This Podcast Will Kill You - Ep 165 Fish Tongue Parasite: Parasite Appreciation Hour

Episode Date: February 11, 2025

Okay everyone, think about your tongue. Maybe move it around a bit, check in with it, consider what it means to you. Now imagine that your tongue suddenly shriveled up and fell off and that in its place is a tongue-sized isopod aka rollie pollie aka pillbug. Just there, hanging out, forever. How are you feeling? Horrified? Disgusted? Hey, we get it. But at least you’ll never be alone again. Believe it or not, this is not some bad creative writing exercise. This is a very real parasite. In fact, it’s not just one but a whole group of them. Fortunately for us humans, these tongue-replacing isopods don’t target mammals but rather various fish species (unfortunately for the fish). And in this week’s episode, we’re getting up close and personal with these bizarre (and dare we say cute?) organ-replacing parasites. If you’re wondering why on earth you should care about a niche parasite like this one or why it might be a bad idea to get rid of all of earth’s parasites, then we warmly invite you to sit back, relax, and enjoy this parasite appreciation hour. Support this podcast by shopping our latest sponsor deals and promotions at this link: https://bit.ly/3WwtIAuSee omnystudio.com/listener for privacy information.

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I'm making this video in the hope that I will be able to watch it at some point in the future, and I'm going to show the world what happened here. I locked myself in the ICU. The CDC stopped taking my phone calls. Called FEMA, help hasn't arrived. I think I now know what is killing people. We were looking for some kind of virus, some kind of viral outbreak. I now know this is not a virus.

This is an organism. It is an organism that has somehow infiltrated people's bodies. The blistering, that's a symptom. That is what threw us off. It is the isopod. It's eating their organs. It's literally eating them from the inside. It is eating their intestines.

It is eating their liver. It goes for the kidneys, lungs, tissue. This is a rapidly growing, accelerating organism. How it's growing this fast, I have no idea. I noticed this rash about 45 minutes ago. and I'm going to continue to take the camera and I'm going to document everything that I see here. If you find this tape, just please get it out.

Erin, it was so hard not to laugh out loud. I loved your rendition of it. Clearly, I am not an actor, but I did the best I could. I disagree. I disagree based on that. Beautiful. And that is my audition tape for The Bay, too. You're hired.

Yes. That magnificent piece of fiction is I pulled from this incredible movie from 2012 called The Bay, which really the creature at the heart of this episode is also the creature at the heart of this creature feature film. Oh, my goodness. It's amazing. Hi, I'm Aaron Welsh. And I'm Aaron Olman Updead. And this is, this podcast will kill you. It's a little different of an episode, perhaps, today. It really is. I think this is one of the first episodes where we were just like, let's do this weird looking thing that we don't know anything about. There's got to be a story there.

Yeah. And there is. And a movie. A whole movie about it. Yeah. We are covering what is commonly known as the fish tongue parasite. Right. or the fish eating louse, tongue eating or tongue eating louse, not fish eating louse.

I mean, there are lice or, well, isopods. Yes. Marine isopods that eat fish. Yeah, this is what they do. So it's an isopod that we're talking about today. You've probably, have you seen the pictures? If you haven't, you will by the end of this episode. It is an adorable little isopod sticking out of the mouth of a fish or it's like in there right

where its tongue should be. and that's because it has eaten the fish's tongue and replaced it. We'll get into it, a lot of it. Oregon replacing parasites. I mean, beautiful. Amazing. Yeah, there is a lot of fun stuff to cover today. But first, it's quarantini time.

It is. Erin, what are we drinking this week? Well, we're drinking Laos got your tongue? Yeah. Because, you know, we just said it. We just said it. That's the name of it. Yes, Laos got your tongue.

It's your standard Mai Tai, which I'm shocked that we haven't actually done before. I know. We haven't. So in your standard Mai Tai is rum, curacao, oria, and lime juice. It's fantastic. Yeah.

So delicious. So delicious. We'll post the full recipe for that quarantini and the non-alcoholic placebo rita on our website, this podcast will kill you.com and on all of our social media channels. So make sure that you're following us. And you can see all the picks and also videos. of this recording that we do that now.

Yeah, we do do that. You can see Aaron's fish shirt, by the way. Yes. Mino Madness. He's got sunglasses. It's a very cute fish. It's not infected. Yeah.

He's drinking what appears to be a Mai Tai. It could be a Mai Tai. It's pretty thrilling. Yes. Oh, yeah. But also on our website, you can find all sorts of things from transcripts. You can find the resources that we use to put together all of these episodes. You can find links to bookshop.org affiliate account, our Goodreads list, music by Bloodmobile, merch, some pretty sweet merch.

Patreon, a contact us form. If you want to reach out, suggest an episode. You can invite us to give a talk. We love that. You can ask us to do a workshop. Or you can say, hey, here's my first-hand account. There's a first-hand account form for that. and other stuff. There's so much stuff there. Yeah. Well, shall we take a quick break and then get

into the creature feature of this week? Let's do it. So the creatures that we're discussing today are isopods. And I thought we should start by what the heck is an isopod. It's a good starting point. Yeah. Isopods are crustaceans, which means that they're arthropods. So they have exoskeletons that they shed between life stages in order to grow. And these are more closely related to like crabs and lobsters, which are also crustaceans, they're decapods, than they are to like bugs or insects, which are other terrestrial arthropods. So the isopods that everyone listening is probably most familiar with, whether you realized it or not, are like roly polis, right? Or also known as wood lice, also known as armadillo bugs, pill bugs. There's so many.

colloquial names for these. Yeah, what did you grow up calling them? Roly polleys. Yeah, me too. Yeah, roly polis. I'm always curious where the different names are. I'm sure there's a map for that, yeah. There probably is. What did you call them, listeners? Yeah. Roli polis, I love that name. It's very cute. But so the roly pollies are terrestrial isopods. Most isopods are not terrestrial. Most of them are marine or freshwater dwelling. There are probably at least 10,000 species of isopod in the world, and they make their home everywhere across the entire globe. And some of these isopods make their homes inside of other creatures, meaning they're parasites.

