How to become a parasite

00:00:09
Speaker 1: Imagine sitting out on a porch on a warm spring evening. You hear the spring peepers calling out to each other in a nearby pond. Birds are chirping in the trees, preparing to turn into their nest for the night. Off in the distance, you see a deer walking with her fawn. It's a beautiful scene, reminding you how wonderful it is to live in Virginia when I hear and see these denizens of the forest, though I see something else as well, walking ecosystems for parasites. You see on average a bird species harbors more than three species of tapeworm nearly three nematode species. Probably call that frog home. Those deer are harboring treematodes, tapeworms, nematodes, and more. Though hidden from sight, parasites are thought to make up over half of the biodiversity of multicellular organisms on our planet. Today, we're going to answer listener questions about the evolution of parasitism. Hope you're not listening to the show while eating dinner. Welcome to Daniel and Kelly's Extraordinarily Infected Universe.

00:01:30
Speaker 2: Hi, I'm Daniel, I'm a particle physicist, and I'm looking forward to meeting alien parasites.

00:01:35
Speaker 1: Hello. I'm Kelly Wiener Smith. I study parasites and space, but not parasites in space. But maybe one day I'll study Yeah, maybe one day I'll be studying parasites in space.

00:01:45
Speaker 2: So my question for you today, Kelly, is what is your favorite depiction of parasites in science fiction?

00:01:51
Speaker 1: Probably aliens don't need to think about it. Aliens, it's a great parasitoid in there. I went through a phase where I was just love all of the zombie movies and I was actually taking notes on like what mechanism was producing the zombies. So for a while, you know, when there were big fears of nuclear bombs getting dropped on the US for example, you know, nuclear material was the cause of zombies. But then it became like infectious diseases, and I've gotten off course. I was thinking about writing a whole book on this actually.

00:02:22
Speaker 2: And why didn't you?

00:02:23
Speaker 1: It sounds fascinating, you know enough people told me that, like, the zombie craze is behind us, and that this is a book nobody would want to read because it would be two of the time. Yeah, two of the moment, right, like when people weren't watching the zombie movies like The Last of Us Sword, which I guess that's running still now.

00:02:39
Speaker 2: Yeah, but zombies aren't a moment, They're a movement.

00:02:42
Speaker 1: You know, that's right. Well, and I gave a talk at the Zombie Apocalypse Medicine Meeting, or I think I for Sam. Yeah, and it's happening in October of this year. I highly recommend folks go to it.

00:02:55
Speaker 2: These are legit scientists.

00:02:56
Speaker 1: These are legit scientists. Yes, wow. And I gave a talk on that topic and it was like the most fun research I've ever done in my life.

00:03:07
Speaker 2: Well, I think I speak for the audience and saying that we all want to read your book on the science of zombies. That sounds fantastic.

00:03:12
Speaker 1: You know. I've got a link to the talk that I could share if anyone wants to see the talk, But I don't think it's going to become a full length book, not with what I've got on my agenda right now.

00:03:21
Speaker 2: All right, Well, today we're not talking about space or science fiction or zombies. We're answering a really fascinating question from a listener about the context, the evolution, and maybe even the future of parasites in our world.

00:03:33
Speaker 3: Take it away, Zach, Hey, they're extraordinaries. The recent episode on the history of Trickinella, I should say, the recent history of trick and Ella got me wondering about the more ancient history of parasites. I assume that being a parasite is a strategy that existed back in the time of the dinosaurs, maybe even further back. I'm curious if we have evidence of that, and if parasites tend to become extinct when their host species does as well, or if we're still living with the same kind of parasites the dinosaurs dealt with. I'm also curious if we know about how parasites evolve, how does an organism go from being not a parasite to a parasite? Do we have any idea of that evolutionary history. Looking forward to hearing more squishy wormy biology from you guys.

00:04:23
Speaker 2: Thank you Zach for sending in this question. If you have questions about the nature of the universe, or the history of parasites, or anything in this extraordinary universe, we want to hear from you. We want to answer your questions. We want to scratch your itch because everybody's curious about the universe, so don't be shy. Write to us two questions at Danielandkelly dot org. Everyone gets an answer.

00:04:44
Speaker 1: And I am currently researching why we scratch itches, which is a question sent by a listener. So you even get those kinds of questions answered.

00:04:52
Speaker 2: Amazing, yep, So dig into this for us, Kelly, tell us about the history of parasites and why we call something a parasite and what fraction of species we could classify as parasites.

00:05:02
Speaker 1: Well, Daniel, let's pretend that you didn't read the outline, which maybe maybe you didn't, although you're usually very well prepared. If you had to guess what percent of multicellular organisms were parasites, what percent would you guess?

00:05:15
Speaker 2: It doesn't seem to me like most things are parasites. Bunnies are not parasites, flies are not parasites. Whales or not parasites, So you know, it seems to me unusual. So I guess one percent is my absolutely no information.

00:05:28
Speaker 1: Guess are you playing along? Are you really guessing one percent?

00:05:31
Speaker 2: I'm really guessing one percent?

00:05:32
Speaker 1: Oh wow, that's fantastic. Okay. I love those moments where you can be like, you're totally wrong. So Dan, you're totally wrong. Yes, yay. So if you look at that rabbit, So, rabbits are mammals, and on average, mammals harbor one to two species of trematodes, one to two species of tapeworms, and four nematode species.

00:05:51
Speaker 2: Well, I didn't want to eat rabbits before, but I certainly don't want to eat rabbits now.

00:05:55
Speaker 1: Well, then let's not talk about more typical food animals like cows and pigs. We'll stay away from those. But it's estimated that this was from a two thousand and eight paper that was estimating about forty four thousand host species with vertebrates and about seventy seven thousand species of trematodes, tapeworms, nematodes, and acanthoscephalins. So this is four kinds of parasites, almost twice as many parasites as there are hosts. And that doesn't even include things like lice and ticks and stuff like that.

