Speaker 1: If you sit on the banks of the estuaries in
Speaker 1: southern California and Baja California, the seat of your pants
Speaker 1: will get super money. But also you'll likely see schools
Speaker 1: of silvery fish swimming through the water. If you watch closely,
Speaker 1: some of those silvery fish will begin behaving rather strangely.
Speaker 1: They'll shoot upwards quickly, breaking the water surface and creating
Speaker 1: these super conspicuous ripples on the surface. Or they'll turn
Speaker 1: on their sides as they scratch their bodies against a rock.
Speaker 1: As they do this, their silvery bellies reflect the sun
Speaker 1: and create this quick and vibrant flash of light.
Speaker 2: Why are they doing this?
Speaker 1: These fish are bite size to the sharks and the
Speaker 1: predatory birds that sometimes roam these estuaries in search of food.
Speaker 1: Shouldn't they be trying to look a little less conspicuous.
Speaker 1: Why are they drawing so much attention to themselves. Well,
Speaker 1: it's me speaking, so you've probably guessed that the answer
Speaker 1: is parasites.
Speaker 2: Today we're going to talk about you.
Speaker 1: Heplurcus california aansis a brain infecting parasite of California chillefish,
Speaker 1: which are the silvery fish dancing through the waters in
Speaker 1: California that we were just talking about. Welcome to Daniel
Speaker 1: and Kelly's extraordinarily infected universe.
Speaker 3: Hi. I'm Daniel, I'm a particle physicist, and I hope
Speaker 3: I'm parasite free.
Speaker 2: Hello. I'm Kelly Wiersmith.
Speaker 1: I study exactly what we're talking about today, and I'm
Speaker 1: okay if I'm harboring a parasite or two, depending on
Speaker 1: what they are.
Speaker 3: All right, Well, that was actually my question for you, Kelly,
Speaker 3: if you had to harbor a parasite, what would be
Speaker 3: your parasite of choice?
Speaker 1: All right, Well, so I probably have some demodex mites
Speaker 1: on my face.
Speaker 2: I'm okay with that.
Speaker 1: You know, they might not count as a parasite depending
Speaker 1: on you know, what they're doing on the given day.
Speaker 3: Are those the ones that live in your pores and
Speaker 3: climb out? And there was uncertainty at whether or not
Speaker 3: they had anuses or if they just exploded when they
Speaker 3: got filled up.
Speaker 1: That's right, but they do have anuses. Science figured that
Speaker 1: out recently. Way to go science.
Speaker 3: What's better that they're taking a dump on your face?
Speaker 3: They're exploding on your face? I don't know. I don't
Speaker 3: feel great about either option.
Speaker 1: Yeah, I think in the end, you know, the waste
Speaker 1: ends up on your face either way. So Daniel almost
Speaker 1: spit his coffee out there.
Speaker 2: It's just great.
Speaker 3: How do you know we're talking about biology because it's
Speaker 3: poop on your face in the first five minutes.
Speaker 2: That's right, this is a chilly lead episode.
Speaker 1: I don't know that we'll get to cannibalism today though,
Speaker 1: so that's too bad.
Speaker 3: See. Yeah, I'll do my best to bring us there.
Speaker 2: Yeah, okay, all right.
Speaker 1: You know, I guess I wouldn't be too upset if
Speaker 1: I had one or two pinworms. I wouldn't mind having
Speaker 1: a small load of pinworms because they probably wouldn't hurt
Speaker 1: me that much.
Speaker 3: Pinworms aren't those the ones that at night crawl out
Speaker 3: and like make you itch in uncomfortable places?
Speaker 1: Yeah, I mean, no, parasite is good, right, Like that
Speaker 1: would defy the definition of parasite. I'm trying to pick
Speaker 1: something that would be least bad, and pinworms are probably it.
Speaker 1: Although not a lot of adults get pinworms.
Speaker 2: What about you? Oh man, you're making a lot of
Speaker 2: curse faces today. I wish we did video.
Speaker 3: There is no parasite I would be happy with. Yes, absolutely, no,
Speaker 3: that was an answer. Yes, that's my answer. It just
Speaker 3: creeps me out. It just bothers me to imagine like
Speaker 3: something living inside of me, treating me like a cave
Speaker 3: or like a habitat or something like. No, I would
Speaker 3: just like I would get a rusty spoon and dig
Speaker 3: it out.
Speaker 1: Okay, hold on, is this a question of magnitude? Because
Speaker 1: you have billions of bacteria living in your gut doing
Speaker 1: exactly what you just described.
Speaker 3: Katrina is convinced to me that they are part of me.
Speaker 3: I contain multitudes, I am multitudes.
Speaker 1: Why can't the demodex and the pinworms be part of
Speaker 1: your multitudes?
Speaker 3: Not invited to the party. No, okay, No, there's a
Speaker 3: fuzzy but bright line between the microbial invaders and then
Speaker 3: non microbial invaders. Maybe it's just because I'm married to
Speaker 3: a microbiologist and not a parasologist. Do you think Zach
Speaker 3: would be happy having parasites within him?
Speaker 2: Absolutely not.
Speaker 1: But but so one of the things that made me
Speaker 1: interested in microbiome science initially was this idea that there's
Speaker 1: a connection between the brain and the gut, and that
Speaker 1: microbes sometimes release neurotransmitters that.
Speaker 2: Can speak to you, know, speak, I'm being.
Speaker 1: Super anthromorphic here, but influence influence, Like you know, there
Speaker 1: are nerves that go from your brain to your gut,
Speaker 1: and the neurotransmitters released by those microbes, you know, might
Speaker 1: be influencing how we feel about certain things. And so
Speaker 1: I became super interested in this idea that the way
Speaker 1: that we respond to the world might be influenced by
Speaker 1: the things that are living inside of us. And my
Speaker 1: advisor at the University of California, Davis Andy C who
Speaker 1: is amazing I adore him. One day was like, Kelly,
Speaker 1: have you heard about toaso plasma gandhi. It's a parasite
Speaker 1: that you can get from your cat that changes your behavior.