And parasitic isopods come in a lot of different flavors, many of which infect fish hosts. Fish are really phenomenal hosts for so many parasites. and many isopods infect fish in a whole bunch of different ways. Some of them might infect fish externally, like attaching on near their eyeballs. Some of them might attach in the gill chambers. Some of them might even burrow their way, like partway into their flesh. And some live in the mouth holes of fish. And that is the isopods, or those are the isopods that we're focusing on today.

mouth hole mouth holes like mouth yeah mouth just wondering with the whole part added i mean yeah the whole the whole whole they're in there and there is not just one of these so-called tongue-stealing parasites no no no there are many in fact they primarily fall in the family simothoidae and within that family, there's a couple genera that seem the most common to specifically do this tongue-sealing thing. That is the genus Sima-thoa and Ceratothoa. But there's several other as well, and I think the phylogeny seems to be in flux. Yeah. The most famous of these, I think the one that has gotten the most popular press, is Sima-thoa egsegua, if that's how you pronounce it correctly. Your guess is as good as my. Which is probably not.

great. So we can use that species as an example, since it's maybe one of the most popular ones, but all of these family of isopods share a pretty similar life cycle. And we definitely don't know everything that there is to know about these parasites. So there's a lot more that we could learn. And because there's so many different species, which infect such a wide range of host fish, We're going to look really broad strokes at what their general life cycle looks like to understand these parasites. So in general, these baby cymothoids, when they're born, they're born as like a live birth kind of because isopods have a brood that's called a marsupium, kind of like a kangaroo or a koala. It's adorable. Isn't it?

And so they hold their young, their eggs, in this little pouch, and then they hatch. And then they develop through several little life stages before they're ready to go off on their own. So baby simothoids, which are also called manka or mankay, that's the like baby form. Oh my God. I have no idea. I know. They have so many weird names. Like don't call them larva.

They're called manka. Wow. Okay. And once they leave their mothers, they are free in the water. and they will first attach to a host. Sometimes they might attach to a host fish that they didn't mean to, like maybe not the one that they really wanted. So then they might take a few nibbles from somewhere on the fish and then drop off.

And eventually the point is to find a suitable host. What species of fish that is will depend on the species of isopod. But the goal is to find a host that it's well adapted to, which is going to be some type of bony fish. And once they do, they will attach to that final host. Every single one of these isopods, these simothoid isopods, isopods, is born a male. So once they attach to their final fish host, they will look around, and if they're the first ones there colonizing this host, they will change their sex into a female.

That's amazing. I know. It's called protandrous hermaphrodism. And it's a form of what's called sequential hermaphrodism. So they're all born with one sex. And then some of them will go on to change sexes over time rather than simultaneous hermaphrodism, which is something like snails, which have both male and female organs at the same time. Right, right. It's fascinating.

But what this does is it allows for these isopods to attach change if needed into a female. and then every other isopod that finds that same host will then be a male that they'll be able to mate with and then make eggs from there, right? That's incredible because I kept seeing like the female isopods are the ones that replace. And I'm like, I wonder why that is. Uh-huh. It just means that she was the first one to attach. Yeah.

Wow. And then this isopod will live essentially the rest of its entire life attached to that host. primarily eating their blood. Okay. Not eating scraps of whatever the fish is eating. Not eating scraps of whatever the fish is eating. Some species, because again, we're talking about a pretty wide range of different cymothoid isopods here.

Some of them, it's thought, maybe feed more, like, on tissue in the fish's mouth. But again, it's feeding on the fish host itself, not on what the fish is eating. but some have different mouth parts that maybe look more like they're sucking pure blood versus eating the fish tissue. But in any case, they're feeding on the fish. And you said the rest of their lives, how long is that? I knew that you were going to ask me that question, Aaron. I can't resist.

I don't know. There, again, there's so many different species. For at least one of these, and the paper that I found wasn't actually looking at one of these mouth dwelling. isopods, but it was a different simothoid isopod that attaches near the eyeball of a fish. Those can live for at least one year. Okay. Does that mean that they all can live at least one year? Do any live longer?

Who knows? Okay. So they spend the rest of their lives. At what point then do they, like, so they are reproducing while replacing the tongue, while acting as the tongue? While acting as the tongue. Yes. Okay.