00:06:25
Speaker 2: What hold on a second, you're telling me that like every species of critter out there has like two unique species of parasites, So there's like a two to one parasite to creater ratio.

00:06:36
Speaker 1: This is a great question. This is where it gets a little bit complicated. So, for example, if you have two different species of rabbit, they might share one of their trematode species for example. So there are some parasites that are generalists and you can find them in more than one host species, but there are also a lot of parasites that are found in one species. And so in general, when we look at how many parasites are generalists and how many are specially listener just found in one host, we get this estimate where we feel comfortable saying that probably more than fifty percent of the multicellular biodiversity on Earth is using the parasitic strategy in some way.

00:07:10
Speaker 2: Oh my gosh, we're in the minority. Yeah, for not sucking on some other critter, we are like unusual.

00:07:17
Speaker 1: We're total chumps, man.

00:07:19
Speaker 2: So we got to rethink this.

00:07:22
Speaker 1: Wow, I know there's a lot of interesting questions that arise from there. So we know that a bunch of species have this strategy of being a parasite, which essentially means that you rely on another organism where you're extracting their energy. It's not benefiting them, and you need them to get the energy.

00:07:39
Speaker 2: Well, then can I ask a technical question like, yeah, is a cow a parasite because it relies on grass and you know it's extracting the energy from the grass. Why isn't a cow listed as a grass parasite?

00:07:50
Speaker 1: So it is eating lots of different blades of grass, and so this is usually an intimate, endurable interaction between two INDs.

00:08:00
Speaker 2: Intimate, wow, intimate.

00:08:02
Speaker 1: Well, you know, if you're living inside, for example, of something and sharing gets dinner with it every night, that's pretty intimate. And so, yeah, cows are eating lots of different grass. We call mosquitoes and lamb preys and ticks and stuff micro predators. So they take little meals from lots of animals. They don't kill them, which predators usually do, but they just take little meals from lots of animals. And so the idea is that to be a parasite you have to be on one organism for a long time.

00:08:33
Speaker 2: Okay, so mosquito's technically not parasites then, right.

00:08:36
Speaker 1: You would be amazing how much fur flies at parasitology conferences when the mosquito and the leech people are like, these are parasites. And then some of the community is like they're micro predators, and the leech people are like, we belong here, and I'm like, they belong here, don't kick them out.

00:08:51
Speaker 2: Well, they're parasites in spirit, even if they don't qualify technically, right, because otherwise, like a giraffe which nibbles on this tree and then the other tree in the other trees basically has the same relation with a tree as a mosquito does with people, right, So.

00:09:03
Speaker 1: That's because it's not killing all of them. Yeah, yeah, And so sometimes you can get into debates with some folks about to what extent herbivores can be thought of as parasites solely. You know, if like an insect lives on the same tree for a long time, is it a parasite or is it a different category altogether because it's a plant And anyway, I guess at the end of the day, nature doesn't care that humans like to have very nice clear boxes to put things in. She's always giving us edge cases to make things complicated.

00:09:31
Speaker 2: And life is an ecosystem, right, It's a web, and there's nothing isolated. We're all munching on each other and eating each other and being eaten by each other. It's a big party, right.

00:09:42
Speaker 1: I don't want to go to the parties you go to, Daniel. Based on what you've just described, we have fun.

00:09:47
Speaker 2: We have fun. But how useful are these categorizations? I mean, does it matter what we call a parasite or not?

00:09:55
Speaker 1: So I think it does. And the main reason I think it does is because often folks will try to model the impact of parasites on populations, or use some modeling to try to understand how to control a parasite population, and those models depend on the assumptions you make about what that organism is doing, how many organisms it's feeding on, how many babies it produces. Like, having these details helps you create models that can describe different groups of strategies, and those models can be important for a lot of things, including epidemiology.

00:10:27
Speaker 2: I see, all right. And also it's nice to get like minded people together, right, And so yeah, you gotta people want to know which conference do I go to? Do I go to the zombie conference or the parasite conference or the zombie parasite conference or whatever? And so we got to put names on things so we can get ourselves organized, even if we admit that the names are fuzzy. You know, is Pluto a planet or not doesn't really matter. We're interested in studying this stuff fluting in outer space.

00:10:51
Speaker 1: I wants organized a symposium called how to Make a Zombie with my friend zen Folks, who has like the coolest name ever, and then it was me, who has the least cool name ever. And it was a very interesting combo. But anyway, it was all about parasites that manipulate host behavior and how they do that.

00:11:06
Speaker 2: Right, right, Yeah, parasites and zombies are connected in the end.

00:11:09
Speaker 1: Sure.

00:11:10
Speaker 2: So the bottom line is that there are lots and lots and lots of parasites out there. I was totally wrong, and there's lots of different kinds of parasites out there. But what about by body mass? Are parasites like a tiny fraction of the mass or if you made a big pile of all the parasites on the planet, would be bigger than the pile of non parasites.

00:11:28
Speaker 1: This is a great question. So I didn't necessarily prepare for you to ask this question, so I don't have all the numbers that I want at my fingertips. But there was a system where this question was studied explicitly and it was the estuaries in southern California. And essentially they took the biomass of all of the free living organisms, and they extracted the parasites from them, and they took the biomass of them. And in these systems, the main predators are predatory birds. They eat the crustaceans, they eat the fish. They're like the main predator out there. And the biomass of trematode parasites was equal to and maybe I think in some cases greater than the biomass of the top predators in the system, which is to say, greater than the biomass of the birds.

00:12:09
Speaker 2: Wow.

00:12:10
Speaker 1: And that's because this system has a lot of trematode parasites that infect the snails, and when the snails get infected, they get castrated and all of the energy that would have gone to making baby snails now becomes parasite biomass. And so the ground it's like a carpet of snails in these estuaries. There's just loads and loads of them.