Speaker 1: And in a very early Listener Questions episode, we talked
Speaker 1: about like how good the science is, what we know
Speaker 1: about that, so you can look for that. But that
Speaker 1: started me on my journey being like interested in how
Speaker 1: parasites impact behavior.
Speaker 2: And so today we're going to talk about you know,
Speaker 2: what I spent a decade.
Speaker 1: It wasn't supposed to be that long, what I spent
Speaker 1: a decade working on, And this is.
Speaker 3: All the research we're going to talk about today already published.
Speaker 1: Yes, because I think my collaborators would appreciate if the
Speaker 1: two papers that we haven't quite finished yet, we're.
Speaker 2: Not highlighted on the episode.
Speaker 1: I don't care, but they are, you know, still hoping
Speaker 1: to get tenures. So I will I will not mention
Speaker 1: those in press or in prep results.
Speaker 3: But yeah, I do agree, that's super fascinating, and it
Speaker 3: makes a lot of sense that the things living inside
Speaker 3: of us don't just live inside of us. They influence us.
Speaker 3: They change the way our bodies work, and that includes
Speaker 3: potentially changing our behavior because we are these biological machines
Speaker 3: and the decisions we make are not divorced from that, right,
Speaker 3: they're completely influenced by that. But still, I find it
Speaker 3: okay to imagine that my microbiome is influencing the decisions
Speaker 3: I make in the way I perceive the world. But
Speaker 3: I do feel hijacked if there's like some critter living
Speaker 3: inside me that's forcing me to like eat that second donut.
Speaker 1: Yeah, well, so I think there's a difference in goal
Speaker 1: in some cases, you know. So like I would like
Speaker 1: to think that a lot of you are my grow
Speaker 1: biome is in sync with you. You know, the microbiome
Speaker 1: wants you to thrive, so that they can thrive. But
Speaker 1: for a lot of the systems that I've ended up
Speaker 1: working in, the host needs.
Speaker 2: To die for the parasite to thrive.
Speaker 1: And so when we're looking at manipulation, which is what
Speaker 1: I sort of specialized in grad school, which is an
Speaker 1: instance where the host's behavior is changing in a way
Speaker 1: that's driven by the parasite in a way that's usually
Speaker 1: bad for the host but good for the parasite. You know,
Speaker 1: that's that's very different than what you're imagining. But folks
Speaker 1: might remember that we did a zombie ant episode a
Speaker 1: little while back, And what I love about these systems
Speaker 1: is that often parasites are changing behaviors of their hosts
Speaker 1: in ways that we couldn't replicate in a lab like
Speaker 1: and by way I mean neuroscientists, you know, So like ants,
Speaker 1: they've got these simple little brains relative to humans, you know,
Speaker 1: but we still couldn't get the ants to do these
Speaker 1: like series of behaviors that this fungus, like operating the
Speaker 1: ant by remote control, can get the ant to do.
Speaker 1: And so you know, through the process of natural selection,
Speaker 1: this fungus has essentially acquired the ability to control ant behavior,
Speaker 1: and it you know, in a very anthropomorphic sense. It
Speaker 1: knows neuroscience better than we do, and so I love
Speaker 1: these systems as a way to you know, maybe jumpstart
Speaker 1: our understanding of how brains work by like studying what
Speaker 1: these parasites have essentially learned about how brains work over
Speaker 1: evolutionary history. And so I get super excited about, you know,
Speaker 1: using parasites as tools to understand how brains work. Although,
Speaker 1: as we'll discover over the course of this episode, neuroscience
Speaker 1: baffles me, and so I mostly focus on the behavior
Speaker 1: stuff and then collaborate with neuroscientists.
Speaker 3: All right, So today we're not talking about the things
Speaker 3: living in Daniel's gut or toxoplasmi or zombie ants or
Speaker 3: any of that kind of stuff. What is the system
Speaker 3: that you and ten years of your life.
Speaker 1: Exploring, oh Man, Okay, So in Baja California, in southern California,
Speaker 1: in estuarys so. Estuaries are areas where the ocean is
Speaker 1: coming in and there's a freshwater system meeting the ocean.
Speaker 1: So it's where freshwater and saltwater are mixing. They tend
Speaker 1: to be super productive areas where like baby fish are
Speaker 1: growing and sharks will come in to eat.
Speaker 2: Some food, and there's a lot of crabs and stuff
Speaker 2: like that. They're super productive systems. They are these tiny
Speaker 2: little fish. They are silvery.
Speaker 1: They're you know, like maybe the adults are a bit
Speaker 1: bigger than the length of your maybe middle finger, but
Speaker 1: they're not as big as like your whole hand, so
Speaker 1: they're you know, somewhere in between.
Speaker 2: They're pretty small. They're pretty drab.
Speaker 1: The males will sometimes have some yellow coloration during the
Speaker 1: breeding season, but in general they're just like these silvery,
Speaker 1: kind of drabfish.
Speaker 3: And they're called killyfish, killyfish, not kellyfish, not killfish, killy fish, killyfish.
Speaker 3: What does that mean? A?
Speaker 2: I don't know. I don't know. Why are they called
Speaker 2: killy fish?
Speaker 3: You only spent ten years studying it and never thought,
Speaker 3: why are they have this silly name?
Speaker 1: Well, stuff doesn't always mean stuff. You know, that was
Speaker 1: really articulate of me. I don't I don't think it's
Speaker 1: I don't know that it means anything.
Speaker 3: See, it's of uncertain origin, but it's likely to have
Speaker 3: come from the Dutch kill for a small stream.
Speaker 1: Nah, if you say so.
Speaker 3: So these sillily named fish.
