The Infinite Monkey Cage - What’s the deal with eels? – Lucy Porter, David Righton and Caroline Durif

Episode Date: November 26, 2025

Fishing rods at the ready, Brian Cox and Robin Ince attempt to reel in a creature that has baffled scientists since Aristotle: the eel. Wriggling in to help them uncover the mysteries of one of nature’s slimiest subjects are marine scientists David Righton and Caroline Durif, and comedian Lucy Porter.How do eels navigate such vast distances so deep under water? Why has no one ever seen them reproduce? And WHY would anyone eat them jellied with pie and mash?! The panel discovers that Spanish eels are always late and that eels from all different countries are thought to meet up somewhere in the Atlantic Ocean for a huge annual orgy.Producer: Melanie Brown Assistant Producer: Olivia Jani Executive Producer: Alexandra Feachem A BBC Studios Production

Hello, 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 relationship. show, I'm afraid to tell you. Well, mustard seeds

are fairy, not an elephant. You see, you have got a little bit of knowledge there. So if we just drop the roundabout Legulus, 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? Careful, I'm not averse to using a gulship spell. No one here knows about the gulship spell. cannot believe totally the wrong ideas. I do you. I don't think you can do it because you'll get the verbal and semantic components required to make a water vessel fly. Oh, I'm going to choose my own adventure. Anyway, so today, what are we discussing? Today 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 behaviorist, a fish ecologist and taking to the surface of the river someone who is the regular hook a duck lady at her school fate and they are i'm david wrighton and i'm a scientist at the ukk center for environment fisheries and 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 display car park. See, Brian, that's like the kind of adventure Legolas would have. I'm Caroline Dreef 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 favorite fish. That's eels and not electric eels. Those are very different. And my weirdest voyage was when I was traveling with my family in Myanmar. And I lost my family and was trying to catch up with them and asked a kid, we were in a village. And I 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. Adorable. 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 until about 7 p.m. And they started handing out the champagne, and we went, oh, this tastes a bit funny.

We're all British people in our 20s. And they said, oh, yeah, because the Americans are all going to. 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 we 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 we were sort of returning from an arctic adventure and it felt like a very long time.

And this is our panel. I love that story. It reminds me of I went to a Jewish wedding in Philadelphia and the waiters are 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. Carol, because I realized 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 going 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 anguillate 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. Just completely different.

The question has to be, then, what is an eel? We might as well start with the definition. So you mentioned their fish already, but what actually specifically is it? How 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? It's whatever the last sandwiches you've had. What do you mean is it's a non-fish-looking fish? Yeah, well, the name of the eels is anguiliforms, which is not really very helpful because it means something in the shape of an eel. So that'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 mora eels or conga eels, or perhaps if any of them, you scuba-dived, you might have seen some things called garden eels and that sort of thing but the the anguilid 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're 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 very primitive fish similar to the hagfish so that would

be our common ancestor from from all those years ago but the anguillid eels that we're talking about are relatively recent in that context. They emerged only about, so 50 to 70 million years ago. And the European eel, which Caroline and I both study, emerged even more recently than that, probably about 3 to 5 million years ago. I love that. What is an eel?

Something that looks like an eel. What gives us an overview of them? What'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. 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, we shall use the measurement of Lucy Porter. Now, of course, this is merely a half a Lucy Porter. but full grown, it can be three Lucy Porter's.

Yeah, so the smallest, it's difficult to say. Probably, Caroline, you might know this one, but one of the tropical eels, I would imagine. But the temperate eels that we were content to 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 meter. I mean, because the smallest would be the juveniles.

So it's sort of fun. That's true. It's a silly question. It's a small. It's just been born. It's the smallest one. But, yeah, the maximum size of, like, the eels we study would be one meter, and you rarely encounter those.

So they're, like, usually 60 centimeters. This might sound like a silly question, but what's the defining characteristic? I mean, it's 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. So they're swimming like a snake? Yes, yes, it's a particular form of swimming called anguilliform swimming

which sounds very much like an eel 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, which 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

I apologize Trevor gets this chair afterwards but is that is it a sort of protective layer or what's the

function of the mucus one very special characteristics about eels is that they can live out of the water

for many hours so Michael Gove one just always have my suspicions that's amazing so they

so yeah and the mucus helps so that they don't dry up and also So 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?