00:12:27
Speaker 2: I'm imagining stepping on them to the crunch, crunch, crunch.

00:12:30
Speaker 1: It's not crunch crunch, crunch, crunch, because it's very muddy out there, so you're like squishing them underground, like they're getting covered in mud. So you do feel bad about that. I did feel bad. It was great exercise working in this system though. This is what I did for my PhD, because every step you take you need to like steal your foot back from the mud. And anyway, good exercise. Point is some systems have really really high parasite biomass. But for example, if you were to collect a deer, the biomass of parasites in a deer is probably a very small percent.

00:13:04
Speaker 2: Like one percent.

00:13:05
Speaker 1: Yeah, I guess you know, probably like one percent. That's where you came from.

00:13:08
Speaker 2: Day I tell what I meant. Yeah, yeah, let's see.

00:13:13
Speaker 1: Totally right. You hit the nail on the head. It was perfect.

00:13:18
Speaker 2: If we keep asking questions, we'll eventually find one that lines up in my answer.

00:13:21
Speaker 1: That's right, And we got there pretty quick. We got there pretty quick. And and this biomass question that you asked depends a lot on what the strategy of the parasite is. And in our next segment we're going to talk about different strategies. But basically, when the strategy involves killing your host, you can take up more of that host biomass because the host is going to die anyway. But now I want to talk about you know, so we've determined that maybe as much as fifty percent or more of the number of species on the planet that are multicellular are parasites. But how many times did you get a transition from free living to a parasitic strategy?

00:13:54
Speaker 2: Right? Like? Did this evolve one time and now we have a huge explosion to parasites or is it a common thing that's been developed independently multiple times, like flight for example.

00:14:04
Speaker 1: I mean, do you want to give a like, guess at how many times parasitism has evolved.

00:14:09
Speaker 2: This time a good sport. I would guess that it's evolved multiple times because it seems like kind of an obvious strategy and it doesn't require like some huge technological innovation like photosynthesis or the engineering a flight. Even so I would guess it's pretty common. I don't know a dozen times.

00:14:28
Speaker 1: When we had Joe Wolf on the show, she pointed out that the crab body plan has popped up I think eight times. I think you undersold the difficulty of parasitism a little bit. So if you are trying to infect a host, you need to find that host you need to survive its immune system. You're now in a completely foreign environment that is actively trying to kill you all the time. But despite those challenges, we think that parasitism has shown up at least two hundred and twenty three times in Kingdom Animalia.

00:15:00
Speaker 2: Sorry, two hundred and twenty.

00:15:02
Speaker 1: Three times, two hundred and twenty three times.

00:15:05
Speaker 2: Like you can identify individual moments when something went from non parasite to parasite in the evolutionary record hundreds of.

00:15:13
Speaker 1: Times, hundreds of times.

00:15:16
Speaker 2: Yes, wow, why aren't we all parasites?

00:15:19
Speaker 1: Well, you know, there's got to be some.

00:15:20
Speaker 2: Hosts, I see. For the grifters to keep drifting, we need some suckers, that's what you're saying.

00:15:26
Speaker 1: And you and I are the suckers, I guess. But so sometimes you get this transition to parasitism that results in a lot of additional species. So I think when you ask people about parasites, they usually think about things like tapeworms and trematodes and nematodes and stuff like that.

00:15:43
Speaker 2: Unfortunately, that is my mental image, and that's why it gives me the willies.

00:15:47
Speaker 1: Yeah, I'm sorry.

00:15:48
Speaker 2: It's intimate, though, it's.

00:15:50
Speaker 1: Very intimate So in phylum plati helminthis, so that's where you get the trematodes and the tapeworms and stuff. They were about transitions to parasitism last time we counted. But in arthropodas, so these are invertebrates with exoskeletons. So we're thinking about like wasps and crustaceans and stuff like that. It evolved one hundred and forty three times. Wow, So lots of transitions and lots of times it happened in insects, and it happened sixty times in flies. So flies you often get a transition from free living two parasitic strategies.

00:16:28
Speaker 2: How do we trace these transitions? Like I understand we can look at the fossil record and see how whales return to the ocean this kind of stuff. But insects, we don't have fossils. How do we know this history.

00:16:39
Speaker 1: We do have some insect fossils. Often they get trapped in things like amber, and amber actually does a really great job of preserving things and making it so you can still see their features. Yeah, but we probably are undercounting because there probably are a lot of like transitions that happened and then we're lost those species went extinct, or a lot of just instances where there was like a free living host that we'd just never found. And so these you should imagine that all of these estimates are very likely to change over time. But we have been lucky to get a fair number of fossils in the record, and the fossils are usually not beautiful exactly what you want fossils. But for example, there's a lot of tapeworms and monogeneans. So these are monogeneans are parasites that you often find on the outside of fish. They have these hooks that they use to hold onto the fish and then they suck like the mucus. Sometimes they suck blood, but they live on the outside.

00:17:32
Speaker 2: The less intimate side, we'll say.

00:17:34
Speaker 1: Yeah, slightly less intimate, but no less durable, and so they use these hooks. And then tapeworms also tend to have these hooks and they use them to hold on inside. So now we've got intimate and durable. But these hooks have very distinctive shapes, and sometimes we can find in fossilized animal poops called coprolites, which we've talked about before, these hooks in like the pairings that you would expect and in the shapes that you would expect and inside of or of, like very adjacent to organisms that you would expect to find them in. And we found evidence wow, of these sorts of relationships from the Mesozoic area, which was sixty six to about two hundred and fifty million years ago. Wow. This includes like the Jurassic period where you know Jurassic Park and the Paleozoic period, which is even later. I think Paleozoic means something like old life. And we found some evidence of these parasites from back then too, so it's totally reasonable to assume that like t Rex was probably carrying around some nematodes. These are ancient interactions.