Speaker 1: Yeah, So, Actually, when I met Zach the person who
Speaker 1: would eventually become my husband. He asked me what I
Speaker 1: studied and I said killy fish. And he said, you
Speaker 1: study kelly fish and I said no, no, killyfish, and
Speaker 1: for a while left that he called me kelly fish,
Speaker 1: which is kind of cute. But anyway, so these are
Speaker 1: super abundant fish. They're social, so they they're you know,
Speaker 1: you off and find them in big schools. And Kevin
Speaker 1: Lafferty and Kimo Morris in the nineties they notice that
Speaker 1: if you walk around in the estuaries and you look
Speaker 1: into all of these like channels, you see schools of
Speaker 1: these fish swimming by, but they're really obvious, Like there
Speaker 1: are other schools of fish that will swim by and
Speaker 1: you sort of like don't really notice them. But when
Speaker 1: the chilly fish swim by, they start doing all of
Speaker 1: these weird behaviors. It's like they're dancing through the water.
Speaker 1: They'll like break the surface of the water and you'll
Speaker 1: see these obvious ripples and then they'll be all these
Speaker 1: flashes of silver because they're they're shooting forward, they're turning
Speaker 1: on their sides, they're rubbing against things, they're contorting in
Speaker 1: the shape of s they're just doing all of these
Speaker 1: like really conspicuous behaviors that the other fish species didn't
Speaker 1: seem to be doing. And then you bring a bunch
Speaker 1: of fish back into the lab and you survey what's
Speaker 1: happening in the estuaries, and you discover that the California
Speaker 1: chillifish have a bunch of parasites on their brain, which
Speaker 1: makes you wonder, is that what's going on?
Speaker 3: So is that the reason they began studying this fish?
Speaker 3: Were they the first people to study this fish, or
Speaker 3: is this like a well known model fish in the community.
Speaker 1: A bunch of people have studied this fish, but not
Speaker 1: in relation to the fact that it had a brain
Speaker 1: infecting parasite. Because it's a superabundant fish in these super
Speaker 1: productive ecosystems, people have studied other things about them, like
Speaker 1: how the heck do these fish survive the fact that
Speaker 1: when the ocean goes out and the river is still
Speaker 1: coming in, the salinity goes from like, you know, almost
Speaker 1: completely fresh water, and then when the ocean comes in
Speaker 1: it's almost completely saltwater? How do they survive that? But
Speaker 1: a lot of people hadn't been studying the parasite stuff.
Speaker 3: Because I don't know if it's widely appreciated. But in biology,
Speaker 3: we have like these model systems where we don't study
Speaker 3: things in the wild. We have like a few things
Speaker 3: that we grow in the lab and study in detail.
Speaker 3: You know, mice and fruitflies and all sorts of stuff.
Speaker 3: And I wonder, like what makes an animal suitable to
Speaker 3: be selected? But these are being studied in the wild, right,
Speaker 3: So it's because of their interesting behaviors and these fascinating questions, right,
Speaker 3: not because they're quick to grow or they eat something
Speaker 3: easy or anything.
Speaker 2: Yeah, right, So I hate these guys.
Speaker 1: So so uh, it turns out so mummy chugs, which
Speaker 1: is Fungilus heteroclitis. I study Fungilius parva pinnis, but a
Speaker 1: closely related species on the opposite coast is very easy
Speaker 1: to study in the lab.
Speaker 2: This is a model species.
Speaker 1: We've got the genome, we've got a bunch of genetic tools.
Speaker 1: You can grow them up super easy. But fungilis part
Speaker 1: of a pinnis You bring into the lab and it
Speaker 1: is really hard to keep them from dying, which is
Speaker 1: crazy because they live in these environments where the salinity
Speaker 1: is changing. You'd like they survive all of these extremes.
Speaker 1: You'd think you bring them in the lab where life
Speaker 1: is easy, they would.
Speaker 2: Thrive, and they do not. And I don't know why.
Speaker 1: And I like, literally years of my PhD was figuring
Speaker 1: out how to keep them happy in the lab. And
Speaker 1: that's probably why they're not a model organism, because they're
Speaker 1: little jerks. You think about them at home at night,
Speaker 1: and they start dying in the lab.
Speaker 3: All right. So these guys Lafferdy and Morris notice these
Speaker 3: fascinating behaviors and they studied these fish and what did
Speaker 3: they learn about them?
Speaker 1: Okay, well, so they brought the fish into the lab
Speaker 1: and they were like, oh my gosh, the brains of
Speaker 1: these fish are like carpeted by this parasite on their brain.
Speaker 1: Like a thousand parasites on the brain of an adult
Speaker 1: is pretty typical.
Speaker 2: Eight thousand can happen sometimes too.
Speaker 1: So there are these tiny little like cysts, these tiny
Speaker 1: little balls. They almost could look like their little glass balls.
Speaker 1: And this is a trematod parasite. So a trematoad parasite
Speaker 1: that you might have heard of is schistosoma. Mansonite causes shistosomiasis.
Speaker 1: This is a problem in places like Africa. This parasite
Speaker 1: is also a trematoid parasite, not necessarily particularly closely related
Speaker 1: to that one, but this is the only other trematode
Speaker 1: you're likely to have heard of, and it has a
Speaker 1: complex life cycle. And let's go through the life cycle
Speaker 1: because it helps you understand why this parasite might want
Speaker 1: to make the fish behave conspicuously.
Speaker 3: And this is the life cycle of the fish, or
Speaker 3: of the parasite, or the harmonious combination.
Speaker 2: Of the parasite.
Speaker 1: All right, and I'm gonna start calling the parasite Youha,
Speaker 1: because you have blurcis California asis is way too much
Speaker 1: to say every time.
Speaker 3: All right, so tell us about Uha's lifestyle.
Speaker 1: Okay, So youh accidentally gets consumed by California horn snails
Speaker 1: that live in the salt marshes. Bad news for the snails.
Speaker 1: The snails get castrated.