So essentially it's like this. One isopod finds a fish. They are the first ones there. So they're going to attach in the mouth, for the ones that we're talking about today, they'll attach in the mouth far back in the buckle cavity of the fish, kind of near the gills, but not quite near the gills. And then they will transform into their final form. And in this case, if it's the first one there, they'll transform into a female. That's the one that's going to take up the whole mouth.

where they attach, generally because they're feeding on blood, what it does is it disrupts the blood flow to the fish's tongue to such a degree that that fish tongue essentially begins to die. And then this isopod is able to grow and fill that entire buckle cavity, the entire mouth hole of that fish, kind of replacing where that tongue used to be. other isopods will also find this fish and they will attach sometimes closer to the gills, sometimes just farther back in that buckle cavity, which again connects to the gills because that's how the fish is breathing. And those will remain as males. The males are much smaller than the females. So they'll be farther back and they won't get as big. And they'll be able to mate. And then that female will have that has that little brood pouch. So it's just holding a bunch of little eggs that are growing and then releasing as they're ready to do so. Tadda.

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Okay. You said that like finding the right host, I know that a wide variety of fishes are affected or like can be infested, I guess, with this with this parasite. And then it will grow. So is its growth limited by the size of the fish's mouth? Or do they find fish that have a big enough mouth? Do they ever grow so big that the fish can't actually feed? Erin, I, I love, you're such an ecologist. Thank you. These are all great questions. Where to begin?

So there are a lot of different species of cymothoids, and they can infect, essentially, like, any fish that you imagine, there's probably a cymothoid that could infect that fish. Some of these species of isopod are more host-specific than others, and some are less host-specific. Okay. The question of like how big do they grow? Why do some grow bigger and some grow, some don't grow as large? We don't really know. Like does that depend on just who they ended up in? Is that part of why there are strong host associations between some of these isopods and some of their hosts? These are all really great questions. They're like evolutionary questions. They're like ecological questions because then it's also like, what is it doing to this fish? So there's a lot.

There's a lot to unpack there that we don't fully know the answer to. Okay. So let's talk a little bit more about like what this ends up looking like and what this ends up doing to the fish. Yeah. Yeah. So first, I want to talk about what this looks like. To give everyone like a mental image, we're just saying like a tongue replacing. What?

What? Google image search it. But then also I'll describe it for you. I also first want to say that a fish tongue is different than our tongue. And this is important for the idea that a fish could have their tongue replaced in function by a parasite. Right. Our tongue is this massive muscle, right? So our tongue is really important in moving food around in our mouth, pushing it from side to side.

It's also important in moving food to the back of our throat so that we can actually engage our swallow reflex. And we also use our tongue to speak, to talk. It's important in our breathing because it has to move in certain ways. But a fish tongue is not like that. A fish tongue doesn't have any skeletal muscle of its own. It's just like a little meaty bit. It's not a muscle. It's just flesh.

What does it do? It's essentially a mechanical device that just helps hold prey up against the roof of the mouth so that the fish can do whatever it needs to do, eat that prey and then swallow it. But there's no muscle. But there's no muscle to it. And if you look, if you Google image search a whole bunch of pictures of fish mouths without parasites, some of them don't even have that much of a meaty tongue.

Some just have like this kind of flat surface. Some have what almost look like teeth all along the bottom like of their palate. So there's a pretty wide variety of what a fish mouth might look like. But there are none of them as complicated as like a muscle that needs to be able to move around. And so the paper from 1983 by Bruska at all that first kind of proposed this hypothesis that the isopod could functionally replace a fish's tongue kind of really brought this to light. Like it's not, it makes sense that this isopod. could serve that same function because it's not that hard.

It's not. Anyone could do it. Anyone could do it. An isopod could do it. So what does it then look like when a parasite is replacing a fish's tongue? It's frankly adorable. It is so adorable. So we might be outliers in this because when I showed John, I was like, isn't this adorable?

And he's like, is that the word? Is that right? Is that the correct adjective? And I'm like, it certainly is. I know. When I was describing it to Brett, he was like visibly shook where he just was like, well, I think because we immediately put ourselves in the position of having an isopod, a large isopod in our mouths forever. Which is not what happened in the bay, by the way. Oh, it's not? No, the tongues were gone. And then they were, everyone was eaten. from the inside out. You know, it's eating the kidneys. It's eating the livers. It's eating much more intense than just the tongue. Yeah. I mean, there wasn't, it wasn't, let's say, the most scientifically accurate movie. Oh, I'm really shocked to hear that. I enjoyed it, though. Okay, so if you see a front-on view of like an open fish mouth that's infected with one of these parasites, the first thing that you'll notice is a pair of black eyes staring at you. It's so cute.

And then this little kind of almost triangular shaped head. It's like a little bit round on top. And then a little pointy little looks like a chin. Yeah. And then they have what looked like almost little hands curled up under their chin, like the way that a raccoon kind of holds their hands across their chest. It's like, hi. Hello.

Hi there. That's how it talks. Oh, hi. I'm just making my little home here. It's cave. Don't mind me. And then you can see what almost look like kind of scales that disappear. back into the fish's mouth.

They're not really scales, but isopods are arthropods, and so they have segmented body parts, like a shrimp or something. And then these guys have seven pairs of these little leggy things. They're called peripods. And they end in these pretty sharp little hooks that they use to attach themselves to the fish. Oh, just like hook in there. If you were to look, so that's what you see, if you look straight on, like,

oh, in an open fish's mouth. If you were looking at a fish in cross-section, like you sliced off the side of their cheek, and then you were looking at what this isopod looked like, you would see something that looks an awful lot like a roly-poly, except it's white, usually. And then based on most of the pictures that I've seen, and I haven't seen one of these in real life, but they are usually quite a bit bigger, depending on the species of fish that they're infecting than most roly polleys in your yard. And they're a little bit more flattened dorso ventrally. So like tummy to back, they're a little more flat so that they can fit in their fish's

little mouth there. Yeah. And this isopod essentially will take up like the entirety of the fish's mouth, the female will at least. And then sometimes if you do that cross section, you might see one, a smaller one further back, like almost halfway into the gill cavity. And that is what it looks like to be infected with one of these isopods. There are some amazing pictures out there.