00:18:36
Speaker 2: I love thinking about when things arose because it's a way to figure out, like how hard is it to happen? And also is it like that happen on alien planets? Like we think about how quickly did we get life after there were the conditions for life on Earth? Pretty quickly? How quickly did we get intelligence after they were their conditions for intelligence? Not that quickly? And it's only end equals one, but it's like a hint as to like is this likely to happen elsewhere, And so how far back is the evidence of parasitism? How quickly did people start to slurp and munch intimately on each other?

00:19:08
Speaker 1: Well, so we are mostly focusing on multicellular organisms going after other multicellular organisms. I think it would be really hard to know if there was like a bacterial parasite going after other organisms in the like primordial soup era of life.

00:19:26
Speaker 2: I guess bacteria phages could be considered parasites and viruses that infect.

00:19:29
Speaker 1: Bacteria, right, Yeah, I meant conferences. We like to joke that the first organism was free living, in the second one found some way to exploit it. But you know, we don't know that for sure.

00:19:39
Speaker 2: This economy that's just happening.

00:19:40
Speaker 1: Yeh, that's right, that's right. And so you know, I think pretty early on, when you start getting multicellular organisms, you get parasites also.

00:19:48
Speaker 2: And remind us how early did we get multicellular organisms For those of us who are not evolutionary.

00:19:53
Speaker 1: Biologists, So multicellular life is probably something like seven hundred million years ago, and I think we have decent evidence of parasites in multi cellular organisms from at least two hundred and fifty million years ago, and I'm guessing that before that it gets even harder to find evidence of parasites, and you have to be super careful that you're not tricking yourself. So, for example, there are these isopods that attached to fish, and sometimes they'll like live on top of their gill arches, so like underneath their gill flap, and then other times they'll live on their tongue. And so if you find an isopod next to a fish, you might be like, oh, there's a parasitic isopod, But how do you know that it wasn't just like fossilized next to the fish. How do you know it was actually a parasitic relationship. So this stuff can be really difficult.

00:20:40
Speaker 2: These glimpses into deep time are super fascinating. One of my favorite things in science. You know, to think that like Earth had life on it for billions of years before it had multicellular life. You know, like so many years, so many seasons, so many days past with just like frothing bacteria and stuff floating in the oceans. Now really interesting, which is also kind of depressing because it means like if you find a planet with life on it. There's a good chance it's just a bunch of single cell organisms nobody really to talk to. It's incredible how rapidly things have changed. Like seven hundred million years ago, we get multi cellular life and then you know, things explode. And only recently have we had intelligence and technology and podcasts.

00:21:22
Speaker 1: Why I promise to not rat you out because you essentially said there's just bacteria and that's not interesting. I promise I won't tell Katrina that you said that, but I agree, Well, this is.

00:21:33
Speaker 2: Our way to find out if she listens to the podcast or not. If I get in trouble over.

00:21:36
Speaker 1: That, that's all right, Well, keep me posted, all right. So when we come back, let's talk about categories that parasites fall into. So we were talking about how do you define and categorize these things. We'll talk about the categories that we think make the most sense.

00:22:11
Speaker 2: All right, we are back, and we are talking about a very intimate topic today. We're talking about the incredible community of folks that live off of you, that you are supporting your parasites and the parasites of everybody else on earth. We're digging deep into the history to understand how often this happens, how it happens, why it happens, and what is the future of parasitism. So let's keep digging into the past. Kelly tell us about like how these things become parasites, How does the species decide from being a maker to becoming a taker?

00:22:43
Speaker 1: I love that phrasing. And the answer is really like, we don't have a great handle on this. So you know, we talked about how this transition has happened probably more than two hundred and twenty three times, and there's this long running belief in the community that the what usually what happens is that an organism starts specializing on dead stuff. So you go from a decomposer to a parasite. And the idea is you get good at living off of flesh when it's dead, and that sort of lowers the threshold to becoming a parasite and living on that thing and taking its energy while it's still alive.

00:23:18
Speaker 2: So first you identify like, hey, these corpses are rich in nutrients, I might as well eat them, and then you start eating the living versions of them.

00:23:26
Speaker 1: Yeah, And so my friend Dan Metz and I wrote a paper. He was leading this paper I was just on the support team, but we wanted to find like the papers that showed this, because this is a thing that you can see found in the literature for like one hundred years or more, but when you look into the literature, it actually hasn't been tested that often. So I think a lot of us have this gut feeling that you know, being a decomposer is the step before becoming a parasite, But folks really haven't done the nice kind of analyzes you need. Where you'd need a tree of life. You'd need to know who the free living organisms and the parasites are are, and you'd need to know what that free living organism was doing before the transition to parasitism, and then look to see if certain kinds of free living organisms were more likely to transition to parasites. And you need some control groups too, and that requires a lot of natural history data. And so you and I have talked in the past about, you know, the value of taxonomy and the value of going out and collecting information, and I think it's critical because a lot of times we'll want to do these sorts of analyzes and then you'll get your tree of life together and you'll be like, oh, we have no idea what any of these pieces are doing in the wild, and so you can't ask these questions that really help us understand, you know, the paths that life take. You need that natural history data.

00:24:40
Speaker 2: And you always need data, right, especially to check your intuitions. Yeah, because you know, science is a story and we're putting together a history of what happened, and when we don't know, we're inclined to like fill in with our intuition, say well, here's an idea. Maybe this is reasonable, and if it sounds natural to us, we accept it. We're like, okay, yeah, I believe that. Yeah, but sometimes the you verse is not intuitive. So you always got to go out there and check, especially when you're telling a reasonable sounding story that doesn't have any actual data to support it. So more science, people.

