Yeah, so hagfish are a group of deep sea animals that are found on the bottom of the ocean, in most of the oceans of the world. We don't tend to find them in the Arctic and the Antarctic, but pretty much everywhere else we find them. They are they're sort of eel-like in their morphology, but that's really the only resemblance to eels. They're long and skinny like an eel, but they are not teleost fish like eels. They don't have fins, they don't have gills, they don't have jaws. So they look almost more like a worm than an eel when you get up close to it.

In terms of their ecology, they are scavengers. Many of the listeners might have seen images or videos of hagfishes swarming around a dead whale. So when something dead falls to the bottom of the ocean, it's usually a hagfish that gets there first - they're really good at the scavenging lifestyle.

In terms of diversity: right now there's about 90 species that are described, and that number increases by one or two every year as we explore more of the deep sea.

And what are they doing when they're not Scavenging? we obviously see them gather at our baited Landers and at Whale Falls and things like that. are they living in Burrows? are they hiding away somewhere?

Yeah that's a good question! So burrowing is something that we're very interested in and it seems to be an important part of the hagfish lifestyle. We don't know how important it is - it seems that some species spend a lot of time in burrows and then other ones, less so. But what is clear is that they have a very low energy lifestyle. Their strategy seems to be: use as little energy as possible and wait until something delicious falls to the the ocean floor, and then be one of the first ones to get there. 

There may be some listeners who are who are struggling to visualise a hagfish: so it's about 1-2ft long, a long slender thing. there's a single nostril at the very snout end, and some.. what's the technical term for the whisker-type things? 

Barbles is the technical term for this. Yeah, they’re sort of sticking out from the face. And as you said, there's a giant single nostril on the the front of the face. And that's the water intake for their gills. They have multiple gill pouches and those are further back on the body. And then there's these little holes called gill apertures/gill pores where the water exits the body after irrigating the gills.

And there's there's one mysterious one isn't there? it has a separate name. one of those Outlets is larger than the others. do we do we know why yet?

Uh, yes so this is interesting! So there's a little tube called the pharyngocutaneous duct and that connects the pharynx (throat) with the last gill aperture. So let's say we're talking about a given species of hagfish that has six gill pouches. That hagfish might have six gill apertures on each side, but on the left side, the last gill aperture might be a little bit bigger because that extra duct fuses with that last gill aperture. And so, the gill apertures look a little bit asymmetrical on the the left side and the right side. 

Now where it gets a little complicated, is one of the Genera Myxini (which is the second largest genus of hagfishes) has multiple gill pouches but only a single gill aperture on each side. And we don't really understand why that is. For some reason, they've gotten rid of individual gill apertures and they have a single aperture close to that final most posterior gill. In those species, the left side opening is much bigger than on the right side because of that pharyngocutaneous duct that fuses with it.

"... this is a quote from the guest "

— Guesty Mc Guest\

An illustration of Atlantic Hagfish (Myxine glutinosa) respiration including the singular gill pore positioned after the gill pouches. Image courtesy of Georgia Wells.

And do we know what the duct is for? Is it for for clearing larger material from the from the gills?

I don't fully understand what it's for because they do seem to be able to reverse flow and cough out their nostril - and they'll do this if they get slime in their face. So I don't fully understand what that duct is there for.

Update:

After speaking with Doug we got an update from him. He said that a lot of the old hagfish books used to say that this pharyngocutaneous duct was a way of ejecting particles that were too big for the gills. (So anything that would have clogged the gills). That was the general consensus, but he personally thinks that doesn't make much sense, because this duct is behind all the gills, so it if it was going to clog them, it would have already clogged them. Plus, hagfish can cough and reverse the flow to dislodge things.

He reckons it's probably more to do with squeezing the food to remove any excess water, before they swallow it and digest it. Because if you swallow lots of water, you dilute your stomach acid and enzymes, and then digestion isn't so effective. Because of the way they feed and rasp and tear at flesh, it actually makes sense for them to ring it out like a sponge before they swallow it, so they're not taking on loads of water. 

Yeah there's something going on. it seems so deliberate but yeah, maybe we've not fully gotten to the bottom of it.

Yeah I'm totally fascinated with the gill anatomy. One of the species we have in the lab has 11 or 12 gills and gill apertures, and then another one from the Atlantic has six gills and a single aperture. There's interesting patterns in different clades of how the gill apertures are arranged. And some of them, they're sort of clustered together almost like they're in the process evolutionarily of fusing together, but they haven't quite. There's one group that used to be in the genus quadratus (which doesn't exist anymore) but it was called quadratus because there were four gill apertures in a square instead of a line. What are the selective pressures that are pushing around these gill apertures? I think it might have something to do with with burrowing but I have yet to come up with a really coherent hypothesis that explains all the variation.