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. If you have a tank, you really have to block all of the little holes. otherwise they'll find the tiniest one and they 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 left them 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

and then they're able to cross over land in wet grass to reach the stream or to reach the sea actually. It's like British people going to Magaloof. We like to reproduce by the sea. 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 with snakes in addition to their anguillid form. Yeah, it's one of the many unique characteristics of eel. But coming back to this form, actually, I mean, one of the things it'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 my goat. They drift through the ocean on ocean currents. They've been sort of transparent as a predator avoidance mechanism, but 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 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. What is the life cycle? I suppose there are so many species

through 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, a 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 millimeters long, and they might grow up to about 100 millimeters long in the course of that three-year journey across the Atlantic. And then they metamorphose into what are called glass eels. So 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. They look like leaf. They're transparent, very fragile. And they were thought to be in other 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. because 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 go, oh, there's mice there now, they must be much.

made out of sawdust. Can I just talk about genitals for a minute? Yeah. I've been so good. You know what? I've the 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 eels?

Just everybody's. If you're going with eel genital yet, I'm happy. 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. And 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-adolescence. So they're not sexually mature, so they hardly have any developed gonads. So Aristotle never found any genitalia. So they leave the rivers of Europe as adolescents basically, Go to that Bermuda, that kind of area, and then never come back. It's only the little lava that come back.

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, is the only purpose

is to make sure more of you come forward to then only have this biology. Yeah, it is biology, yeah. It's a whole lot of biology. Exactly, and it can seem from a human perspective, you know, not having a hobby or, you know, all those other things that are just like, it can

feel very unfair as far as as I'm concerned. It raises so many questions, Dave, because that migration for a start, I mean, that's a monot butterfly style migration. So what do you say? It's 8,000 Yeah, at its maximum, it would be sort of 8,000 kilometres or so from the easternmost point in which yours 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, Johann Schmidt, discovered that the spawning area was in the Saegas O.C. 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 because 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. But the average age of eels in the UK is probably sort of 15 or so, or, 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, their dorsal side. And they have special adaptations for migrating across those ocean distances. So when an individual 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. 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 relationships. So how do they migrate? You mean how do they navigate? Navigate, yeah. It's a huge challenge. When you think about they're distributed all over Europe in 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, and it's our gaseous sea. They 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. Yes. And then the Norwegian ones leave earlier in the season. And it's regulated by, we think, by photo period.

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, but 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. In 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. And 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 sargastas. 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?

Well, 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 trawled 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 surgasso 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 studies come in and so we know pretty much where they are migrating where they're spawning but we've never been able to observe it. And 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, they're never sexual, we don't know what triggers sexual maturation. So there's been experiments where we inject them with hormones, but we've never really found the natural triggers for sexual maturation. So if they stay in continental waters, then they're eternal adolescence in a way. It's unbelievable to me because it's such a common thing.

You know, when you say, we don't really understand the life cycle of this, you think of some rare animal or plant that aren't many of them around, but these just everybody knows these things. And have, you know, historically, like, I was reading about Ely in the Fens, which is called Ely because it's like Eil 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 a bit. No, 80,000. You used to get 80,000 eels a year, the Bishop of Ely. What year was that? Because I'm thinking of inflation and stuff. It feels like, and I've eaten, I hate to say it, because you're not allowed to, you know. But I have eaten jelly deals, which it was about 25 years ago, and it remains the worst thing I've ever eaten.

Which is quite impressive. 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. and part of our English cuisine, obviously. How can we make this a bit more mucousy? Yes. So what was it like? I've never had jelly deals. Oh, completely foul.