00:25:10
Speaker 1: Right, So much more science, Yes, so much more science. Okay. So we were talking a little bit earlier about why you would want to try to create boxes to describe what parasites are doing, what different animals are doing, and I was arguing that you need those boxes so that you can create models to help understand what's happening. And there's a couple of different folks who have come up with categories that they think keep popping up as strategies that parasites take. And I saw a quote in one of these papers that made me think, oh, man, this is for Daniel uh oh, so here it is. This is from Poola twenty eleven. It seems safe to speculate that if life exists on other planets, as long as alien ecosystems show some basic similarities to ours, then parasitism will not only have evolved, but alien parasites would be using exploitation strategies very close to the ones to find.

00:26:01
Speaker 2: Here well parasites in the universe.

00:26:07
Speaker 1: So he was really going out on a limb here, very strongly worded. He believes these categories explained like truths about paths taken by life.

00:26:16
Speaker 2: I think it's fascinating when scientists do that. When they find something here, they understand it, and then they say this must be universal. We found it here, and therefore it's everywhere. It seems like it could be the only way, And that's so often just reveals like a lack of the broader context, like you're within a certain set of box of ways of thinking about the universe and it's hard to imagine. Outside that box doesn't mean that outside the box doesn't exist. And like, as I was just saying, like this is why we got to go and find alien parasites and blow earthling parasitologists minds about ways that you can suck on other people. Right, there may be even more intimate ways to be a parasite we never even imagined, totally true, as you know. And that's why I wrote this whole book about how aliens might do science, trying to think outside the box.

00:27:01
Speaker 1: Daniel, what is that book called.

00:27:03
Speaker 2: It's called Do Aliens Speak Physics? And it's for sale in November. Check it out at.

00:27:07
Speaker 1: All fine bookstores. Yeah, no, it is a fantastic book. I got to read it.

00:27:11
Speaker 2: So I love that this parasitologist is thinking about aliens. But I'm a little skeptical that our earthly categories of parasites are the only ones in the universe. But anyway, walk us through the kinds of parasites we have found here on Earth, and everybody out there, if you're eating dinner, maybe finish up before we dig into all of these intimate details.

00:27:31
Speaker 1: Okay, So, one of the exciting things about these strategies is that organisms from very different locations on the tree of life seem to have ended up with very similar strategies, which suggests that there's something critical about these boxes that we're creating.

00:27:47
Speaker 2: Yeah.

00:27:47
Speaker 1: So one of the strategies, which we've already talked about, is parasitoids. So the idea here is that you infect one host species, you use all of its energy. You have to kill it as part of your life cycle, and so we get parasit wasps that do this. We get nomatomorphs. So these are sort of nematode looking things that live in cricket So have you ever seen that video of a cricket that jumps into a pool and a worm that's like ten times the length of the cricket, maybe even more, glides out of its back end.

00:28:17
Speaker 2: Mm hmmm.

00:28:18
Speaker 1: Nightmare fuel, Yes, absolute nightmare fuel.

00:28:20
Speaker 2: There's one with the praying mantis also, I think yeah, yeah.

00:28:23
Speaker 1: Yeah, yeah. So sometimes those animals survive actually after they jump into the water, which blows my mind because you would think something that big comes out of you, you're a goner.

00:28:31
Speaker 2: Yeah, I think that some mornings. Yes, oh god, I think there's an intimate detail for everybody.

00:28:41
Speaker 1: That's right intimate, I hope not durable. I hope we can all forget that. Okay, So you get those like wormy things that eat almost everything inside of the organism, and it's kind of amazing because they've managed to take everything from the organism without killing it, so they're able to like avoid the critical organs, but all the like fat has been slurped out. Anything that's not absolutely critical has been slurped out and turned into parasite. And those zombie ants that you see with the Cordyceps fungus, they also are exhibiting a parasitoid strategy. They kill the ant and then they use all of that energy to make their own babies.

00:29:16
Speaker 2: So explain to me the word here parasitoid, because as a non expert, I hear that, and I think that's sort of like a quasi parasite or a semi parasite or something oid. Makes me think, like, not quite, what does the word mean parasitoid?

00:29:31
Speaker 1: This is a great question. I think the original phrase came from a German scientist, and that doesn't necessarily mean that, like their prefixes and suffixes meant something different. You're right, I'm looking up Merriam Webster and the first definition of parasitoid is resembling a parasite, which must be the oid thing the.

00:29:50
Speaker 2: Way that like our jokes resemble humor but aren't always right on the money. You know, it's humoroid.

00:29:55
Speaker 1: That's right, that's right, we're humoroid kind of podcasts, which sounds maybe too close to hemorrhoid.

00:30:01
Speaker 2: But uh so a lot of intimate jokes today, yes.

00:30:05
Speaker 1: Right, so many, my guess would be and I this is total spitball. Okay, So there are no parasitoids that infect humans, and so I think that when we think of parasites, we think of things that are like living in your guts or you know, like we think of the kind of parasites that we have. And so it's possible that when parasitoid was first named, they were like, oh, like, it's living in another thing. We're familiar with that. But then it like kills them and bursts out of them, and it's this is something different. So let's call it parasitoid. And I'm totally spitball. Like you have asked me yet another question. I don't know the answer too, but I'm gonna double down on my answer.

00:30:43
Speaker 2: All right, So these guys don't live in humans. They infect their host, slurp up all the in crucial bits, and then eventually kill them.

00:30:51
Speaker 1: Yep, that's a strategy, all right.

00:30:53
Speaker 2: And so if that wasn't gross enough for you, let's hear about the next category, which I shiver to even say. Parasitic castrators.

00:31:02
Speaker 1: Yes, lovely. So they don't kill the hosts, but they do reduce their evolutionary fitness to zero. So essentially they make it so that that organism will never be able to pass down its genes again. It won't be able to have any more of its babies.

00:31:18
Speaker 2: Wow.