Very cool. you can sort of see a shadow of eyes too. there's light receptive patches where we usually associate eyes right?

Yeah and and pushes into flesh as well!

Yeah, but then the Atlantic hagfish doesn't have a transparent window. They still have this vestigal eye. If you dissect the skin away from the head, there it is. But there's no transparent window, there's just skin and then layers of muscle. There's just no way that light is coming through in a way that would allow them to form any kind of image in those species.

Is the same true of the juveniles? we see that in some of the cusk eels, they use their eyes less and less as they get bigger. and they kind of skin-over as they age.

Oh that's interesting. I not sure about that, I've never noticed in the really little ones. But that's a really good thing to look for. I'm pretty sure it's the same in the little ones. There is one species that we just described a few years ago. It’s a new species from the Galapagos and it's a completely albino hagfish and so we gave it the name The Ghost Hagfish because it just looks very strange and it's white. It has zero pigment in its skin and in the young ones, they're basically transparent. The skin is completely transparent, you can see the slime glands, you can see the heart beating, you can see the blood going through the gills. So that's kind of interesting - it's not a transparent window it's just the entire skin is transparent.

Yeah and the ancestral State seems to have eyes right? there was some fossils found recently where the it looks like they used to have more more well-developed eyes.

Yeah, I think that's becoming clear; that they probably had pretty decent eyes if you go back far enough. It was probably this adoption of a burrowing lifestyle that led them to start losing the eyes. But I should say that they are light sensitive. So even the ones that have this thick skin over the eyes,  they're clearly light sensitive. So when we open our tanks, they respond to the light. So what's going on? Do they have photo receptors in the skin? That's my guess… but not much is known about that.

And I suppose one of the things they're famous for (if we're talking gross anatomy) is being agnathan - not having a true jaw. they have a very interesting apparatus at the business end, could you tell us a little bit about how they feed and that jawless structure?

But they're sharp enough and powerful enough that if it's soft flesh they can actually tear off a chunk and burrow into flesh as well, don't they?

Yeah so that's one of the amazing things that hagfishes do: they will burrow into a carcass and hang out there and eat their way out. Which physiologically for most animals, that would not be possible beacause there's just no oxygen inside a rotting carcass. So that's another really cool thing about hagfishes is their ability to go without oxygen.

And are they are they coping without oxygen? one of my little factoids is that they can ventilate through the anus. is that a thing - like sea turtles?

Oh I've never heard that. I mean, it's not impossible. Now that I think of it… so I mentioned burrowing before, we've been doing some experiments on burrowing in the lab and we finally managed to get hagfish to burrow in captivity. We've managed to get them to do it in clear substrates, so we have a paper and review right now on hagfish burrowing in gelatin.

Oh wow - that must look amazing!

It is amazing! So for a long time, we've known about hagfish burrowing. You guys go down with your cameras and occasionally you see a hagfish driving its head into the mud. Anyway, the reason I bring this up is that they make these u-shaped burrows and sometimes their tail is poking out a little bit, so I wonder if it's possible that they're doing gas exchange across their tail when they're in a burrow. 

A bit of a bum-snorkel.

Hahaha exactly! One other thing I should mention about hagfish, yet another weird thing, is they have what's known as a subcutaneous sinus that is quite large. So basically their skin is not very well attached to the rest of their body and there's all this space in between. There's quite a bit of blood there… so it's estimated about 30% of their blood volume is in that subcutaneous sinus and some have have suggested that this blood is involved in cutaneous respiration.

Wow okay. and would that also be a repository of oxygen as well? a bit like myoglobin.

Yeah possibly!

There's so many unusual quirks to them. I'm so glad they're still around.

I know, I mean they've been around for a long time if you think about the extinction events that they have survived, it's really quite remarkable.

"... their skin is not very well attached to the rest of their body and there’s all this space in between. There’s quite a bit of blood there… so it’s estimated about 30% of their blood volume is in that subcutaneous sinus... "

 — Doug Fudge

Yes that was one of the questions actually. how have they survived these events? are they a bit like the Chimera are they almost a prototype that used to be abundant in the shallows that have been preserved in the deep sea?

I don't know. You know, we only have two hagfish fossils, so there's not a lot to go on. 

There's a lot of extrapolation for a whole group. 

Yeah but you know, I don't see any reason why they couldn't have started out in shallower water and then moved into deeper water. One of the ways that I understand their current distribution (which tends to be on the deepish side) usually 100m down to about 3,000m. And we'll talk about the slime later I'm sure, but the slime is is really effective at protecting them from fish predators. But it has zero effect on air breathing predators. So, whales, seabirds that can dive that deep, pinnipeds: they just slurp up hagfish very happily and they don't mind the slime. So I think that really explains a lot about where we find hagfish because they're essentially immune to being eaten by fish, but they're very vulnerable to marine mammals and birds. So I think that drives them down and keeps their abundance pretty low in the in the upper depths.

