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

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Speaker 1 Hello, I'm Robin Ince. And I'm Brian Cox.
And this is the Infinite Mustard Seed Cage.

Speaker 3 The Infinite Mustard Seed Cage.

Speaker 1 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.

Speaker 3 Elva.

Speaker 3 Yeah, elf. Baby eel.

Speaker 1 Well, that's not the questions we've got for you today. This is a lot more of a DD-related show.

Speaker 1 I'm afraid to tell you.

Speaker 3 Well, mustard seed's a fairy, not an elf.

Speaker 1 Yeah, see, you have got a little bit of knowledge there.

Speaker 1 So if we just drop the roundabout Legolas, Elrond, and Hawke 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.

Speaker 1 Anyway, so

Speaker 3 He's very odd, isn't he?

Speaker 1 You're odd. Careful, I'm not averse to using a gull ship spell.
No one here knows about the gullship spell. I cannot believe it, totally the wrong idea.

Speaker 3 I don't think you can do it because you will get the verbal and somatic components required to make a water vessel fly.

Speaker 1 Oh, I'm going to choose my own adventure. Anyway, so today, what are we discussing, Brian?

Speaker 3 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?

Speaker 1 And why are jellied eels not as popular as they used to be?

Speaker 1 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 fate.

Speaker 3 It's Jane. And they are.

Speaker 3 I'm David Wrighton and I'm a scientist at the UK Centre for Environment, Fisheries and Aquaculture Science.

Speaker 3 I've been studying European eels for nearly 20 years including tracking them across the Atlantic to the Azores and beyond.

Speaker 3 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.

Speaker 3 And I've never been so glad to see a pay and display car park.

Speaker 1 See, Brian, that's like the kind of adventure Legolas would have.

Speaker 13 I'm Caroline DeReeiff, 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.

Speaker 13 That's eels and not electric eels. Those are very different.

Speaker 13 And my weirdest voyage was when I was traveling with my family in Myanmar.

Speaker 13 And I lost my family and was trying to catch up with them. And asked a kid, we 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.

Speaker 13 And he took me to his home and showed me his family, which was very sweet.

Speaker 13 Very awkward.

Speaker 9 Adorable.

Speaker 16 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.

Speaker 16 So, the longest water journey I ever did was on the River Thames, and it was when I was in my 20s. My friend was marrying an American woman, and we got on a boat at the Thames embankment at midday,

Speaker 16 and they set sail. The couple were going to get married on the boat, and we were going to sail up and down till about 7 p.m.

Speaker 16 and they started handing out the champagne. We all went, Oh, this tastes a bit funny.
We're all British people in our twenties.

Speaker 16 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.

Speaker 16 Now, if you know British people

Speaker 16 in their twenties, we spent seven hours, people

Speaker 16 one person attempted to jump off when they spotted a park,

Speaker 12 and we

Speaker 16 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.

Speaker 1 And this is our panel.

Speaker 1 I 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,

Speaker 1 we don't need any more wine. Apart from that table, they're all from Europe.

Speaker 1 Carol, 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 going to be talking about.

Speaker 1 So, what is that? Apart from obviously the charge, what is the difference?

Speaker 13 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

Speaker 13 they live in both habitats, so in freshwater and saltwater. Whereas electric eels, it's

Speaker 13 actually

Speaker 13 not really an eel.

Speaker 3 The question has to be: then, what is an eel? We might as well start with the definition. So, you mentioned a fish already, but what actually specifically is it?

Speaker 3 How are they related to a salmon or something? Where do you go? Where's the common ancestor?

Speaker 1 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?

Speaker 1 Salmon seems to be it's whatever the last sandwich is you've had.

Speaker 9 That seems to be

Speaker 3 what you mean is it's a non-fish-looking fish.

Speaker 3 Yeah, well, the name of the eels is anguilliformes, which is not really very helpful because it means something in the shape of an eel.

Speaker 3 So that's

Speaker 3 that's that's 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.

Speaker 3 You'd think of things like moray 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.

Speaker 3 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.

Speaker 3 If you were to try and go back and find a common ancestor with us,

Speaker 3 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.

Speaker 3 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.

Speaker 3 They emerged only about 50 to 70 million years ago.

Speaker 3 And the European eel, which Caroline and I both study, emerged even more recently than that, probably about three to five million years ago.