00:31:19
Speaker 1: But now the parasites get to.

00:31:20
Speaker 2: Live cute little parasite babies. You know.

00:31:25
Speaker 1: The parasite studied for my PhD has these two little eye spots, and it has like a little suckery kind of mouth that opens and closes. And by the end of my very long PhD it took a really long time to finish, I kind of thought that they were cute. So I am capable of thinking some parasites are cute.

00:31:42
Speaker 2: They've infected your brain. See, they're working their magic on you.

00:31:45
Speaker 1: That's right, that's right.

00:31:46
Speaker 2: So tell us about what these guys do.

00:31:48
Speaker 1: All right, So at the top of the show, we talked about those snails where a bunch of the biomass of those snails is actually going towards the production of parasite babies. That's an example of castration. All of the energy that would have gone TODs making snail babies is now making parasite babies. And those snails can live for like I think it's like a decade after getting infected. And sometimes when the tide comes in on a warm day, they can release up to two thousand of those parasites into the water column. And so we think of them as essentially like extended versions of parasites. At that point, they're not working towards their own evolutionary goals anymore. Also, side note, the parasites that live in the snails have social behaviors, Like do you know how ant communities have different jobs for the different ants, and the ants have different shapes. Yeah, you've got like a million mile stare right now? Is this too much?

00:32:38
Speaker 2: No, I'm thinking about ants?

00:32:39
Speaker 1: Oh all right, okay, I was thinking.

00:32:41
Speaker 2: About that time they poured aluminum into an ant farm. I remember, like the shape of it.

00:32:45
Speaker 1: That's a mean but informative way of getting that information. Okay, So ants have individuals who have different jobs, and they have different shapes, and they have some individuals who never have their own offspring. Their job is just to support the colony. The same thing is happening inside of these snails. So there are attacker morphs that are essentially just like giant mouths that go around and when a parasite tries to invade the snail, because sometimes a new parasite species will come in invade a snail that already has parasites living in there, and it could kick out the parasites that are living in there and then take over the gonad.

00:33:22
Speaker 2: So it's like parasite versus parasite battle inside the snail.

00:33:25
Speaker 1: Yes, wow, yes, And some kinds of trematode parasites have essentially these mouths that never reproduce on their own. Their whole job is to just patrol the snail, and if a parasite comes in, they go and they suck a hole into the side of the tree with toad and like suck out the insides and kill it that way.

00:33:43
Speaker 2: Wow, this is like gang violenceers. This is our territory.

00:33:46
Speaker 1: Yes, yeah, they have like more interesting social lives than I do. They're fascinating.

00:33:50
Speaker 2: But how high bar is that?

00:33:52
Speaker 1: Kelly tell me, Oh my god, it's like it's like on the ground. You don't have to lift your foot up much at all. I don't go out much. But anyway, okay, parasitic castrators, trematodes. Let's do one other example. There are barnacles and so like if you've been to the ocean, you've probably seen barnacles. There are barnacles that stick to the sides of like whales and turtles. There are also barnacles and isopods. These are these little crustacean things that are both able to castrate crustaceans like crabs. So the barnacle in particular does this very interesting thing where it has a sack where female crabs would usually have their babies, and it sets up shop there, and females usually will use their claws, so this is on the underside of what you'd think of as like the crab's belly. They use their claws to like pass water over it to make sure that their eggs stay oxygenated. They will do that when the barnacles down there to keep the barnacle oxygenated, and males will start doing that too, So it looks like not only do they castrate male and female crabs, but they get the males to start acting like females to take care of the barnacles. We call this feminization, and it's kind of poorly understood, but anyway, So barnacles are castrating crabs, trematodes are castrating snails, and there's insects castrating other insects. This is another very common strategy.

00:35:12
Speaker 2: You just said the word castrating way too many times from that country. Let's move on.

00:35:18
Speaker 1: There's a total of six categories, but rather than making you all sit through all six, let's pick one more to give you a flavor of the amazing strategies that parasites have taken on.

00:35:30
Speaker 2: Let's do vector transmitted parasites. Because that sounds physics, y vectors.

00:35:35
Speaker 1: Ooh, all right, Well you can try to connect this with physics at the end, we'll see. But so, what we mean by vector transmitted is that you have one animal that is bringing a parasite to another animal and essentially injecting it into that other animal. Probably the most famous vector transmitted parasite is malaria. So mosquitoes will take a blood meal from a person who has malaria, they suck the parasite up. A parasite does some developing inside of the mosquito and eventually goes to live in the mosquito's salivary glands. And then when a mosquito goes to take a blood meal from another human who isn't infected yet, the parasite ends up in that person's blood, and then that person gets infected and the cycle starts again.

00:36:16
Speaker 2: I think it's fascinating. You call this a blood meal. Makes it sound so wholesome, you know, like it's just going home to have dinner after a long day of work. You know it deserves it, But like, this thing is sucking my blood, you know.

00:36:28
Speaker 1: I've read stories from the early nineteen hundreds in the Journal of Parasitology where they sort of mentioned in an offhanded way that like the mosquitoes and the lice in the colonies were fed blood meals by volunteers, which means somebody was like sticking their arms in the mosquito cage. Anna Phillips was telling us in the Leech episode that now you can like fill medical gloves up with blood from you know, a hospital or something and feeds them that way. But I love the faces you make when we talk about parasites. It's so entertaining for me. But Mike, you know I if I I tell you my big goal here, I feel like when you start learning about parasites, you can't get over the disgust hump Ye. It's just like so upsetting. But the more you learn about it. This doesn't happen for everybody, but maybe you'll get there. I feel like the more you learn about it, the more kind of amazing it is that evolution has resulted in this super complicated strategy. Like I mean, imagine being able to live in environments as different as a mosquito and a human, like to conquer both of those hosts in a way that like human ingenuity has just not been able to outsmart. There's good evidence that malaria is able to essentially like change the proteins on the outside so that our immune system can't recognize it. Like we start to get a handle on it, and then it shows up again in like a whole different outfit, and our body is like, where did the malaria go? But it's still there and it's like racing us and you know we have We've created all of these vaccines that keep you know, not quite keeping up. These are kind of incredible creatures. You can hate them, and I wish there was no malaria because I don't want to see any child with malaria ever again. But you kind of have to respect sort of the amazing strategies that they've been able to come up with with the help of natural selection.