Yeah that makes sense. well, let's talk about what they're most famous for which give them some of their common names ‘slime eels’ or ‘snot eels’. so what is the structure of hagfish slime and what makes it so unique?

Yeah, so it's probably best to start with the anatomy of the glands that make it. So, a typical hagfish has let's say 180 slime glands. So ~90 slime glands on each side of its body. They tend to occur just in a line down the body. There's really no place on a hagfish where a predator can bite without being near a slime gland that is going to shoot out slime in response. So these are smallish structures, a couple of millimeters in diameter. The glands are surrounded by muscle and when that muscle contracts, it basically squeezes the contents of the gland very forcefully out into the surrounding sea water - maybe even into the predator's mouth who's biting them. And something happens when that material (we call it exudate) mixes with seawater, it expands about 10,000 times to form this remarkable material that is really good at clogging gills of fishes. So let's unpack that a little bit. So there's two components that are made within the glands. One of them is a fibrous component and and these are silk-like fibers. They're about 15 cm long when you lay them out end to end. They're about 1 Micron in diameter, so a little thinner than your typical spider silk.

Wow! and from observation, it seems to be almost reflexive. so if you touch a hagfish or pinch it at one point, only those glands seem to fire.

Yes, so for a long time we didn't realise that. So when you catch hagfish on a boat, and maybe you've experienced this Thom, the slime is coming out everywhere.

Yeah the whole thing's angry!

You can just see this milky stuff pouring out of the slime glands, and it wasn't until we filmed hagfish in captivity under semi-realistic conditions and pretended like we were biting them, that we realised that it's only in the vicinity of where they're getting bitten that they release the the slime. So that's interesting. It obviously makes sense as the slime is probably a pretty big energy investment for the hagfish, so it doesn't make sense for the whole animal to be shooting slime out in every direction. 

I didn't know that about the the mucus component; that there isn't a bladder or a reservoir of it. the cells themselves are sacrificed. they can produce a lot and it's spectacular, but it's got to take quite a long time to recover? if the cells have got to be regrown rather than just producing more mucus?

Yeah, exactly! So I had a PhD student look into this question of slime gland regeneration. So this was Sarah Schorno’s work at the University of Guelph, and the answer she came up with, was that it basically takes a month for them to refill a slime gland that has been emptied. So again, another reason not to shoot them all off at once because a hagfish without slime would be very vulnerable.

"... It basically takes a month for them to refill a slime gland that has been emptied. "

— Doug Fudge

These sharks come in and bite hard on a hagfish and then they’re just swimming away with a mouthful slime streaming out of its gills. Yeah the obvious question is, what happens to that shark? Well, we don't know because in all these videos, the predator swims away and we don't see it anymore. But I can tell you a little bit about what we know from our lab experiments working with the slime. 

So you know we can collect exudate out of slime glands from anaesthetised hagfishes and we can work with the slime and measure its properties. So, we've confirmed that the slime is really amazing at clogging. We tried to compare it to other materials that you might think would also be good at clogging - like really high molecular weight polymers, food thickeners like xanthan gum, and the slime is more than two orders of magnitude (more than 100 times) better than all the materials we tried, in terms of how good it is at slowing the flow of water across a mesh.

So how does that relate to your question of: does the shark survive? So in those experiments, we were really interested in understanding how the mucus was contributing and how those really fine fibers were contributing. I had always assumed that the fibers were really important for the clogging function and so we did an experiment where we physically separated the mucus and the threads and measured the clogging performance separately. The amazing thing we found was that the mucus alone is pretty much as good as the whole slime with the fibers in it. So it's really the mucus that's that's doing the clogging. 

So that led to another interesting question which is: what the heck are the fibers even doing there? They're clearly really complicated to make; the cells that make them are one of the most interesting cells you can imagine. They spin this ball of yarn in their cytoplasm that is just an an incredible structure. Clearly natural selection has invented these things and perfected them over millions of years for a reason. And so, our current understanding is that the fibers are there to keep the slime on the gills.

So we did another experiment where we tried to wash the slime away… and this relates to your question of what happens to the shark; is the slime there forever or can they flush it away? So we tried chasing the slime with just seawater. And what we found is that if you leave out the fibers, the first go of the mucus alone is is really good at slowing the flow. But if you flush it again, you pretty much reduce the clogging effect to almost nothing. So in other words, it's really easy to wash it away if you don’t have the fibers in there. On the other hand, it's really hard to wash the fibers or wash the mucus away. So you can flush it with about 20 volumes more of sea water and the performance stays pretty similar. So again your question of how long does the slime remain on the gills… does the shark suffocate? Can it get it off? We don't know exactly…

…but it doesn't look good.