They're really, really incredible. If you're not following us on social media, you'll have to Google search for yourself. You should just follow us. We posted pictures. Now's the time. If you were wondering, when's the time? It's now. Okay.

I have a question, though, about the effects. Because we basically said, okay, well, you know, a job so easy an isopod could do it, as in replacing the tongue. But it's not just replacing the tongue. It's also taking blood. So are the fish negatively impacted by that aspect? That is a really important question. A lot of the studies that have looked at what the effects are on the fish that are infected

are primarily in aquaculture settings with farmed fish, which is logical because not only is that a place where you can really study things like survival and growth and length and all these things, but also fish and aquaculture seem to. to be particularly susceptible to infection with these parasites. Okay. I don't know exactly why. It is thought that in aquaculture settings, it's almost always not a species that typically infects those fish.

Okay. And they're introduced by wild fish that then come in contact and then they're able to infect like all the whole entire. Right. Opportunity. Opportunity knocks. Yeah. Right.

But there are also some really incredible, sound really difficult to do ecological studies that have looked at these isopods in more natural settings. What are the effects on like population dynamics even, but also survival, reproduction? Long story short, overall, and again, it varies. Species to speciese to specy da-da-da-da-caviots. Fish that are infected with these tongue-replacing parasites do seem to be negatively affected to one degree or another. We see things like anemia. we see evidence of tissue damage and of the host's response to that tissue damage, so things like inflammation where the parasite was attached. We also can see inhibited growth and a reduction in weight and length of fish that are infected versus not infected. In some cases, we have increased

mortality of infected fish compared to non-infected fish. And in a lot of studies, we see a reduction in egg production or in egg quality in fish that are infected with a parasite compared to not infected. Okay. So yeah, it's not great for fish to have their tongue replaced by an isopod. But Aaron, getting back to some of the questions that you had asked about, like, why does this isopod infect the mouth? Like, what is the strategy there?

And then, like, what are the tradeoffs between a mouth infection versus an infection of an isopods somewhere else, like on the gills or on the external body or whatever. So there was a really interesting paper. I mentioned it already by Bruska et al from 1983. And that was the first one that really was like, hey, this isopod is essentially functionally replacing the tongue. The one that said, it's not that hard. And one of the things that they pointed out that's really interesting is that by making a niche in the mouth of this fish, the isopod in a lot of cases can grow to a significantly larger size than it could in say the gills of a fish because necessarily the size that isopod grows to essentially is displacing fish tissue, right? It has to like eat away a hole in

the gills, which is going to more negatively, affect the fish if it can't breathe as good, then by replacing a space in its mouth that's already a potential space to inhabit, right? Okay, okay. And so that is one of the big ideas as to like how this relationship works. It allows for the females to grow to a larger size, which theoretically means they can hold more brood so the isopods can

reproduce more readily or have greater fitness. And in the fish, it's a relatively less negative impact. And they even said in this paper, which I thought was really interesting, that like, well, we think that sure, maybe there's a negative impact for an isopod to replace your tongue, but it's less negative than just having your tongue eaten without anything to replace it. So that in itself is really interesting. Isn't it? It's so, so interesting. So these isopods essentially were pre-adapted to attach to a fish host and consume tissue. And so there could have been many different areas. And it was like, oh, the tail region, not great.

Not a whole lot of tissue there. The fish can't swim. It's just going to sink and die. And then it's like what area is going to promote longevity for both the fish and the isopod? And that happens to be the mouth. Yeah. for these species. For these species. And like, we know biologist evolution doesn't quite work like that, but yes. Right. And is a greater size always a good thing, not necessarily. Yeah, exactly.

Right. Isn't that so interesting, though, Aaron? I just, I love it. I just think these, I think these little guys are fascinating and fun. And they've, they've really been a parasite that I have thought of ever since the day I first saw them. They're one of your, what? Roman empires. I guess Roman Empire. Yeah. One of my parasite Roman empires. Parasite Roman empires.

I mean, yeah. Same, honestly. And like, every time I re-remember them, I'm like, wow, what a thing to exist. I know. I'm glad that we did this deep dive. Yeah. Well, tell me, speaking of deep dives, Aaron. I have so many.