00:38:10
Speaker 2: Yeah, I totally appreciate you leaning into the wonders of the universe, and you know, that's our whole brand, and the universe is extraordinary and these things are incredible. And I guess I'll try to think about that next time I see a mosquito biting Meia be like, wow, think about all the amazing science going into this itch that I'm going to be suffering for the next three days.

00:38:29
Speaker 1: Well, that's a micro predator, Daniel, I'm not interested, just kidding.

00:38:36
Speaker 2: All right, So let's take another break and we come back. Let's hear some good news about how parasites can go extinct. Okay, we're back, and Kelly is grossing me out with intimate details of how parasites infect their hosts, castrate them, and pass on diseases. Yay, we're all supposed to think. Yay. So let's turn to some good news. We heard about how parasites have evolved many many times in the record. Do parasites also go extinct? We have evidence for parasites disappearing. How does that happen?

00:39:26
Speaker 1: They do go extinct, and so, you know, we talked about how parasitism as a strategy popped up, you know, over two hundred times in the tree of life. It appears it's also been lost a few times.

00:39:36
Speaker 2: Ah, I know.

00:39:38
Speaker 1: So sometimes it's hard to get a handle on those losses because if an animal loses it and then doesn't end up in the fossil record, it can just be kind of complicated. But yeah, parasites go extinct, and in fact, some parasites are more susceptible to extinction than their hosts. And I know that's not sad for you, but we're losing diversity. So one parasite strategy that we didn't talk about in the last section are trophically transmitted parasites.

00:40:02
Speaker 2: What does trophic mean?

00:40:03
Speaker 1: So, like, think about trophic levels in a food web, and so trophic transmission is essentially a predator consumes an animal that was infected, and the parasite gets into the predator. So the trophically transmitted system I studied for my PhD was California killifish. There are these small fish in southern California estuaries and they get eaten by predatory birds like egrets, and the parasite goes from living on top of the fish's brain to living in the egrets gut.

00:40:30
Speaker 2: I wouldn't want to live in either of those places. I mean, you're in southern California. There's lots of beautiful places to live. Why would you choose a fish brain? But hey, you do you?

00:40:39
Speaker 1: I mean, I'd rather live in Virginia. But that's all right. So everyone knew that was coming. But this parasite actually has an even more complicated life cycle. So parasites can have different strategies depending on where they are in the life cycle. So there you've got trophic transmission from fish to bird. The parasites then find love in the bird's gut and they may eggs that pass with the bird species into the salt marsh, where they're accidentally consumed by those snails that we talked about that get castrated and then this is.

00:41:10
Speaker 2: Such a horror movie. This whole thing is a horror movie.

00:41:13
Speaker 1: I love it, I eat it up. My mom loved horror movies. Maybe that's what's set me up for all of this.

00:41:17
Speaker 2: It's her fault.

00:41:18
Speaker 1: Yes, yeah, yeah. So anyway, in order for this parasite, this trematode parasite, to complete its life cycle, you need to have the fish, you need to have the birds, and you need to have the snail. If you lose any of those hosts in the cycle, or any of them falls to numbers that are small enough where it's hard for the parasite to find them, then that animal can go extinct. And so parasites have all of these extra requirements that depend on what's happening with their hosts, and so in some cases we think that parasites might be more susceptible to extinction than a lot of free living animals because there's so many different components that need to work well for the parasite to complete the life cycle.

00:41:54
Speaker 2: It's kind of a fragile system, a bit of.

00:41:56
Speaker 1: A fragile system, and so parasites can be used as what we call ecological indicator. So if you go to an estuary and you find a lot of parasites, that probably means a lot of host species are there, and they're there in pretty high numbers, and so a lot of parasites can be indicative of a healthy ecosystem, which I think is counterintuitive to some people but makes perfect sense to my parasitologists heart.

00:42:18
Speaker 2: Like if you go to a new neighborhood and you discover there's lots of gangs there, you're like, oh, wow, this must have a thriving economy for the gangs to be siphoning money off of.

00:42:30
Speaker 1: Wow, where do I go with that? I mean, I guess it means that the police aren't really doing their job, which maybe means there's immune systems and hosts that are compromised, perhaps by pollutants. But maybe we're pulling this analogy a little bit too tight, so let's move on.

00:42:45
Speaker 2: All right. So they do go extinct, and maybe even more often than their hosts because they rely on like a complex system and getting passed from one to the other. But then they're everywhere and they're very common and they've evolved many many times. How do we understand that altogether?

00:42:58
Speaker 1: I mean, there's a lot of hosts all and you can be losing a lot of hosts, but you still look around and there's a lot of birds outside and stuff. It's just, you know, hard to keep track of all of this stuff. But if I can, just for one second try to convince you that you should care about parasites going extinct, and I won't.

00:43:12
Speaker 2: You can try.

00:43:13
Speaker 1: Yeah, So, parasites are important parts of ecosystem. So we mentioned that those snails that are castrated can release up to two thousand parasites into the water. Those parasites are free swimming bags of energy that get eaten by a lot of organisms, so they provide food to the system. Also, you know, we talked about those crickets that have those long worms in them, and the crickets are forced into jumping into the river and then the worms, the nomatomorphs, come out the back of them. Those crickets are terrestrial organisms that jump into creeks, And in Japan there's this endangered trout where a lot of the food that it gets comes from crickets that are jumping into the creek that they wouldn't have access to otherwise. And so these parasites often make lots of food available to other animals or make that food easy to catch. They play important roles in ecosystems that we don't always understand very well, and if you lose them that could be bad in some cases.