Speaker 1 I love that. What is an eel? Something that looks like an eel.

Speaker 3 Give us an overview of them. What's the smallest one? What's the biggest one?

Speaker 3 Well, the biggest anguillid eel

Speaker 3 is the mottled eel, which can reach about 200 centimeters in length, two meters, and weigh up to about eight kilograms in weight. So that's quite a sizable individual.

Speaker 16 It's taller than me, but it weighs less.

Speaker 15 I'll just put that.

Speaker 17 But I would try and fight one.

Speaker 1 I like the fact 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.

Speaker 1 Now, of course, this is merely a half a lucy porter, but full grown, it can be three lucy porters.

Speaker 3 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.

Speaker 3 But the temperate eels that we work on tend to tend to be relatively large compared to some of the tropical species.

Speaker 13 Yeah, so you would think the maximum of the smallest eels, which is the maximum length, would be one meter. So, I mean, because the smallest would be the juveniles.
So, it's sort of

Speaker 1 like the car has just been born.

Speaker 17 Yeah. The smallest one.

Speaker 7 Of the adult.

Speaker 13 But yeah, the maximum size of the eels we study, it would be one meter, and you rarely encounter those. So they're like usually 60 centimeters.

Speaker 3 This might sound like a silly question, but what's the defining characteristic? I mean, it's this thing that's basically a fish with no fins. Is that what's the specification? Well, they do have fins.

Speaker 3 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.

Speaker 3 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.
Like a snake. Yes, yes.

Speaker 3 It's a particular form of swimming called anguilliform swimming,

Speaker 12 which

Speaker 1 sounds very much like an eel.

Speaker 9 Yes, yes.

Speaker 3 But they also they also have sort of pectoral fins.

Speaker 3 Those are the the the fins at the at the side of the body which are used for for changing their sort of angle to move upwards or or move downwards or break or or whatever. So they're not finless.

Speaker 3 Um they have very, very tiny scales and they they do produce a lot of mucus to cover those scales, which means that they they have this sort of characteristic slipperiness.

Speaker 16 I read that the some of them use the mucus to sort of protect themselves from scratches. So I've tried that tonight.

Speaker 16 I apologise to ever get this chair afterwards, but

Speaker 16 is it a sort of protective layer, or what's the function of the mucus?

Speaker 13 One very special characteristic about eels is that they can live out of the water for many hours.

Speaker 16 So, there's Michael Gove one.

Speaker 16 Just always have my suspicions, but that's amazing.

Speaker 13 So, they so, yeah, and the mucus helps so that they don't dry up, and also also it allows them to breathe through their skin somehow to at least to get some oxygen.

Speaker 7 So

Speaker 16 how long can they be out of the water for?

Speaker 13 Well, if it's cold, if it's not

Speaker 13 in the sun, right in the sun, then they could stay. I mean, I've sometimes, because they also have this ability to escape,

Speaker 13 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 then just like

Speaker 13 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.

Speaker 13 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

Speaker 13 and then they're able to cross over land if in wet grass to reach the other the stream or to reach the sea actually.

Speaker 16 It's like British people going to Magaloof.

Speaker 13 We like to reproduce by the sea.

Speaker 13 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.

Speaker 3 Yeah, 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.

Speaker 3 So when they are in their larval phase, they don't even look like eels at all.

Speaker 3 They're transparent. They have very tiny heads with very pointy teeth.
They have a tiny little tail.

Speaker 9 And they

Speaker 12 drift.

Speaker 3 They drift through the ocean on ocean currents.

Speaker 3 transparent to as a predator avoidance mechanism.

Speaker 3 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 these juvenile glass eels and then the yellow eels in rivers and streams and so on.

Speaker 3 What is the life cycle? I mean, 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,

Speaker 3 when do they mature?

Speaker 3 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 of them.

Speaker 3 So when they hatch as 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.

Speaker 3 And then they metamorphose into what are called glass eels.

Speaker 3 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.

Speaker 3 By transform, how abrupt is that transformation? How does it happen?

Speaker 3 It's a matter of days, isn't it? I think, Caroline, it was

Speaker 3 an accidental discovery.

Speaker 13 Yeah, so the connection David is talking about between

Speaker 13 these larvae that are called leptocephalus. Lepto means leaf, and cephalis means head.
So they look like leaf. They're transparent, very fragile.
And they were thought to be another species.