Speaker 2: By the parasite. What I know, it's crazy.
Speaker 3: Chemically or like surgically, I mean, are we losing bits
Speaker 3: of the body here or are they just becoming deactivated?
Speaker 1: Well, okay, so all of the gonads, I'm not quite
Speaker 1: sure how you're imagining this, but the gonads are.
Speaker 3: All you don't want to know how I'm imagining.
Speaker 1: Yeah, no, I didn't ask you to explain it. And
Speaker 1: so the gonads are inside of the snail. All of
Speaker 1: the gonad tissue ends up being taken up by parasite tissues.
Speaker 1: The parasite start replicating, replicating, replicating, replicating, replicating, and they
Speaker 1: produce a stage that will swim out from the snail
Speaker 1: to go off in search of fish.
Speaker 3: This is nightmare fuel already, Kelly.
Speaker 1: It is, and these snails can still live for like
Speaker 1: a decade or more with this parasite. And when the
Speaker 1: tide comes in in the summer, up to two thousand
Speaker 1: of the free swimming stages of this parasite can emerge
Speaker 1: from the snail to go off in search of fish.
Speaker 1: It's crazy, and okay, parasites have social lives, all right.
Speaker 2: So look, I'm gonna be honest everybody.
Speaker 1: This is gonna be a little bit of a ranty
Speaker 1: episode because I love this parasite and I've just spent
Speaker 1: a decade like falling in love with its weirdness.
Speaker 3: Okayyi, this warning is fifteen minutes too late, Kelly.
Speaker 1: Guys and Daniel tried to get me on track before
Speaker 1: the start of this episode. He's like, Kelly, I can
Speaker 1: tell you're gonna get ranty. Let's focus, and I'm here.
Speaker 1: I am being like, sorry, Daniel.
Speaker 3: All right, ran away really quick.
Speaker 2: Okay.
Speaker 1: So, once you're inside of a snail and you've taken
Speaker 1: control of a gonad, there are other trematodes that would
Speaker 1: like to come in and usurp the gonad that you're
Speaker 1: living in. And so you have Lurcis Californian sas and
Speaker 1: other trematode species out in the estuary make a stage
Speaker 1: called a soldier, and the soldier is essentially just like
Speaker 1: a giant mouth that patrols the snail, and if another
Speaker 1: stage comes in, it will go. At this stage in
Speaker 1: their life, they're essentially just giant bags of fluid, and
Speaker 1: so it will like try to pop the other bags
Speaker 1: of fluid so that they can't come in and take
Speaker 1: control of the snail gonad. And so anyway, they've got
Speaker 1: reproductive stages that are like making this free swimming stage
Speaker 1: of the parasite reproducing, reproducing, reproducing, and then they're making
Speaker 1: a bunch of soldiers that are patrolling the snail to
Speaker 1: keep everything safe.
Speaker 2: It's just absolutely amazing to me.
Speaker 3: That there's this like grand battle for the snail gonad exactly, yes, incredible.
Speaker 3: And so these parasites are working together, they're like cooperating
Speaker 3: with each other against the snail and against competing parasites.
Speaker 1: Yes, but they're also they're reproducing asexually, so they're like
Speaker 1: all clones. So it's like the exact same individual clone clone, clone,
Speaker 1: clone clone, but some of them look like soldiers, some
Speaker 1: of them look like reproductives. But like if you were
Speaker 1: to sequence them genetically, they're all like the same.
Speaker 3: All right, So it's like the snail gonad hive. Mind. Ye,
Speaker 3: that sounds like the title of.
Speaker 2: A book I'd like to read, sure, yeah, or the
Speaker 2: book I'd like to write.
Speaker 1: And so all right, when the tide comes in in
Speaker 1: the summers, when the water is nice and warm, thousands
Speaker 1: of these parasites leave the snail and each one of
Speaker 1: them has a very low probability of finding the fish,
Speaker 1: and they only live for like twenty four hours. They're
Speaker 1: like little sacks of energy and they're gonna run out
Speaker 1: real fast. But if they do encounter the fish, we
Speaker 1: think what happens is they burrow through the fish's skin,
Speaker 1: they find a nerve and they follow that nerve up
Speaker 1: to the fish's brain and then they go on top
Speaker 1: of the brain and they form a cyst on the brain,
Speaker 1: so they're not in the brain tissue. They're like resting
Speaker 1: on top of the brain.
Speaker 3: And why do they want to be on top of
Speaker 3: the brain.
Speaker 2: Ah, there's a couple of different ideas there.
Speaker 1: So one of the ideas, and this was like a
Speaker 1: very early idea, is that if you're on top of
Speaker 1: the brain, you're kind of protected from the immune response,
Speaker 1: because if the brain's immune system overreacts, then that could
Speaker 1: be really.
Speaker 2: Bad for the fish. That could kill the fish.
Speaker 1: The fish could start like swimming on its side, then
Speaker 1: it could get eaten by another fish. And so by
Speaker 1: being in an area where the host can't allow the
Speaker 1: immune system to attack too strongly, you sort of increase
Speaker 1: your odds of survival.
Speaker 2: But it could also be because it's.
Speaker 1: A pretty good place if you're going to be trying
Speaker 1: to manipulate behavior, that's a good location from which to
Speaker 1: do that.
Speaker 3: Yeah, that makes sense. And so there are multiple of
Speaker 3: these parasites crawling up the fish nerve to the brain.
Speaker 3: Are they all working in tandem like they did in
Speaker 3: the snail gonet. Are they now fighting each other for
Speaker 3: who gets to like ratitude drive this fish?
Speaker 1: That's a great question, so hard to say. We think
Speaker 1: that they are working together because usually you see signs
Speaker 1: of competition between parasites. But when we studied this, we
Speaker 1: saw some evident that they were cooperating. And so the
Speaker 1: way that we measured this was we looked at the
Speaker 1: volume of the parasites. As the density of parasites increased.