It was in a pie and mash restaurant and actually had some American friends over and I mean, the only thing can say, I think they serve the jellied 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. The eel is probably the most widely distributed fish in Europe. Any country with a coastline, you know, 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

manner to be paid by their serfs in sticks of eels. 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. It'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.

can 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. And 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 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, but, 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 managed 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. Wow. 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. So there's a lot of fishing pressure also on the glass yields 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. So could you describe how we do that and how much success we've had? Yeah, so there's been a number of different phases in technology really that's helped that. And the very early work was done with tags that will 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. You follow it. So basically follow it in a boat. You'd have a couple of operators on the boat

listening for the ping of the tag, and then they would navigate and maneuver the boat to follow the eel sometimes for days. I think the longest, the longest is about a week long, you know, a few tens of kilometres. But that technology obviously has its limits. I love the fact that you always 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... Oh, 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 is. 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 beam down into our laboratories and so on. And 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. And this particular example recorded the migration of an eel for about 1,000 kilometres from the west of Ireland in about 2010. And at about 600 metres, one day in that eels 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. And then the depth sensor was recording then surface-based dives of the mammal that had eaten it. You know, here is, you know, an example of predation, 600 metres.

There's no light, really, at 600 metres 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. Can we just... Lucy, could you describe it? Because, yeah, if there's a 50-pound reward, we should say, right at the end it says 50-pound reward. So for all beachcombers who were after 50 quid,

what should they... Tell us what you think it looks like. Well, it kind of looks like a... I mean, I would say suppository, is the word that springs to mind. I don't know if any of it. But it's three bright orange small tablet shapes, and then one with chips

and all the gubbins in but yeah if I found that on a beach I would presume that it was some kind of intimate toy it's quite big though it's what is it about I don't know

10 centimetres something like that I would call it like a little string of sausages 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, don't get him confused.

But even with all of this, you still... So how come we never find the spawning area? Well, it's a long migration. It's 5,000 kilometres. I 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 his 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 still remains mystery. And I think that mystery that we're working on 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 there just came out of putrefaction?

And you go, Aristotle was right. It's ridiculous. Are there expeditions, Caroline? So are there missions every year, expeditions that go out to the spawning grounds? 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. So the people, the first scientists who did that that was in the 60s in Paris, in actually where Pierre and Marie Curie worked, and they used urine of pregnant woman to, now we don't do that anymore.

But to inject it 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. And then the bone almost dissolves because it goes through osteoporosis, just like we do. And so the Germans have actually injected eels. with the car pituitary extract. This is what we use instead of urine of pregnant women.

And they released the sexually mature eels in the water and with the hope that they would start, you know, saying, oh, this is, here are my buddies and we're going to spawn now, but 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 an unknown life cycle of something that's so common my unfair question was can you is there anything else as biologists can you think of of anything else that's so common and yet so mysterious no

right 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. Another enigmatic and mysterious. 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. And they'll actually time their ascents, to shallower water with dusk, and they will then dive again at dawn. So even though they're at 1,000 metres or 500 metres,

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 watercolumn every day. This is what drew me in to study eels

when I saw some of the first ever traces of this. What is this fish to it? 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 saga. Why are they rising up? Is it temperature-based? Why do they come up and then... That's another mystery there's...

I'm beginning to think you two might have been lazy. There'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 kind of calibrate their magnetic compass. It 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 so 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, you know, as currently 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. So there's still plenty of mystery, I think, left in the EO Behaviology. 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 there's something that's so common.

And we know so, we know a lot about it, but then such huge gaps. I don't know. I've eaten the most mysterious thing. Yeah. I always thought, you remember that time you ate that panda? Yes. I thought she's not. She is not going to top that, Lucy Porter.

But the quest continues. Giant squid next. What for you was the most astounding? Because I know you also, you know, you 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. I can see everyone at home now moving their jar of eel blood onto a higher shelf. This is genuinely wreathy.

Do not put eel blood in your eyes. We've saved lives today. We've saved lives. Now, we ask the audience a question as well. The question that we asked them today is, What is the slipperiest thing you've ever encountered? Lucy, what are 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 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? That definitely sounds like a punchline to a joke, doesn't it? That is very 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 you got there? I've got Jim, I just like this one from Jim. The slipperest thing I've ever encountered is my brother when it's his turn to buy around. 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. At the weekend, I like to dress up as a group of non-mature eels. That's right. I'm an elvers impersonator. Oh. Feels good to have got that out of my system.

guys thanks so thank you very much to our panel David Wrighton Caroline Durif and Lucy Porter next week we're doing deuterium

goes to neutrons and some helium oh fusion again yeah anyway bye bye nuclear fusion again goodbye goodbye

Do you know that nice again? Hello, 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.

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