I don't know where to begin with trying to understand the history of this, the evolution of this. The, what? Yeah, yeah, yeah, let's just take a break and then I'll begin. Anyone who works long hours knows the routine. Wash, sanitize, repeat. By the end of the day, your hands feel like they've been through something. That's why O'Keefe's working hands hand cream is such a relief. It's a concentrated hand cream that is specifically designed to relieve extremely dry, cracked hands

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If scaling your business feels like start starting over, you need the Intuit ERP. Intuit Enterprise Suite is the AI-Native ERP solution that consolidates, migrates, and automates, all in one place. Learn more at Intuit.com slash ERP. It probably won't come as a tremendous shock that the history of the fish tongue replacing isopod, specifically, the species that I feel like gets a lot of the press, the Simothoa exigua, that history is a little thin. The history in general is a little thin. Right. It's like, hey, we found this thing in 1979 and well, yep. I mean, so this genus of parasitic isopods actually goes back farther than 1979. So Simaithoa was described, depending on who you ask, it was either Linnaeus or Fabricius

and in the late 1700s. And then Simotho Exigua got its recognition or its name, I think, in 1884, when two naturalists Skioti and Minart included them in a big monograph about the subject. I probably said those names wrong. But nearly 100 years would pass after this monograph before anyone would pay significant attention to these bizarre creatures. And in 1981, in 1983, Bruska, like you said, Aaron, we mentioned these papers a couple times, published first

a monograph and then a paper, the second paper, the 83 paper, was with Gilligan, describing this isopod in detail, including some absolutely incredible pictures with the isopod in the mouth of a fish, the spotted rose snapper specifically. Also, side note, in this paper is where I learned that there is a fish species whose species name is... Boop. Boops. Boops. Aaron, I almost, I almost included that as a fun fact, too, because I loved it so much.

I loved it. I was like, this can't be some type of sea bream, right? Boops, boops. And I was like, what's a seabream? I don't know, but I love it. It's incredible. Also, this episode made me feel really depressed about how much knowledge your brain can just leach out because I used to know so much about fish, Aaron.

Same. I took an ichthyology class in grad school. I couldn't tell you anything. Same. I'm like, how to re-look up like teleost, Aaron. Oh, yeah. I mean, I'm married to a fish biologist, and I still don't know very much. Listen, we all have our strengths.

Yeah. But anyway, so since this 1983 paper, researchers have mapped the general distribution of this critter and other related critters. We've gained a better but incomplete understanding of its life. cycle, and we've measured the impact of these isopods on their fish hosts. But perhaps the most exciting development in the history of this tongue-eating isopod is the 2012 movie, The Bay, which is where our firsthand account came from. I really do think that it, like, increased awareness of this parasite and pollution in the Chesapeake Bay. I don't know. Maybe. Maybe. The Bay is a found

footage, Mockumenter. I just, I feel the need to include this because I watched it last night, so you don't have to, like you meaning general audience, Erin, I still want you to watch it. I am going to watch it. Don't worry. I'm going to watch it tonight. I just felt it didn't have time last night. Oh, man, it is ridiculous. And I just, I, it has a shockingly high rating on Rotten Tomatoes, 76%. And I'm, I'm going to, I just want to read you one more little quote from it, because how does it have a 76%? I want also all, everyone listening to know how many times I've heard Aaron say that it has a 76% on rotten. You could tell me that it has 76% you'd be, that's, that's going to be your Roman Empire is that the Bay has a 76% rating on rotten tomatoes.

Okay, here we go. I don't think we can rule out a foodborne virus or anything airborne, but this looks like a water vector. The blistering looks like a kind of coccosis. I don't know if that is spelled correctly or anything, but that's how I wrote it down exactly from the subtitles. The lesions could be mycobacteria marinum or schistosomyasis. I mean, Jesus, there could be cholera in there. If the water is being polluted by anything chemical on top of the bacteria, we can easily see a new form evolve. Maybe a fungal bacteria? What? Maybe a mutated tapeworm? Who knows? Oh, we would be so annoying for most people to watch movies with, I think. Oh my gosh. I mean, this is why when you and I watch something together, we drive other people away.

Yes. Um, yes. I, I love it. I mean, a fungal, a fungal bacterium mutated tapeworm? What? I know. What does that mean? A new form? A new form. That doesn't make someone want to watch this movie. Um, yeah. Okay. But amazing movie aside, that's pretty much it when it comes to the history. of these tongue-eating isopods. They haven't played a major role in world wars. They don't feature in the Hippocratic texts or ancient Egyptian papyri. They aren't associated with any major developments in medicine.

They do have this incredible creature feature about them, which is more than you could say for most parasites. I will give it that. True. True. But most people wouldn't place them high on a list of quote-unquote important parasites. True. Even amongst fish parasites, I couldn't find enough papers that gave them the credit I feel like they deserve. Exactly. And to that, I would say, yeah, most people are wrong.

Our human-centric perspective prevents us from grasping the significance of parasites and pathogens that don't directly or even indirectly impact us, like livestock diseases. And even if we do acknowledge the role that these underappreciated parasites might play in an ecosystem, it's largely from a parasites are bad and cause disease perspective. This is especially the case when it comes to conservation, where parasites are more often than not seen as a barrier to conservation efforts rather than a focus of conservation itself. So if you think of any wildlife conservation program, what animals come to mind? Big, like,

big charismatic mammals. Yes. I don't know. Big cats. Big cats, giant pandas, elephants, polar bears, sperm whales. These gorgeous and charismatic megafauna. Uh-huh. You probably don't picture the sperm whale roundworm that can grow up to nine meters long.

Nine meters? Nine meters. That's like, I, I, you. When you said it, it actually, like, didn't even register because I was like, that doesn't, it's not, wait, wait, wait, hold on. Yeah. Yeah. Yeah.

Yeah. Yep. Or you probably don't think of the protozoan parasite that infects blackfooted ferrets or the fish tongue eating louse. Parasites that would go extinct if their host when extinct. Like, for example, the California condors, lice, which did go extinct. Yes. Yes. I have it in here.