Speaker 1: So usually what you see is that as you get
Speaker 1: more and more parasites crammed into the same size of
Speaker 1: a space, they start getting smaller, and that's probably because
Speaker 1: they're competing for food resources. But what we saw was
Speaker 1: that the more parasites you got crammed onto the brain
Speaker 1: of a fish, the bigger the parasites.
Speaker 3: Seemed to get because they're cooperating.
Speaker 1: Yeah, which suggested to us maybe they're secreting some compound
Speaker 1: that suppresses the immune system, or they're creating some compound
Speaker 1: that manipulates behavior, and the more of them that are present,
Speaker 1: the less each one needs to create in order to
Speaker 1: accomplish the same goal.
Speaker 3: So even though they're not identical, they can still cooperate.
Speaker 2: Maybe so these were fish that were caught in the wild.
Speaker 1: This was just an observational study, So this is indirect
Speaker 1: evidence that maybe they are cooperating, and we didn't see
Speaker 1: the same thing happening when we look at trematodes that
Speaker 1: we're living in their liver. So it looks like, you know,
Speaker 1: maybe there's some cooperation happening. We don't know for sure.
Speaker 3: All Right, these ideas have parasited my brain and they
Speaker 3: are now gratituding me to suggest that we should take
Speaker 3: a break, so everybody can go off and cleanse their
Speaker 3: mind from snail gonads, and when we'll come back, we'll
Speaker 3: discover what these parasites do to these poor little killyfish. Okay,
Speaker 3: we're back. Welcome to the Kelly Rants about Parasites episode.
Speaker 3: So you've been telling us about how these parasites grow
Speaker 3: in the snail gonads and then crawl up the nerve
Speaker 3: into the brains of these killyfish and work together to
Speaker 3: influence them. But you're also talking about this nineteen ninety
Speaker 3: six paper by Lafferty and Morris, the sort of the
Speaker 3: seminole work on this. What did they find initially about
Speaker 3: how these parasites affected the behavior of these fish.
Speaker 1: Yeah, but in order to understand the results of the
Speaker 1: Laffarty and Morris study, you need to understand where the
Speaker 1: parasites have to get after they're in the fish.
Speaker 3: Oh wait, the fish is not the end of the cycle.
Speaker 3: There's more.
Speaker 2: There's more, there's more.
Speaker 3: The nightmare continues, people, it does, it does.
Speaker 1: Okay, So all right, so you've got like a bunch
Speaker 1: of parasites on the brains of the fish. But actually
Speaker 1: the last host that the parasite needs to get to
Speaker 1: are predatory birds.
Speaker 2: And so once a California killyfish gets eaten by a.
Speaker 1: Predatory bird, the digestive juices in the predatory birds start
Speaker 1: breaking down the fish. That breaks down the cysts that
Speaker 1: the parasites are in. The parasites break out of the cysts,
Speaker 1: they find love, they find a mate, they produce eggs.
Speaker 3: So being eaten by the bird is bad for the fish,
Speaker 3: but good for the parasite exactly.
Speaker 1: Yes, the parasite cannot complete its life cycle unless the
Speaker 1: fish gets eaten by a bird.
Speaker 2: And if the fish gets eaten by a shark, that's bad.
Speaker 1: For the parasites. They have to get eaten in a
Speaker 1: certain way. There are wrong ways to get eaten and
Speaker 1: right ways to get eaten.
Speaker 3: I gotta say, these parasites are bad engineers. This whole
Speaker 3: thing is every time I hear about a parasite life cycle,
Speaker 3: it's some ridiculously complicated Rube Goldberg machine where everything has
Speaker 3: to go just right.
Speaker 1: Yeah, well, in another episode, we should talk about the
Speaker 1: evolution of these life cycles, because we think that they
Speaker 1: came about because like, you know, so say it started
Speaker 1: at the snail, and you got the parasites that leave
Speaker 1: the snail, and maybe they were going off in search
Speaker 1: of another snail. The idea is that so many of
Speaker 1: them were getting eaten by fish it made more sense
Speaker 1: to capture fish as a host in your life cycle
Speaker 1: than to you know, just keep getting eaten by fish.
Speaker 1: And in this way complexity arose, so you know, this
Speaker 1: complexity might be better than what was happening before.
Speaker 3: It's also the kind of example that makes me just
Speaker 3: like sort of squish up my eyebrows when somebody says
Speaker 3: nature is so obviously designed, because I'm like, nobody would
Speaker 3: design this. This is a mess.
Speaker 2: This is a mess, it really is. It's wild, all right.
Speaker 3: So now they're happy that they're in these birds and
Speaker 3: they've killed these poor chilly fish by feeding them to
Speaker 3: these birds.
Speaker 1: Then what happens, okay, and then when the bird poops,
Speaker 1: they poop the parasite eggs out into the salt marsh,
Speaker 1: where the snails accidentally eat the eggs and the cycle
Speaker 1: starts again.
Speaker 2: So that is the life sign.
Speaker 3: Oh my gosh. Wow. All right, so it really is
Speaker 3: a loop.
Speaker 2: It really is a loop.
Speaker 3: Snails eat them, they grow in the gonads, then they
Speaker 3: crawl up the fish into the brain influence. The fish
Speaker 3: get eaten by birds, and then the birds poop them
Speaker 3: out and they're eaten by snails and the cycle continues.
Speaker 2: Yes, right, okay.
Speaker 1: So Kevin Lafferty and Chemo Morris were out in the
Speaker 1: estuaris in southern California and they were looking at these
Speaker 1: California killyfish populations, which they knew they had a bunch
Speaker 1: of parasites on their brains.
Speaker 2: And they saw these fish just like.
