44And so we know pretty much where they are migrating, where they're spawning, but we've never been able to observe it. And that's, I think that's fascinating. Like, we've never found eggs. So it's still a question, and some, actually, some scientists believe that there are spawning areas in the Mediterranean. So this, I think for me, that was a species where there's so many mysteries. Also, there are never sexual... We don't know what triggers sexual maturation. So we've...

29:15There's been experiments where we inject them with hormones, but we've never really found the natural triggers for sexual maturation. So they're... If they stay in continental waters, then they're eternal adolescents, in a way. It's unbelievable to me, because it's such a common thing. You know, when you say, well, we don't really understand the life cycle of this, you think of some rare animal or plant. There aren't many of them around. But these are just... Everybody knows these things.

29:45And have, you know, historically... Like, I was reading about Ely in the Fens, which is called Ely because it's like Eel Island. And they used to just sort of scoop them out the water. And the Bishop of Ely was paid in eels. He used to get 16,000 eels a year. It was pretty good going for him. But no, 80,000. He used to get 80,000 eels a year, the Bishop of Ely. What year was that? Because I'm thinking of inflation and stuff.

30:16It feels like... And I've eaten... I hate to say it, because you're not allowed to, you know. But I have eaten jelly eels, which... It was about 25 years ago, and it remains the worst thing I've ever eaten. Which is quite impressive. Well, this is a big question that we have, which is around Europe and in Japan. They are eating eels, or they did eat eels. They had an enormous number of really delicious recipes. But again, we seem to focus very much on the mucus element in the UK. Which is a very common part of our English cuisine, obviously.

30:47How can we make this a bit more mucus-y? Yes. So what was it like? I've never had jelly eels. Oh, completely foul. It was in a pie and mash restaurant. And I actually had some American friends over. And I mean, the only thing I can say, I think they serve the jelly eels to make the pie and mash seem comparatively appealing. So Lucy was just saying that you're not allowed to eat eels anymore. Is that correct? Well, it's not quite, no. There are still some fisheries for eels, but it's now highly regulated because the European eel is classified as critically endangered.

31:21The eel is probably the most widely distributed fish in Europe. Any country with a coastline will have eels within its borders. And so it used to be obviously one of the most common fish that people would encounter if it's in every river, every stream and so on. And that's why, you know, the Bishop of Ely was receiving 80,000 eels a year as payment. And that's, you know, this was a common thing. For lords of the manor to be paid by their serfs in sticks of eels. So why are they endangered? If you were to sort of put together a blueprint, if you like, or a design plan for a fish species that would be impacted by the various challenges of the Anthropocene, the age in which we live, the eel would probably be it.

32:01It's a long-lived species that reproduces only once. So it can bioaccumulate pollutants, for example, which can be damaging the risk of disease, obviously, you know, the risk of encountering disease or parasites. It lives in a habitat that is now highly modified in which there are many barriers, dams, power stations and so on. And it has a life stage in its larval form, which could be impacted by, for example, climate change and changes in ocean currents. But also in its adult and migratory form, which can also be impacted.

32:34And, of course, there is the big issue of fishing and fishing rates as well, which can impact upon population sizes. So it is critically endangered, but there are management plans in place. The difficulty is, you know, for the eel recovery plan, the eel management plan, is that eels are so long-lived, their generation time is so long, that actually it will take quite a long time for that plan to come into action. So the people who are designing the plan and putting the plan into action may not actually be around to see the recovery of the eel stocks to their former high levels.

33:06But, you know, there are some encouraging signs. Just wanted to add also that we don't know how to farm or do aquaculture of eels because we manage to fertilize them and to get larvae, but we don't know what to feed them. So they die. And that's a big problem. So everybody relies on fishing glass eels and having on-growing farms, aquaculture. We don't know what to feed them. We don't know what to feed the larvae.