Speaker 13 And then it 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.

Speaker 13 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.

Speaker 13 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.

Speaker 1 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.

Speaker 16 Can I just talk about genitals for a minute?

Speaker 12 Yeah.

Speaker 9 I've been so sorry. You know what?

Speaker 1 Every show you've ever been on, you have eventually.

Speaker 9 It's just a bit early.

Speaker 3 Aristotle's or the eels.

Speaker 16 Just everybody's.

Speaker 1 You're going with eel genitalia.

Speaker 16 But did he not check and couldn't find any genitals and then kind of went, oh, well, then that's...

Speaker 13 Yes, that was the other part of the story, is that he never found any eggs.

Speaker 13 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-adolescent, so they're not sexually mature, so they don't, they hardly have any developed gonuts.

Speaker 13 So Aristotle never found any genitalia.

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Speaker 3 So they leave the rivers of Europe as adolescents, basically. Go to

Speaker 3 that Bermuda, that kind of area,

Speaker 3 and then never come back. It's only the little larva that

Speaker 13 they die, probably.

Speaker 13 Most likely, because they do not eat during that long journey, which can be up to 8,000 kilometres.

Speaker 9 So they have sex and then die.

Speaker 13 So they better not have a headache.

Speaker 14 Yes, that's it.

Speaker 1 It is this incredible level of just going this is the only purpose yeah is to make sure more of you come forward to then only have

Speaker 1 biology yeah it's biology yeah

Speaker 1 exactly and that and it can seem from a human perspective you know not having a hobby or you know all those other things

Speaker 1 it can feel very unfair as far as I'm concerned.

Speaker 3 It raises so many questions, Dave, because that migration for a star, I mean, that's a monarch butterfly style migration. So what do you say? It's 8,000.

Speaker 3 Yeah,

Speaker 3 at its maximum, it would be sort of 8,000 kilometres or so from the easternmost point in which yields are found. The obvious question is: how?

Speaker 3 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 Sargasso Sea.

Speaker 3 And that journey across the Atlantic Ocean is something that's motivated generations of scientists, possibly since Aristotle,

Speaker 3 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.

Speaker 3 They start as a leptocephalus, they turn into a glass eel, they then become elvas 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.

Speaker 3 The oldest ever recorded was about 90 years old.

Speaker 3 But the sort of average age of eels in the UK is probably sort of 15 or so,

Speaker 3 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.

Speaker 3 And they have special adaptations for migrating across those ocean distances. So that when

Speaker 3 an individual sort of chooses, let's say it's the wrong word, but when it begins its migration, so that varies.

Speaker 3 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.

Speaker 3 Yes, yes, or even older. And this is

Speaker 3 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.

Speaker 3 So, how do they migrate?

Speaker 13 You mean how do they migrate? How do they navigate? Navigate, yeah. It's a huge challenge when you think about they are distributed all over Europe and North Africa.

Speaker 13 So, eels from northern Norway will have to congregate with eels swimming from Morocco, for example, and find a common place. And because of, we see that the genetic structure is just a big mix.

Speaker 13 So this is what happens. It's a big orgy in the Sargasso Sea.
They all meet.

Speaker 13 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.

Speaker 13 So generally, they migrate during the fall,

Speaker 13 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, then the Norwegian ones leave earlier in the season.

Speaker 13 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.

Speaker 13 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 bows.
So they have a tiny brain.

Speaker 13 But the part of the brain that is responsible for smell is huge. So that probably plays a role and it increases also at the reproductive stage.

Speaker 13 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.

Speaker 13 Partly, because we know that many other organisms like turtles, birds, butterflies use

Speaker 13 the Earth's magnetic field. In the magnetic field, you have

Speaker 13 three characteristics that you can use.

Speaker 13 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.

Speaker 13 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.

Speaker 13 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 sargassum.

Speaker 16 It's amazing how mysterious it all is, isn't it?

Speaker 1 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?

Speaker 13 Well, for one, we've never found the spawning area. I mean, literally, we that was going to be my question.

Speaker 3 How accurate are they?

Speaker 13 Yeah,

Speaker 13 the only reason we know is that because this Danish oceanographer trawled the Atlantic Ocean in the 19 something,

Speaker 13 and he measured leptocephalus larvae for 20 years, and he saw that the smallest ones were in the Sargasso Sea.