Speaker 1: Doing all of these weird, conspicuous behaviors, and they knew
Speaker 1: that this parasite had to get to birds next. So
Speaker 1: they thought to themselves, are all of those weird behaviors
Speaker 1: that the fish are doing things that they're doing to
Speaker 1: try to draw the attention of the predatory birds, so
Speaker 1: that the predatory birds will eat the fish that are
Speaker 1: behaving conspicuously. So they went to a population that doesn't
Speaker 1: have the parasite so that they could get naturally uninfected fish.
Speaker 3: Are there populations that don't have the parasite?
Speaker 2: Yes?
Speaker 3: Why is that?
Speaker 1: So there's populations that have the parasite and populations that don't,
Speaker 1: and they differ in actually kind of a lot of ways.
Speaker 1: So the way you get a population that doesn't have
Speaker 1: the parasite usually is that the population becomes landlocked in
Speaker 1: some way. So for example, on the campus of the
Speaker 1: University of California, Santa Barbara, there is a lagoon that
Speaker 1: has California killeyfish in it and because it isn't tidally influenced.
Speaker 2: Or maybe for some other reason.
Speaker 1: I don't know a lot about snails, but for whatever reason,
Speaker 1: there's not snails in there, and so this environment doesn't
Speaker 1: have the snaw. Without the snails, you don't get the parasite,
Speaker 1: you don't get the life cycle. But you also don't
Speaker 1: have tides, which means the fish aren't like you know,
Speaker 1: going out into the ocean. Sometimes they're not comming. Like
Speaker 1: the whole system is different in a bunch of ways.
Speaker 1: And so these fish differ not only in that they
Speaker 1: don't have the parasite, they also differ in a lot
Speaker 1: of like their daily activities, the predators that they encounter,
Speaker 1: blah blah, blah, blah blah.
Speaker 3: And immediately this makes me wonder if they're a good
Speaker 3: control sample, right, because they're different, not just in the
Speaker 3: fact that they don't have the parasite, but in all
Speaker 3: these other ways. And somebody out there might be thinking
Speaker 3: that's bad science. But my reaction is like, well, this
Speaker 3: is where the experimental science comes in, right, and understanding
Speaker 3: those differences and can you draw conclusions and what can
Speaker 3: you do to quantify your uncertainties about them? So what
Speaker 3: kind of conclusions could they draw from this other sample,
Speaker 3: a slightly different fish.
Speaker 1: Well, so they were very clear about the limitations in
Speaker 1: the study. They you know, they pointed out, it's a
Speaker 1: different population, it differs for a variety of reasons. This
Speaker 1: should be the start of our studies in this system,
Speaker 1: not the end of our studies in this system. But
Speaker 1: I'm going to tell you about the rest of the
Speaker 1: study that they did, and then I'm going to tell
Speaker 1: you that like two decades later, I did the follow
Speaker 1: up work to address this problem.
Speaker 3: As is often the case, the nuance is lost and
Speaker 3: the bigger story sort of dominates the lore.
Speaker 1: Yeah, I'm gonna jump ahead a little bit and tell
Speaker 1: you that this paper has been cited over seven hundred
Speaker 1: times and continues to clock a bunch of citations. And
Speaker 1: my paper that was the boring study that was like
Speaker 1: super painstaking, is not getting anywhere near that many citations.
Speaker 1: But anyway, it's still interesting and you're gonna have to
Speaker 1: listen to it, everybody, thank you, so, all right, So
Speaker 1: they got fish from a population without the parasites. Yea,
Speaker 1: they got fish from a population that had the parasites,
Speaker 1: but they also had a bunch of other parasites, because
Speaker 1: the fish in these systems, they're not just infected by
Speaker 1: the parasite on their they're infected by something like seven
Speaker 1: other truma toad.
Speaker 2: Parasites as well.
Speaker 3: Oh my god.
Speaker 2: Yeah, they're just like riddled with paras lousy with parasites,
Speaker 2: lousy with parasites. All right.
Speaker 1: So they brought them into the lab and what they
Speaker 1: noticed was that the fish that had parasites on their
Speaker 1: brain were doing about three to four fold more conspicuous
Speaker 1: behaviors than the fish from the population without the parasites,
Speaker 1: So they were doing these conspicuous behaviors a lot more often.
Speaker 1: And then they set up enclosures in a lagoon where
Speaker 1: the fish would be out in these enclosures, and predatory
Speaker 1: birds could wade in and eat whatever fish they wanted
Speaker 1: and then come back out again. And then after about
Speaker 1: fifty percent of the fish had been eaten or something,
Speaker 1: they went out and they looked to see which fish
Speaker 1: had been eaten and which fish were left behind.
Speaker 3: So these enclosures paint me a picture of them. I
Speaker 3: was imagining first that like sectioned off various parts of
Speaker 3: the lagoon. But are these things open from the top?
Speaker 3: Why did the birds have to wade in?
Speaker 1: So these were along the shoreline, and it was a
Speaker 1: net that sort of went out from the shoreline, traveled
Speaker 1: along the shoreline, and then came back to the shoreline.
Speaker 1: So it had like three netted sides, and they did
Speaker 1: that twice, and one side they covered so that they
Speaker 1: could just measure how often the fish were just sort
Speaker 1: of like escaping from the net. And then the fish
Speaker 1: could either just sort of drop from the top in
Speaker 1: if they wanted, or they could walk along the beach
Speaker 1: and wade in.
Speaker 2: Oh, I see along that way.
Speaker 3: So the death from swooping down to gobble your lunch
Speaker 3: is still an option, yep.
Speaker 1: Still an option, okay. And so what they found was
Speaker 1: that the fish that had more parasites were more likely
Speaker 1: to be eaten. And the way they inferred that was
Speaker 1: when they did the dissections afterwards. They would have expected
Speaker 1: to see, you know, something like ten fish with a
Speaker 1: thousand parasites on their brain, but they didn't see any
Speaker 1: fish with a thousand parasites on their brain. Most of
Speaker 1: the fish that were left had very few parasites on
Speaker 1: their brain. And so they end up concluding that the
Speaker 1: more parasites you have on the brain, the more conspicuous
Speaker 1: behaviors you do, and the more likely you are to
Speaker 1: be e and by predatory birds.