I have that in here as an example. Yes. Oh, my gosh. Several conservation plans include directly ridding a target species population of their parasites. So they choose one species over another. They choose the free living organism over the parasitic species, even if that means the extinction of that parasitic species. Right, because no one's worried about extinction of parasites. Exactly.

Exactly. And maybe out there you're thinking, okay, but like what's the problem with that? like parasites cause disease. They are bad. They're gross. Like, why wouldn't we want a parasite-free world? Mm-hmm. And that's kind of what I want to spend the rest of this history section talking about.

Like, why we should care about parasites, why they matter in ecosystems, and why conserving parasite biodiversity rather than reducing it should be a goal of conservation programs. I love this so much. This is like such our roots. I know. I know. Who knew? Who would have been?

thought we found our roots in the fish tongue-eating louse parasite. And so maybe at the end of this, I won't have convinced you to love and adore parasites, but at the very least, I hope that I leave you with a little bit more appreciation for them. We're going to call this the Parasite Appreciation Hour. Yes. Yeah. This is a planet of parasites. And I don't mean that in the sense of like humans are parasites because we're exploiting all these natural resources and destroying ecosystems and killing the planet and we are the parasites ourselves. You know, that is true. But I mean that on this planet, parasites dominate. Parasitism is the most common consumer strategy on this planet. Nobody, can you say that again? Because nobody appreciates that. Parasitism is the

most common consumer strategy on this planet. Parasites make up 30 to 50% or more of all living species. That's a lot of species. That's a lot of species. They may be the most abundant and the most diverse group of multicellular animals on Earth. I mean. And yet. And yet.

And yet. Compared to free living organisms, they get a sliver of the attention and the funding. In a 2020 paper by Colin Carlson at all, one of my favorite authors of scientific papers to read because they're just so... I have a quote from... Is it from the same paper? It might be. It might be. Yeah. So in this paper, the authors estimate that there are between 100,000 and 350,000 parasitic helmin species, the vast majority of which 85 to 95% are still unknown. Oh my gosh. Hundreds of thousands of species unknown. Unknown.

Researchers who study free living organisms massively outnumber those who study parasites. I mean, and we can attest to this being like the minority in our grad program of people who study disease ecology or parasites at all. And many large-scale ecological survey programs like Neon, the National Environmental Observatory Network, barely make an effort to characterize the diversity of parasites. in an ecosystem, despite the fact that they have been found in some communities to make up the most biomass. I mean, where's the caring? Where's the caring? The bias against parasites is clear.

But what is also clear is their incredible importance in ecosystems. Conservation costs money. And the goals of conservation programs sometimes conflict or appear to conflict with the needs of a region or community or the interests of a corporation. And so one big challenge that conservation organizations face is justifying why conservation is important. Like, why should we invest time and resources into preserving ecosystems and restoring biodiversity? Right. What's it going to get us as humans? Exactly. Especially if that comes at the cost of human economic development.

It goes without saying that this is a complex issue and that the balance of tradeoffs or even whether tradeoffs exist at all might be different depending on who you ask and the timescale and landscape scale that you're looking at. But the bottom line is that conservation must be argued for. And convincing people that we need to conserve charismatic species like blue whales or snow leopards, that's one thing. But persuading them that wormy parasites or parasites that replace the tongue of a fish that we don't really think that much about, that these are. also worthy of conservation, that's a whole other can of worms, pun intended. Yep.

In their 2023 paper, Conservation of Parasites, a primer, authors Limbary and Smith lay out three broad, not mutually exclusive categories that most pro-parasite conservation arguments fall into. So like, why should we conserve parasites? Here are the three general categories. Number one, intrinsic value. Parasites are worthy of conservation because they are living things. and like all living things, should be protected because all of life has value.

Number two, their ecological role. Parasites are key species in all ecosystems, and their removal could have unforeseen or foreseen and disastrous consequences. And number three, parasites can tell us how healthy or unhealthy an ecosystem is. In other words, parasites are valuable, number one, because they are. Number two, because they are vital in ecosystems. And number three, because they are important to humans. These are not the only reasons why parasites are important.

For instance, if we lose parasite diversity, we also lose opportunities to study the incredibly varied ways these creatures have adapted to this lifestyle, which could give us insights into the evolution of novel traits, the transition from a parasitic to a free-living life cycle, and even the evolutionary history of certain host species, which we could assess by looking at parasite genetic diversity. But for today, I want to chat a bit more about just those three I mentioned. And actually, just two and three, because besides the true parasite enthusiasts out there, I'm not sure a whole lot of people are going to be convinced that parasites have intrinsic value.

As we learn in school, and as Webster Dictionary defines, a parasite is, quote, an organism living in on or with another organism in order to obtain nutrients, grow, or multiple. supply often in a state that directly or indirectly harms the host, end quote. So why would something that directly or indirectly harms another organism be valuable to keep around? The answer to that question comes down to perspective and scale. To an individual elk infested with tapeworms, that's not going to feel good. You're not going to like that. You're not like, yay, more tapeworms. Wow, this is wonderful. Love that. But to the wolves who can more easily take elk infested with tapeworms, that's great. And the fact that not all elk in an ecosystem are infested with tapeworms or have different parasite burdens, that introduces diversity

into this dynamic, influencing which elk survive and which don't, and potentially driving the evolution of this population. Parasites are well known to mediate predator prey relationships like this. I love, I love parasites in predator prey relationships. They're just so good. There's so much there. It's just like life is tradeoffs. All of life is tradeoffs. And it's all these interconnected tradeoffs and relationships. And we don't understand it all. And that's what I love about it. Yeah. Also, there's just so many beautiful examples, including of fish parasites that like when you're infected with these parasites, you're far more likely to get eaten by a bird. And then that parasite is going to infect the bird. And then it's just so good. It's so good. And then it's not.