00:44:08
Speaker 2: Yeah, I totally appreciate that. And it's sort of curious to me, like, why do parasites have this ick factor that like predators don't, you know, like a jaguar, it's still like finding another critter and taking it and eating its resources. Why is this somehow grosser to be like living inside as you consume somebody for some reason, it is definitely grosser though like morally it's not any better or worse. So I don't know, there's just some sort of like human ick factor there. And maybe when we meet the aliens they'll be like, yay parasites, and Kelly will be like, I found my people.

00:44:40
Speaker 1: That's right, that's right. I understand you guys, you no, I agree. There's definitely a judgment called people think of parasites as like these super degenerate organisms. They have fewer sort of abilities to like sense the exterior environment, but they do manage to thrive in really complicated systems where they're being attacked. And yeah, anyway, people do tend to find parasites to be much gross to the lions. I get that some people could look at a lion and be like, that looks much nicer than a tapeworm. But if you know what to look for in a tapeworm, some of them are quite beautiful. I've got my tapeworm picture right here.

00:45:12
Speaker 2: Oh wow. Do you know anybody who like has a coat made out of tapeworms or something like that, You know, tapeworms. Maybe we need a fashion industry, you know, to replace fur with parasites somehow.

00:45:23
Speaker 1: I think that any parasite themed fashion that is available is found at the American Society of Parasitologists Student Travel Auction. We bring it in and we all pay way too much for it to try to pay for student travel so that they can come to our conferences. But anyway, we're getting off track. We've got one more question from the listener, and the question is do parasites go extinct with hosts? So there's a number of different things that can happen when hosts go extinct or when hosts sort of split into multiple species, and so yes, often when a host goes extinct, the parasite can go extinct. Also, if the parasite was infecting lots of other host species, then maybe it will be fine. But we already talked about complex life cycles where if you lose one host in the system, you can lose the parasite too. But there's also examples where you get these splits in the tree of life, and as you get those splits in the hosts, the parasites also split into new species over time because you now get like separations of parasite populations, and over enough time they become their own species. And so there's a sort of famous example where pelicans, as they sort of speciated over time, they're lice speciated with them. So pelicans, if you like create the tree of life for pelicans and then you create the tree of life for their lice, you could almost lay them on top of each other. They follow very similar paths. But you can also get cases where the parasites don't seem to be tracking what's happening with their hosts. So, for example, guppies, they are these these tiny little fish they get infected by monogenians. These are these external parasites we were talking about that have the hooks and they hold onto the outside of the fish. They don't seem to be following the evolutionary trajectory of their hosts. And right now we don't have a really good framework for when you should expect the tree of life for parasites and species to essentially mirror one another, and when you shouldn't. But we get all of these different options because nature is amazing and it depends, but we don't.

00:47:15
Speaker 2: Always know on what, and the incredible variety of strategies we see helps us explore like the space of possibilities and maybe even helps us think outside the box to like what might be happening elsewhere in the universe.

00:47:26
Speaker 1: Absolutely, yes, I would love to find a planet that was rich with free living hosts and dissect a bunch of them to see if they have parasites, but only if it was humane and ethical.

00:47:40
Speaker 2: All right, aliensf you're listening, come on by. Kelly wants to look inside you.

00:47:44
Speaker 1: Kelly wants to say huh.

00:47:46
Speaker 2: In an intimate sort of way.

00:47:48
Speaker 1: All right, Well, thank you so much to Zach for sending in his questions. Let's see if Zach feels like we've answered the many questions he posed about the evolution of parasitism.

00:47:57
Speaker 3: Hey, they're extraordinaries. Thank you so much for this credible primer on parasites and their history. I feel like I got way more than I bargained for, which is awesome. You definitely answered my essential question that, yes, t Rex probably had tapeworms or something like it, that we can't be totally sure. I also really enjoyed hearing about all the different categories of parasite strategies, and really appreciated hearing that there are no parasitoids or castrating parasites for humans, so I can sleep a little better at night. I was also definitely convinced that parasites have an important role to play in our ecosystems, so I'll try to be a little bit more appreciative and a little less grossed out. I am still a little curious. Seems like it's still an open question how something goes from being a free living organism to being a parasite, even if we can count when it happens. I'm also curious if anything has ever gone from being a parasite back to being a free living organism. In any case, you guys rock. Thank you again so much.

00:48:57
Speaker 1: All right, thank you so much to Zach for all of his many questions. If you would like to send us questions, we would love to hear from you. Send them to questions at danieland Kelly dot org. And I hope that you enjoy your meal or you ate it before we began this conversation.

00:49:11
Speaker 2: And before you heard Kelly say the word castraid uncountable number of times.

00:49:15
Speaker 1: I don't think there's a limit on how many times you can say castrat. It's a great word.

00:49:20
Speaker 2: All right, everyone, thanks for joining us on this tour of one more aspect of our extraordinary universe.

00:49:32
Speaker 1: Daniel and Kelly's Extraordinary Universe is produced by iHeartRadio. We would love to hear from you.

00:49:38
Speaker 2: We really would. We want to know what questions you have about this extraordinary universe.

00:49:43
Speaker 1: We want to know your thoughts on recent shows, suggestions for future shows. If you contact us, we will get back to you.

00:49:50
Speaker 2: We really mean it. We answer every message. Email us at Questions at Danielankelly.

00:49:56
Speaker 1: Dot org, or you can find us on social media. We have accounts on Instagram, Blue Sky and on all of those platforms. You can find us at D and K Universe.

00:50:06
Speaker 2: Don't be shy write to us