Speaker 3: And I feel like that's a story I've heard before,
Speaker 3: that parasites manipulate host behavior makes it more likely for
Speaker 3: them to be eaten. Was this like the first time
Speaker 3: that had been observed in detail? Was this a new
Speaker 3: story just in this animal or more.
Speaker 1: Broadly, this was not the first time, like, you know,
Speaker 1: since the seventies, I think people had been talking about this,
Speaker 1: but this was one of the first times it had
Speaker 1: been well studied in a vertebrate and one of the
Speaker 1: first times there was a really elegant experiment that showed
Speaker 1: that not only was the parasite associated with the behavior
Speaker 1: that seemed intuitively like it really should be increasing risk
Speaker 1: for the host, but also then they showed that, like, yeah,
Speaker 1: the predatory birds are actually eating the infected fish. The
Speaker 1: authors were like really good about explaining all of the caveats,
Speaker 1: like all of the limitations of the study, and like
Speaker 1: this paper got me. You know, I spent the next
Speaker 1: decade following up on this paper. I found this study
Speaker 1: super exciting, and I moved to Santa Barbara for two
Speaker 1: years to study underneath the lab that did this work.
Speaker 3: Wow.
Speaker 2: Yeah, this is like a now classic example of manipulation.
Speaker 3: And in their first paper, do they come up with
Speaker 3: a causal mechanism to explain this or is it more
Speaker 3: just correlational? More parasites means more behavior, and therefore we're
Speaker 3: inferring that the parasites are causing the behavior.
Speaker 2: It's correlational.
Speaker 1: Yeah, and so there's another parasite that they quantify that
Speaker 1: is living in the liver, and they look for correlations
Speaker 1: between this liver trematode and conspicuous behaviors as well, and
Speaker 1: they find some correlations, but the correlations are stronger with
Speaker 1: the brain parasite, and so they hypothesize that from its
Speaker 1: location in the brain, this parasite is probably hijacking behavior,
Speaker 1: and so there's probably something about being in the brain
Speaker 1: that helps the parasite do that. But you know, since
Speaker 1: they hadn't actually done experiments on mechanisms, they leave that
Speaker 1: to future work, and their post doc Jenny Shaw, actually
Speaker 1: would go ahead and follow up on that, which way
Speaker 1: will talk about a little bit later in.
Speaker 3: The r tradition of leaving the hard questions to future work,
Speaker 3: as we all do in our papers. Ooh, that's important
Speaker 3: weakness in my study. I'm going to say, that's future work.
Speaker 2: Yeah, well, you know, you can't do everything all at once.
Speaker 3: No, you certainly can't. I do that all the time.
Speaker 3: But I guess that leaves us open to the possibility
Speaker 3: that the parasites are not causing this behavior, but there's
Speaker 3: some third unknown thing which is causing both the parasites
Speaker 3: and the behavior, for example, which would induce a correlation
Speaker 3: as well.
Speaker 2: Yes, right.
Speaker 1: Okay, So first of all, there's the problem of them
Speaker 1: coming from different populations and the populations differing in a
Speaker 1: lot of ways. Second, there's the problem that not all
Speaker 1: of the parasites in the wild fish were quantified. It
Speaker 1: could be some other parasite or some combination of parasites
Speaker 1: that we're causing the problem. And so this is not
Speaker 1: an ideal study design, as noted by the authors. Right, Ideally,
Speaker 1: what you'd want to do is, you know, get fish
Speaker 1: from a population that has like an evolutionary history with
Speaker 1: the pairs site, bring them into the lab, like maybe
Speaker 1: hatch them in the lab, grow them up in the lab,
Speaker 1: infect some of them with the parasite, and a lot
Speaker 1: of folks when they infect fish with parasites, they'll infect
Speaker 1: them like once with like five thousand parasites, whereas actually,
Speaker 1: when they're in nature, they start acquiring parasites like almost
Speaker 1: as soon as they hatch, and they acquire like two
Speaker 1: or three every day. And you can imagine a brain
Speaker 1: would respond very differently to like getting yeah, like slammed
Speaker 1: with five thousand parasites and one day, you know, relative
Speaker 1: to picking up a few every day. So yeah, ideally
Speaker 1: you'd hatch fish in a lab, infect them a little
Speaker 1: bit every day, leave some fish as controls where they
Speaker 1: don't get infected, and then observe how their behaviors change
Speaker 1: over time.
Speaker 3: And that gives us some more direct answer to this question,
Speaker 3: because we're inducing the effect, so we're not open to
Speaker 3: the possibility that something else is causing both the parasite
Speaker 3: and the behavior. That's right, all right, So let's take
Speaker 3: a break and when we come back, we'll hear all
Speaker 3: about Kelly's painstaking slog through through the questions of the killyfish.
Speaker 3: All right, we're back, and we've sort of set up
Speaker 3: the question. Now we understand what the killyfish is and
Speaker 3: the life cycle of this parasite, what people had studied,
Speaker 3: the sort of clickbaity result that's gotten a lot of attention,
Speaker 3: and now Kelly, as is her wont is going to
Speaker 3: pour cold water all over all that understanding.
Speaker 1: Since you said clickbaity, you know, Kevin Lafferty, one of
Speaker 1: the authors, is a good friend of mine, so I
Speaker 1: don't feel like he clickbaited it. He was very clear
Speaker 1: about the limitations, but it took off and had a
Speaker 1: life of its own.
Speaker 3: Right now, it sounds like they were responsible and clear
Speaker 3: about the limitations and didn't overstate their conclusions.
Speaker 2: Yes, perfect, thank you. Okay, But as is.
Speaker 3: Often the case, if you have nuance in your paper,
Speaker 3: that's some times overlooked in the coverage. Yes, even if
Speaker 3: it's not the author's fault.