Speaker 13 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.
And so we know pretty much where they are.

Speaker 13 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.

Speaker 13 So, it's still a question, and some actually, some scientists believe that there are spawning areas in the Mediterranean.

Speaker 13 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.

Speaker 13 So, we've there's been experiments where we inject them with hormones, but we've never really found the natural triggers for sexual maturation. So

Speaker 13 if they stay in continental waters, then they're eternal adolescents

Speaker 13 in a way.

Speaker 3 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.

Speaker 3 There aren't many of them around.

Speaker 3 But these just, everybody knows these things.

Speaker 16 And have, you know,

Speaker 16 historically, like I was reading about Ely in the Fens,

Speaker 16 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, used to get 16,000 eels a year.

Speaker 16 It was pretty good going for a bit. No, 80,000.
You used to get 80,000 eels a year, the Bishop of Ely.

Speaker 3 What year was that?

Speaker 1 Because I'm thinking of inflation and stuff.

Speaker 16 It feels like, and I've eaten, I hate to say it because you're not allowed to know,

Speaker 16 but I have eaten jelly deals, which it was about 25 years ago, and it remains the worst thing I've ever eaten.

Speaker 1 This is a big question that we have, which is around Europe and in Japan, they are eating eels, or if they did eat eels, they had an enormous number of really delicious recipes.

Speaker 1 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.

Speaker 16 How can you make this a bit more mucus-y? Yes.

Speaker 1 So, what was it like? I've never had jelly deals.

Speaker 16 Well, it was 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 deals to make the pie and mash seem comparatively appealing

Speaker 3 so what is that 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 uh fisheries for eels but it's now highly regulated because the the european eel is classified as critically endangered the eel is probably the most widely distributed distributed fish in Europe.

Speaker 3 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.

Speaker 3 And that's why

Speaker 3 the Bishop of Ely was receiving 80,000 eels a year as payment. And this was a common thing for lords of the manor to be paid by their serves in sticks of eels.

Speaker 3 Why are they endangered?

Speaker 3 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.

Speaker 3 The eel would probably be it. It's a long-lived species that reproduces only once, so it can bio-accumulate pollutants, for example, which can be damaging.

Speaker 3 The risk of disease, obviously, you know, the risk of encountering disease or parasites.

Speaker 3 It lives in a habitat that is now highly modified, in which there are many barriers, dams, power stations, and so on.

Speaker 3 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.

Speaker 3 And of course there is the

Speaker 3 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.

Speaker 3 The difficulty is

Speaker 3 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.

Speaker 3 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.

Speaker 13 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.

Speaker 13 So,

Speaker 13 they die and that's a big problem. So everybody relies on

Speaker 13 fishing glass eels and having on growing farms, aquaculture.

Speaker 1 We don't know what to feed them.

Speaker 13 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.

Speaker 13 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.

Speaker 13 And also, since their

Speaker 13 drift takes two to three years to reach the coastline, then their diet probably changes. And we're not able to reproduce that.

Speaker 13 So there's a lot of fishing pressure also on the glass seals to have some kind of aquaculture.

Speaker 3 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.

Speaker 3 So, could you describe how we do that and how much success we've had?

Speaker 3 Yeah, so

Speaker 3 there's been a number of different phases in

Speaker 3 technology, really, that's helped that.

Speaker 3 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.

Speaker 3 As you follow it, so basically, follow it in a boat.

Speaker 3 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

Speaker 3 the longest track is about a week long, you know, a few tens of kilometers. But that technology obviously has its limits.

Speaker 1 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.

Speaker 1 Here is a new Christmas number one: it's eel ping. And

Speaker 3 well, I do have some ideas about how that could be possible, but getting back to the.

Speaker 1 Oh, if you can, no, no, no, no, no, no, this sounds like there's money here.

Speaker 1 Let's work out how we can turn these eels into recording artists, because once they start making money, they'll definitely survive.

Speaker 3 That's the way the world works. Well, what we do now is we use a type of tag called an archival tag.

Speaker 3 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.

Speaker 3 And then, after a particular period of time, some six months or a year, that tag is is then released from the eel, and the data can be transmitted to us by satellite.

Speaker 3 So, communicated through satellite and then beamed down into our laboratories and so on. And we can reconstruct the life of that eel.

Speaker 3 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.

Speaker 3 I've got a little example here, if you're interested.