just predator prey relationships, right? Like there's also competition among members of a species. And so in these ways, parasites can affect how energy and resources flow through an ecosystem. So take camel crickets and grasshoppers, which when infected with a certain parasite, quote, are 20 times more likely to jump into a stream where their biomass constitutes up to 60% of the energy intake of endangered fish populations. Whoa. Oh. Isn't that so cool? I love that. I know. Wow. The cascading effects of parasites in an ecosystem are difficult to measure, but it's kind of like, as Joni Mitchell says, you don't know what you got till it's gone. Parasite removal from an ecosystem is kind of like what we saw with the removal of apex predators to protect livestock, which led to an explosion in some populations, like some herbivore populations. decimation of others, and an overall vastly changed landscape that regained stability once those

apex predators, like wolves, were reintroduced. Parasites play similar roles in ecosystems, helping to organize, stabilize, and promote genetic diversity. Parasites can also stimulate a host's immune system, so some studies have shown that parasites can protect hosts from a novel pathogen or damages from heavy metals. See our allergies episode for more on that kind of hypotheses. Zoomed in to like an individual level, it's very easy to see why the word parasite has such negative connotations. But taking in the big picture of an ecosystem, these are vital and so underappreciated parts

of this beautiful, intricate machine, one where we don't fully understand how it runs and what might happen if we fiddle with this. knob or adjust that lever. And let me remind you again, this isn't like a handful of parasites playing an important role in a few interactions here and there, even though I've only given a few examples. This is everywhere. Every free living organism on this planet has parasites. Yeah. Again, yeah, parasites might be the most dominant life form on Earth. Period. And so maybe you're still not fully convinced that we should conserve parasites. I know you are, Aaron. I had you at intrinsic value.

Yeah, you did. But what if I told you that we can use parasites to assess whether an ecosystem is healthy or unhealthy? Certain species of parasites actually accumulate pollutants more readily than their hosts, and so they can set off early warning bells about a new pollutant or one that's on the rise in a particular ecosystem. And parasites with complex life cycles involving multiple hosts also tend to be more sensitive to environmental change. So if humans alter a habitat or introduced pollutants or if the climate gets warmer and drier, these parasites might be the first ones to feel those changes, which can be helpful for us to forecast potential downstream effects. Although it might seem like a contradiction, a healthy ecosystem is one with parasites, not one without. But as human-mediated change keeps on trucking, as this extinction crisis keeps ongoing, we're at risk of losing this key component of ecosystem function.

As a group, parasites are among the most, if not the most overall, threatened with extinction, with estimates ranging from 3 to 33% at risk. Wow. Because when a free-living species goes extinct, it takes with it. it's species-specific parasites uniquely adapted to that species. And most of these parasites have never been characterized in the first place, which makes it that much harder to track their disappearance. Most conservation aims don't specifically include parasites, and very, very few parasites are on any endangered species lists, which doesn't mean that there aren't any endangered parasites because there absolutely are. What we need is a shift in how we

perceive parasites. We need to do a better job at recognizing their value, characterizing their diversity, understanding their role in ecosystems, and developing concrete goals for their conservation. And very importantly, let me underline this, these goals are not preserve all parasites no matter what. Because I know some of you out there are like, wait a second, I thought we were trying to eradicate draconculiasis caused by this parasitic worm. Should we? Save that? No, no, absolutely not. We are trying to eradicate for concholiasis, and we will get there eventually. Shout out to Jimmy Carter and the amazing work of the Carter Foundation. The research groups that have put together these roadmaps for parasite conservation make it very clear that there

are exceptions. No one is a parasite extremist. Parasites that are excluded from these plans include those that present a disease risk to human health, livestock health, or threaten the existence of a wildlife species, like the nematode that infects giant pandas and can actually lead to their death. So no, this is not a call to save all the parasites, but it is a call to acknowledge their incredible diversity and underappreciated significance, and maybe just to reflect on our own bias when it comes to parasites. Save the whales, absolutely, but also save the whale tapeworm.

Save the fish-tong louse parasite. So with that, Erin, I'll get off my soapbox and hand it over to you to tell us what's going on in the world of fish tongue replacing isopods today. We'll get into it right after this break. Oh, Erin, that was so much fun to just think about parasites in a much larger context. That's also where I will end. But to bring it back for a moment to Sima Thaithoa Dysopods. Like, what are we talking about again? What do we do?

What's this episode about? One of the things that I wanted to underscore here, because I think that these parasites, specifically Simahtoa exigua gets the most attention as like the one. It is not the one. There are so many of them. I fell into that trap too. I was like, this is the one that I see the papers about. It's an understandable trap to fall into.