33:38Wow. We think that in nature, in the wild, they feed on what we call marine snow. And they have these, like David was describing, these very long teeth. And they don't really swim, so they drift with the Gulf Stream after they hatch. And they probably latch on to marine snow and feed on that. And also, since their drift takes two to three years to reach the coastline, then their diet probably changes. And we're not able to reproduce that.

34:11So there's a lot of fishing pressure also on the glass eels to have some kind of aquaculture. You mentioned in your introduction that you've been involved in an expedition that went, so trying to track them, essentially, across to the Azores and then onwards.

Eel Tracking

34:25So could you describe how we do that and how much success we've had? Yeah. So there's been a number of different phases of technology, really, that's helped that. And the very early work was done with tags that would make a little sound, a little ping, what are called acoustic tags. And people would track eels that were tagged with an acoustic tag. And they would have a hydrophone at the sea surface. They'd let the eel go. And they would try and track the eel. As you follow it. So basically, follow it in a boat.

34:55You'd have a couple of operators on the boat listening for the ping of the tag. And then they would navigate and manoeuvre the boat to follow the eel, sometimes for days. I think the longest track is about a week long, a few tens of kilometres. But that technology obviously has its limits. I love the fact you also called it the longest track, because that sounds like they were recording it. And then took it to Abbey Road and released it. Here is a new Christmas number one. It's eel ping. Well, I do have some ideas about how that could be possible. But getting back to the...

35:26Oh, if you can... No, no, no, no. No. This sounds like there's money here. Let's work out how we can turn these eels into recording artists. Because once they start making money, they'll definitely survive. That's the way the world works. Well, what we do now is we use a type of tag called an archival tag. That's a tag that records the environment that the eel is in, the depth and the temperature and the light. And those data are stored within an internal memory in the tag. And then after a particular period of time, some six months or a year, that tag is then released from the eel. And the data can be transmitted to us by satellite, so communicated through satellite and then beamed down into our laboratories and so on.

36:03And we can reconstruct the life of that eel. So the tag's acting a bit like a sort of black box in an aeroplane. And we can try and understand where it is and how it's been behaving. There are other types of tags that we've used. I've got a little example here, if you're interested. So this is an example of another tag we use, which is an archival tag. It doesn't communicate, but it drifts. So when the eel dies, this tag will come out of the eel, and that will then drift back. And in some cases, they'll drift back and land up on beaches, on shores, and they'll be recovered by people who are walking on beaches.

36:38And this particular example recorded the migration of an eel for about 1,000 kilometers from the west of Ireland in about 2010. And at about 600 meters, one day in that eel's life, it was eaten. It was eaten by a long-finned pilot whale. And we know that because the pattern of data that we get from the tag allows us to look at the depths and the temperatures. Obviously, the temperature increased above water temperature as soon as the tag was eaten to 37 degrees.

37:11And then the depth sensor was recording then surface-based dives of the mammal that had eaten it. You know, here is an example of predation, 600 meters. There's no light, really, at 600 meters that we could ever distinguish. And so, really, it's a fantastic example of what goes on in the deep ocean and how you can find these things out. Lucy, could you describe it? Because, yeah, there's a £50 reward, we say. Right at the end, it says £50 reward. So for all beachcombers who are after 50 quid, what should they – tell us what you think it looks like.

37:46Well, it kind of looks like a – I mean, I would say suppository is the word that springs to mind. I don't know if anyone – but it's three bright orange small tablet shapes and then one with chips and all the gubbins in. But, yeah, I would – if I found that on a beach, I would presume that it was some kind of intimate toy. It's quite big, though.

38:17What is it? About, I don't know, 10 centimeters, something like that? I would call it like a little string of sausages. Yeah. Yeah, you see, I was going to say it looks like three licorice torpedoes. The fact that you went straight for suppositories shows how different our minds are. I'm always thinking confectionery. Anyway, so – Don't get them confused. But even with all of this, you still – so how come we never find the spawning area? Well, the – it's a long migration. It's 5,000 kilometers.