It doesn't look good. But I'll say one more complication which is: we were adding water from the top of our clogging assay, whereas a shark has the ability to back flush. So we weren't back flushing so maybe a shark, if it really forcefully back flushes, might be able to recover enough gill function to survive.

Which seems to be the first response. in that classic video, it immediately coughs, and coughs out this big mess of slime. 

Yeah, so that video came out in 2011 and we had published paper in 2006 where we argued that the prime function of the slime was to clog gills. We had never seen it in the wild, we had never seen it in the lab. We did some crazy experiments where we sort of added slime to isolated fish heads that we got from a fish market and we just tried to get at that question that way. So to see those videos from the group in New Zealand… it was just amazing for us. I show that video every time I give a talk, that shark coming in, biting a hagfish and swimming away in horror.

Is there a noxious element because I feel like I've almost seen them claim dibs on a meal by sliming it, without any sort of Predator interaction. just to keep everyone else away from this food fall they found.

Yeah that is something that we've seen several times now as well. A hagfish feeding around a carcass or some bait that you put on the bottom. They very often will slime even though nothing is biting them and the result is: you can end up with quite a bit of slime accumulating around that food and I do think that has a deterrent effect on other scavengers and predators.

Are they vulnerable themselves to this defence? I've seen too many in too small a container and Choke themselves.

And your second question: can they suffocate themselves? I think that's true if you have hagfish in a bucket and they're stressed out and they turn all of the water in the bucket to slime, which they're very capable of doing. If there's enough of them, then yeah eventually they will suffocate because they can't breathe slime. Another amazing thing that hagfishes do, that many of your listeners will know about, is knot-tying. They do this for various reasons, but one of the reasons is: if they happen to get caught in their own slime (which is is pretty rare under natural conditions) but certainly in a bucket if they have nowhere to go, they might end up inside a big mass of slime. But if that happens what they do is they tie their body in a knot and then they pass their body through that knot and that has the effect of essentially wiping the slime off of their body.

And they can use that knot as an anchor as well can’t they? that almost gives them leverage to push off from a burrow or from a carcass.

Yeah exactly, you might have seen videos of hagfish feeding, maybe on a whale carcass and they'll tie their body in a knot. They'll brace their head up against the carcass with a sort of loop, like an overhand knot and then use that to more forcefully tear chunks away from the carcass.

So with how unique and useful this slime is, there's been a few human applications hasn't there?

When I did my PhD, I was in a biomaterials lab where most of the research going on around me was on spider silk. There was a a lot of interest at the time about spider silk as a material and biometic efforts to make synthetic spider silk. Around that time, I was doing mechanical testing on these very fine fibres from hagfish slime and one of the things that came out of that work was the realisation that the material properties of these hagfish are actually quite good (not as amazing as spider silk but pretty close). So there's been quite a bit of interest in doing a similar thing that was already going on with spider silk, but with hagfish fibers. And that work continues. I dabbled in it for a while with my PhD supervisor John Gosline, but there's a group at Utah State who's very actively pursuing this line of research with funding from the Navy. They're basically using microbial vectors to make large quantities of the proteins that make up hagfish fibres, the material properties are so interesting.

One of our listeners noticed the colour variation in hagfish. we associate them as pale or slightly grey, but there's quite beautiful oranges and Pinky looking ones. and then here in New Zealand, we have a quite a beautiful mottled one. what is the variation because it's really quite striking. they're quite pretty some of them.

Yeah this is fascinating. So it might seem quite shocking to people that we only just now have the first hagfish genome published. The reason is that they do this thing called chromosome elimination which is very rare. One of the first things that we're taught in biology is that every cell in our body has the same genome, and isn't that interesting: how is a heart cell so different from a liver cell even though they have the same genome? It turns out that's not true in hagfish. They will just eliminate entire chromosomes from certain tissues. So if you're hoping to sequence a hagfish genome and you take a muscle sample, you're going to miss a whole bunch of chromosomes. So you need gonads to do it and have all the chromosomes represented.

It's crazy! it goes totally against everything we're told. that is really really strange.

But, you know, in many ways it kind of makes sense, right? Like I find the other situation sort of surprising. We waste a lot of energy transcribing a load of nonsense, so it's kind of smart I think.

Is it tissue differentiation or is it ontogenetic? are genes related to the juveniles that they then drop? or is it once I've decided to become muscle tissue I'm just going to carry muscle information?

Yeah that's a great question. I don't know. It's just yet another data point in the adage that ‘hagfish are just weird in every way’. No matter what aspect of their biology you're looking at, expect to be surprised. And that just happens over and over again. I don't know why they're so weird, but they are. 

"... I don’t know why they’re so weird, but they are. "

— Doug Fudge

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