Speaker 3 So,

Speaker 3 this is an example of another tag we use, which is an archival tag. It doesn't communicate, communicate, but it drifts.
So, when the eel dies,

Speaker 3 this tag will come out of the eel, and that will then drift back.

Speaker 3 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.

Speaker 3 And this particular example recorded the migration of an eel for about a thousand kilometers from the west of Ireland

Speaker 3 in about 2010. And

Speaker 3 at about 600 meters, one day in that eel's life,

Speaker 3 it was eaten. It was eaten by a long-finned pilot whale.

Speaker 3 And we know that because the pattern of data that we get from the tag

Speaker 3 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.

Speaker 3 And then the depth sensor was recording then surface-based dives of

Speaker 3 the mammal that had eaten it. You know, here is

Speaker 3 an example of predation 600 meters.

Speaker 3 There's no light really at 600 meters that we could ever distinguish.

Speaker 3 And so, really, it's a fantastic example of what goes on in the deep ocean and how you can find these things out.

Speaker 9 Can we just Lucy?

Speaker 1 Could you describe it? Because, yeah, there's a £50 reward, we say, right at the end, it says £50 reward.

Speaker 1 So, for all beach combers who are after 50 quid, what should they tell us what you think it looks like?

Speaker 16 Well, it kind of looks like a,

Speaker 16 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

Speaker 16 small tablet shapes, and then one with chips and all the gubbins in. But

Speaker 16 yeah, I would, if I found that on a beach, I would presume that it was some kind of intimate

Speaker 16 toy.

Speaker 3 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.

Speaker 1 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.

Speaker 1 I'm always thinking confectionery, and you're anyway.

Speaker 17 So

Speaker 9 don't get them confused.

Speaker 16 But even with all of this, you still.

Speaker 16 So, how come we never find the spawning area?

Speaker 3 Well,

Speaker 3 it's a long migration, it's 5,000 kilometres. I mean, what I would say

Speaker 3 is the spawning eel is the Higgs boson of biology

Speaker 3 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.

Speaker 3 So we still have not observed a spawning eel in the wild.

Speaker 3 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.

Speaker 3 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.

Speaker 1 What if it does turn out they all just came out of putrefaction? You go, Aristotle was right, this is ridiculous.

Speaker 3 Are there expeditions, Caroline? So, so

Speaker 3 do you are there missions every year, expeditions that go out to the spawning grounds?

Speaker 13 Yeah, every other year. There's a German team who sends a boat

Speaker 13 cruise and

Speaker 13 they trawl and

Speaker 13 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

Speaker 13 they've even tried to because the the way now we can we can make an eel sexually mature by injecting hormones. So the the people, the first scientists who did that, that was in the in the 60s in

Speaker 13 in Paris, in actually the

Speaker 13 where Pierre and Marie Curie worked. And they used urine of pregnant women to, now we don't do that anymore.

Speaker 13 We use, but to 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.

Speaker 13 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.

Speaker 13 And so

Speaker 13 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

Speaker 13 with the hope that they would start saying, oh, this is... Here are my buddies and we're going to spawn now.
But it didn't work, so unfortunately.

Speaker 16 I think it's nice that they're keeping their reproductive activities to themselves.

Speaker 16 I think more people should do that, I really do.

Speaker 3 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.

Speaker 3 I mean, my unfair question was: can you, is there anything else as biologists? Can you think of anything else that's so common and yet so mysterious?

Speaker 13 No,

Speaker 1 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?

Speaker 3 I think I'd go for giant squid. Oh, yeah.
Yeah, no, no. I like 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.

Speaker 3 Eels do live, when they're in their migratory phase, they do live really, really deep. So

Speaker 3 they will spend their days

Speaker 3 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

Speaker 3 their ascents into shallower water with dusk, and they will then dive again at dawn.

Speaker 3 So, even though they're at 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.

Speaker 3 So, it's another amazing aspect of their biology that they are one of the best detectors of light.

Speaker 3 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.

Speaker 3 This is what drew me in to study eels when I saw some of the first ever traces of this.

Speaker 3 What is this fish doing?

Speaker 3 Why is it behaving like this?

Speaker 3 The extreme environment that they're experiencing, going from 100 atmospheres of pressure to only 50 atmospheres of pressure, that's quite extreme.

Speaker 3 It's amazing the tags can even measure that, that they survive that.