38:48I mean, what I would say, you know, is the spawning eel is the Higgs boson of biology. Wow. Because it's an enigmatic particle that has actually proved elusive and impossible to find in the history of those that have asked questions about its existence. So we still have not observed a spawning eel in the wild. No eel egg has ever been found in the wild, in the ocean, I should say. And it's one of those things that, you know, still remains mystery.

39:20And I think that mystery that we're working on, you know, today is the same when it connects us back to the scientists of thousands and hundreds of years ago. What if it does turn out they all just came out of putrefaction and you go, Aristotle was right. This is ridiculous. Are there expeditions currently?

Expeditions

39:35So do you – are there missions every year, expeditions that go out to the spawning ground? Yeah, every other year. There's a German team who sends a boat cruise and they trawl and they haven't caught any eels. They catch larvae and so they add to the database of the size of the larvae, but they've never – they've even tried to – because the way now we can make an eel sexually mature by injecting hormones.

40:11So the people, the first scientists who did that, that was in the 60s in Paris, in actually the – where Pierre and Marie Curie worked. And they used urine of pregnant women to – now we don't do that anymore. We used – but injected an eel, which for the first time became sexually mature. And it's very impressive because you never see that. It's like a balloon. It just looks like – yeah, like a balloon with just a little head and a little tail.

40:47And then the bone almost dissolves because it goes through osteoporosis, just like we do. And so the Germans have actually injected eels with another – with the carpituitary extract. This is what we use instead of urine of pregnant women. And they released the sexually mature eels in the water with the hope that they would start, you know, saying, oh, this is – here are my buddies and we're going to spawn now.

41:19But it didn't work, so, unfortunately. I think it's nice that they're keeping their reproductive activities to themselves. Yes. I think more people should do that. I really do. Of all the years that we've been doing monkey cage, I don't think I've heard of a more complex and unknown life cycle of something that's so common. I mean, my unfair question was, can you – instead of anything else, as biologists, can you think of anything else that's so common and yet so mysterious?

41:51No.

41:53Right, so you're not allowed to study eels, right? So that's the thing, right? They say, no eels. What are you going for, then? I think I'd go for giant squid. Oh, yeah. Yeah, no. Another enigmatic and mysterious species. Because they're very rare and deep down in the ocean. You can kind of understand why we don't know about them. Eels do live – when they're in their migratory phase, they do live really, really deep. So they will spend their days at about 1,000 meters depth, something like that. And at night, they will rise and spend their time in shallower water, maybe about 500 or 400 meters.

42:26And they'll actually time their ascents into shallower water with dusk, and they will then dive again at dawn. So even though they're 1,000 meters or 500 meters, they can still detect light, which is about the sort of the limit of light-based vision for fish in general. So it's another amazing aspect of their biology that they are one of the best detectors of light. But if you think about the pressure changes that they experience and the temperature changes they experience on a daily basis, they're moving up and down in the water column every day. This is what drew me in to study eels when I saw some of the first ever traces of this.

43:01What is this fish doing? Why is it behaving like this? The extreme environment that they're experiencing, going from 100 atmospheres of pressure to only 50 atmospheres of pressure, that's quite extreme. It's amazing the tags can even measure that, that they survive that. But these eels are doing it on a routine basis for day after day, month after month, on that romantic journey to the Sargasso Sea. Why are they rising up? And is it temperature-based? Why do they come up and then...

43:31That's another mystery we've said. I'm beginning to think you two might have been lazy.

43:39There's several other fish that do that. Salmon and tuna. And we think it might have to do with the navigation also, that they have to calibrate their magnetic compass. Might be involved also in their sexual maturation again. That there's been experiments where they put eels, a very important depth, to see if that triggered some hormone production. But didn't really.