But there are so many of these parasites, which are mouth-dwelling parasites. And they're literally everywhere across the entire globe. In some studies that I found that we're looking at, you know, like specific species of this particular isopod in specific. specific species of fish, right? So like one paper on this species, another paper on this species. Across the board, in some of these prevalence of these parasites was as high as like 45%. That's so high. I know. And it does seem to vary depending on the size of the fish. So smaller fish, prevalence seems to be higher in a lot of cases and less prevalence in the larger fish. Why is that? I don't know. Is it that once they get larger, has the isopod already?

died. I don't have an answer for that. But it also varies geographically. Though these isopods are present across the entire globe, they do seem to be at higher prevalence in warmer tropical waters, as well as in places where we have a lot of aquaculture. Because in some studies, they have found prevalence of these fish-tonged parasites in aquaculture that are as high as like 98%. And there have been some cases of like relatively high rates of mortality in aquaculture species of fish. And that's not usually due to like a typical host parasite interaction, but maybe like a parasite that doesn't typically infect that species of fish. Right. So there's not like a general like conclusion that I can draw from all of this because there's so many different species of these

parasites that infect so many different species of fish. Aaron, what about, geographic range. Like, are these parasite species sort of distributed globally across the world? Or they increase in prevalence or incidents or diversity as you go closer to the equator? That's a good question. I didn't look at whether you have like a increase in diversity with latitude and stuff like that. I would assume similar to a lot of other parasite and, you know, species in general that you probably see higher diversity in tropical areas that are warmer, etc. A lot of the papers that tried to look at like overall diversity of these parasites were very region-specific. So we have papers that are like, here's the diversity in the Indian Ocean.

Here's the diversity in this region. Here's the diversity around Australia and stuff like that. So I didn't find any that were, well, there was a couple actually that was like global diversity, but they're just like really, really broad. And also don't tend to be specific to just the tongue replacing cymothoids, but are looking at simothoids more broadly. Because again, these can also infect fish in other areas, not just in their tongue. In any case, there's a lot of them. They're everywhere. So where do we go from here, right?

There's so many open questions. How many species are there really? Because we don't know. Are they changing in distribution? Like, are they moving around? And if so, why? What is it that's driving changes in distribution? Why is it that some species are much more host-specific than others?

What are the factors that are driving this host specificity? I don't know. So many questions. So many questions. And so I don't have answers to any of those questions. There are so many people doing work to better understand the natural history, the evolution, the ecology of these parasites. But I also wanted to end this episode with some bits from, I think, the same paper.

I have three column Carlson papers. So. Yeah. Well, it's not just Carlson. No, Colin Carlson at all, a longtime friend of the pod, I've decided we're friends, never met. But this was the paper from 2020 that was published in proceedings. of the Royal Society, B. And I just really appreciated this paper not only for its thoroughness,

it was like a really long, detailed paper that essentially makes the case for a real need to get a handle on the existence of parasite diversity across the globe. Yeah. And this paper uses worms, mostly worms that do infect humans, as a case study in this. And it also goes into a lot of detail on like, how does one, how do we as a scientific community go about actually accomplishing this? And what does it mean for understanding our planet and the health of our planet both now and, of course, under conditions of climate change in the future? And so I just want to, I don't have like profound things to say as conclusion of this paper. But like, I just do think that it's so this parasite, which is charismatic in its own way.

Right. More than other parasites, if we are allowed to be the judge. Yes. Much more than other parasites because it is, it is very cute. It is very startling. Like, it makes you feel things, even if those things aren't like the warm and fuzzies. I think that it gives us the opportunity to really think about. parasites in a way that most people just don't ever think about parasites or try not to think about

parasites. And so I want to end with this one little quote from this paper, quote, though some consider the task of cataloging parasite diversity a testimony to human inquisitiveness. It is also a critical baseline for understanding biological interactions in a world on the brink of ecological collapse, end quote. And I feel like that's an important thing to keep in mind. Parasites have a lot that they can teach us, and we should learn from them. Yeah. Agreed.

Case closed. Case closed. Not taking questions, thank you. Goodbye. Comments only. But we have lots of places that you can learn more about these parasites, the simithoid isopods, so cute, and so many other parasites and their roles. in our ecosystem. So let's hit you with some sources. So many sources, Aaron. Okay, so I will shout out

once again that Brusca and Gilligan paper from 1983 that has a great description of one of these parasites. And then when it comes to the importance of parasites in ecosystems and why we should conserve parasites, I have a million papers. I really enjoyed one by Limberry and Smith from 23 titled Conservation of Parasites, a Primer, which I also shouted out in the history section. But there are so many ones out there that are not reviews, but like specific papers about this parasite in this ecosystem or in this population. And it's just honestly really enlightening reading. I love it.

I have a number of papers more than I expected for this episode. I also loved that Bruce Gann-Gilligan paper. there was one from 1998 by Bunkley Williams and Williams called Isopods Associated with Fishes, a Synopsis and Corrections, and then a one I really loved from 2014 by Smit at All, global diversity of fish parasitic isopod crustaceans of the family Simothoidae. So there's a few other papers that are more broadly about the Simothoids, and then a bunch of like specific ones about this species versus that species, etc. But as always, we'll post the full list of our sources from this episode.

episode and every single one of our episodes on our website, this podcast will kill you.com. Check it out. Thank you to Bloodmobile for providing the music for this episode and all of our episodes. Thank you to the film director of the Bay. Just kidding. Thank you to Tom Brifogel and Leanna Squalachi for the audio mixing. Thank you to everyone at exactly right. And thank you to you, listeners. We hope that you enjoyed this episode. We hope you love parasites a little bit more than you did before. Yeah. And we hope you learned something new. And a special thank you and shout out, as always, to our fantastic patrons.

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