Speaker 3 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.

Speaker 3 Why are they rising up? And is it temperature-based?

Speaker 3 Why do they come up and then?

Speaker 9 That's another mystery.

Speaker 1 I'm beginning to think you two might have been lazy.

Speaker 13 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.

Speaker 13 It might be involved also in their sexual maturation again.

Speaker 13 That there's been experiments where they put eels a very important depth to see if that triggered some hormone

Speaker 13 production, but didn't really. So

Speaker 7 again,

Speaker 3 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.

Speaker 3 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.

Speaker 3 It doesn't always work because there are predators that can see using sound.

Speaker 3 There are things like squid in the depths which might also be able to detect them at those low levels of light. So

Speaker 3 there's still plenty of mystery I think left in the ELB.

Speaker 16 They're doing it just because they know you're watching and they really want to mess with you.

Speaker 12 Yeah.

Speaker 3 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.

Speaker 1 Ask him, he does quantum mechanics and you know the way that they behave differently when observed. So, you know.

Speaker 3 I must say this has been absolutely fascinating.

Speaker 3 To me, as we've said it before, but that idea that there's something that's so common and we know so we know a lot about it, but then I know I can't believe I've eaten the most mysterious thing that there is.

Speaker 1 I always thought, you remember that time you ate that panda? Yeah. I thought she's not.

Speaker 1 She is not going to top that, Lucy Porter.

Speaker 16 But the quest continues. Giant squid next.

Speaker 1 What for you was the most astounding thing? Because I know you also, you know, you did some reading beforehand, because I was fascinated in the fact that eel's blood was poisonous to human beings.

Speaker 3 I think, is that that's correct, isn't it?

Speaker 13 It's true. They have toxic proteins in their blood.
That's why you never eat raw eels. Actually, sushis,

Speaker 13 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.

Speaker 1 I can see everyone at home now moving their jar of eel blood onto a higher shelf.

Speaker 18 This is genuinely wreathy.

Speaker 9 Do not put eel blood in the face.

Speaker 1 We've saved lives today.

Speaker 19 We've saved lives.

Speaker 1 Now, we asked the audience a question as well. The question that we asked them today is: what is the slipperiest thing you've ever encountered?

Speaker 1 Lucy, what have you got there?

Speaker 16 Gemma says, a water slide because things can only get wetter.

Speaker 1 See, I was wondering when this came out, because we never look at these before, but I thought the must I thought because it was going to be fish-based, it would be things can only get batter.

Speaker 1 But I'm so glad that, yeah.

Speaker 3 What is the slipperiest thing you've encountered? A wet guinea pig in a bath.

Speaker 1 That definitely sounds like a punchline to a joke, doesn't it?

Speaker 8 That is very

Speaker 1 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?

Speaker 16 I've got Jim. I just like this one from Jim.
The slipperiest thing I've ever encountered is my brother when it's his turn to buy around.

Speaker 15 Very sweet.

Speaker 16 Can I tell my joke that I wrote?

Speaker 19 Yeah.

Speaker 1 Okay. It would seem mean for us to make you sit there for 70 minutes and not allow such a thing.

Speaker 16 I mean I'm doubting it now, but it is a joke.

Speaker 16 At the weekend, I like to dress up as a group of non-mature eels. That's right.
I'm an Elvis impersonator.

Speaker 16 Feels good to have got that out of my system. Thanks, guys.
Thanks.

Speaker 1 So, thank you very much to our panel. David Ryton, Caroline Durif, and Lucy Porter.

Speaker 3 Next week, we're doing deuterium and tritium goes to neutrons and some helium.

Speaker 1 Oh, fusion again.

Speaker 9 Yeah, anyway.

Speaker 3 Goodbye.

Speaker 15 In the infinite monkey cage

Speaker 19 till

Speaker 20 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.

Speaker 20 We've done the American War of Independence, we've done Empress Matilda and the medieval anarchy, we've done Alexandre Dumas, the French writer, the Kellogg brothers, and their health farm.

Speaker 20 We looked at the lives of Viking women, Renaissance-era beauty tips. We jumped to 18th-century India and also to ancient Alexandria.

Speaker 20 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.

Speaker 20 Plus, there's 140 episodes in our back catalogue, so if you want to laugh while you learn, the show is called You're Dead to Me and you can find us first on BBC Sounds.

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