44:09So, again... Another failed experiment. But one thing we know that they're not doing when they're making these big migrations is feeding, because they don't have a stomach that is active. So, it's got to be something to do with that migration and, as Caroline says, perhaps navigation. Perhaps it's driven by light. Certainly in the day, they're trying to avoid predators. It doesn't always work, because there are predators that can see using sound. There are things like squid in the depths, which might also be able to detect them at those low levels of light.

44:43So, there's still plenty of mystery, I think, left in the eel-behaving biology. Could they be doing it just because they know you're watching and they really want to mess with you? Well, that's one thing we haven't tested yet. But I'm not sure how we would be able to see if they did it when we weren't watching. Ask him. He does quantum mechanics. And you know the way that they behave differently when observed. So, you know... I must say, this has been absolutely fascinating. To me, as we've said it before, but that idea that there's something that's so common. And we know a lot about it, but then...

45:15I can't believe I've eaten the most mysterious thing that there is. I always thought, you remember that time you ate that panda? Yes. I thought, she is not going to top that, Lucy Porter. But the quest continues. Giant squid next.

Eel Facts

45:30What for you was the most astounding thing? I know you also did some reading beforehand. I was fascinated in the fact that eel's blood was poisonous to human beings. I think that's correct, isn't it? It's true. They have toxic proteins in their blood. That's why you never eat raw eels. Actually, sushis, eel sushis, they're not raw. They're grilled. They're called unagi in Japan. And if you have a sore and put eel blood, it will get infected and really turn bad. Also, do not put it in your eyes.

46:02I can see everyone at home now moving their jar of eel blood onto a higher shelf. This is genuinely reethical. Do not put eel blood in your eyes. We've saved lives today. We've saved lives. Now, we asked the audience a question as well. The question that we asked them today is, what is the slippierest thing you've ever encountered? Lucy, what have you got there? Gemma says, a water slide because things can only get wetter. Yes! See, I was wondering, when this came out, because we never look at these before, but I thought

46:38the must, I thought because it was going to be fish-based, it would be things can only get batter, but I'm so glad that, yeah. What is the slipperiest thing you've encountered? A wet guinea pig in a bath? Aww! That's great. That definitely sounds like a punchline to a joke, doesn't it? That is very, uh, freshly laid goose poo on the wooden deck at my outdoor swim centre. I had somersaulted into the water before I could say, what the? That's from Lifeguard Lee. And what else have you got there? I've got, uh, Jim, I just like this one from Jim.

47:10Uh, the slipperiest thing I've ever encountered is my brother when it's his turn to buy a round. Very sweet. Can I tell my joke that I wrote? Yeah. Okay. It would seem mean for us to make you sit there for 70 minutes and not allow such a thing. I mean, I'm doubting it now, but it is a joke. Um, at the weekend, I like to dress up as a group of non-mature eels. That's right, I'm an Elvis impersonator. Aww!

47:41It feels good to have got that out of my system. Thanks, guys. Thanks. So, thank you very much to our panel. David Wrighton, Caroline Durif, and Lucy Porter. Thank you very much.

48:03Next week, we're doing deuterium and tritium goes to neutrons and some helium. Oh, fusion again. Yeah, anyway. Bye-bye. Nuclear fusion again. Goodbye.

48:16In the infinite monkey cage. Turn that nice again.

48:26Hello, I'm Greg Jenner. I'm the host of You're Dead to Me. We are the comedy show that takes history seriously. And then we laugh at it. And in our latest series, we've covered lots of global history. We've done the American War of Independence. We've done Empress Matilda and the Medieval Anarchy. We've done Alexandra Dumas, the French writer, the Kellogg brothers, and their health farm. We looked at the lives of Viking women, Renaissance-era beauty tips. We jumped to 18th century India, and also to ancient Alexandria. We looked at the life of Hannibal of Carthage, who fought the Romans. And we've done Marie Antoinette, and a big birthday special for Jane Austen.

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