Speaker 2: Yeah, yes, right, that absolutely happened. Okay.
Speaker 1: So, working with Ryan Heckinger and oven Overly, we decided
Speaker 1: that we were going to try to do a bit
Speaker 1: of a better design to try to nail down exactly
Speaker 1: what this brain infecting parasite was doing. And so what
Speaker 1: we did was we went out to an estuary and
Speaker 1: during the full moon when the killy fish breed over
Speaker 1: the summer, we went out there and we said.
Speaker 2: Excuse me, mister and missus killy fish, may we please
Speaker 2: have some gammeats?
Speaker 3: Why did the killy fish only breed during the full moon?
Speaker 3: I mean, I yet, it's romantic, but like the system
Speaker 3: was complicated enough without introduced like astronomical coincidences.
Speaker 2: I don't know.
Speaker 1: It would have been great if we could have gone
Speaker 1: out there more often. But it's synced up to.
Speaker 3: The moon, all right, So there's some were wolf connection
Speaker 3: here we don't know about.
Speaker 2: That's right, that's right.
Speaker 1: But I was like six months pregnant at the time,
Speaker 1: and so we were like, you know, very gently getting
Speaker 1: the fish to release their gam meats into a bucket
Speaker 1: for us so that we could fertilize eggs. And I
Speaker 1: was trying really hard to not puke into the gam
Speaker 1: meat bucket, as like so many science moms before me
Speaker 1: have had to try to avoid doing.
Speaker 3: So you're pleasuring these fish by hand to get them
Speaker 3: to produce these gam meats.
Speaker 2: Nice, That's not how I'd put it, But all right, this.
Speaker 3: Is the glamorous work of science.
Speaker 2: It's the glamorous work of ecology.
Speaker 1: But so we were able to actually hatch a bunch
Speaker 1: of fish in the lab, and then we went out
Speaker 1: and we collected snails and we figured out which snails
Speaker 1: were infected by HU and you can get them to
Speaker 1: repeatedly give you the same, you know, genotypes of the
Speaker 1: parasites over and over and over again. But we collected
Speaker 1: a bunch of the snails so we could get a
Speaker 1: bunch of different genotypes of the parasites. We kept those
Speaker 1: snails in the lab. Unfortunately, we had to get snails
Speaker 1: from like one estuary down. We had hoped to get
Speaker 1: everything from the same estuary. That's a long story, but anyway,
Speaker 1: so we were able to get our parasites, able to
Speaker 1: get our fish. They were hatched, and then twice a
Speaker 1: week every week we did controlled infections in their tanks. Wow.
Speaker 1: And so we would like extract some parasites, we would
Speaker 1: put it in a vial, we would slowly lower the
Speaker 1: viol into the tank, and we'd leave it in there overnight,
Speaker 1: so they slowly built up infections. And at one point
Speaker 1: we went out into the wild and we collected wildfish
Speaker 1: and we were able to confirm that we had about
Speaker 1: the same number of parasites in our fish as you
Speaker 1: found in the wildfish at the same time. So we
Speaker 1: had sort of mimicked what was happening in the wild,
Speaker 1: which was a ton of work good for us.
Speaker 3: And again, the goal of mimicking what's happening in the
Speaker 3: wild is to have a little bit more control over it.
Speaker 3: And you have some fish with the parasites and some
Speaker 3: fish without the parasites to get a better understanding of
Speaker 3: like the actual mechanism here.
Speaker 1: Well, yeah, and because if we eventually want them to
Speaker 1: have like two thousand parasites on their brain, like they
Speaker 1: would have in the wild. We didn't want to slam
Speaker 1: them with two thousand parasites all at once because that
Speaker 1: could kill them, and so we were trying to sort
Speaker 1: of slowly build it up so that what was happening
Speaker 1: in the wild was also happening in the last and
Speaker 1: so we did that. We did have a slightly higher
Speaker 1: density because our fish didn't grow quite as fast as
Speaker 1: the wildfish.
Speaker 2: We don't know why that's annoying.
Speaker 3: Why won't biol as you'd just obey my commands, right,
Speaker 3: I know, I know.
Speaker 1: And so anyway, then we measured the behavior of the
Speaker 1: fish at three, seven and eight months of age, and
Speaker 1: we found out that there were a bunch of different
Speaker 1: conspicuous behaviors that were measuring. The effect that was the
Speaker 1: most pronounced was that the fish that had parasites on
Speaker 1: their brain darted.
Speaker 2: About twice as much as the control fish. So that
Speaker 2: was the main effect.
Speaker 1: I thought the main effect was going to be scratching,
Speaker 1: because to me, the most obvious conspicuous behavior when you're
Speaker 1: standing on the shoreline is when they flip on their
Speaker 1: sides and the sun reflects off of their belly. That
Speaker 1: really draws my attention. But the behavior that seems to
Speaker 1: be impacted is what we call darting. So this is
Speaker 1: when a fish sort of like out of nowhere, it
Speaker 1: will shoot forward really quick, like something really scared it,
Speaker 1: and then it's like it forgot that it was afraid
Speaker 1: and it just goes back to its normal behavior.
Speaker 2: And it's really weird.
Speaker 1: So darting is a behavior that fish often use when
Speaker 1: a predator is around. But when they dart, they're darting
Speaker 1: like behind a piece of vegetation, or they're like darting
Speaker 1: into like a turbid zone, like you know, there's like
Speaker 1: a plume of sand and they're darting into the middle
Speaker 1: so that they can hide. It's very strange to see
Speaker 1: a fish dart forward and then immediately resume normal movement.
Speaker 1: And so the idea here is that if they dart forward,
Speaker 1: they draw the attention of the predatory bird, and then
Speaker 1: if they're not hiding afterwards, they're very easy to like
Speaker 1: hone in on. Maybe we haven't actually like done videos
Speaker 1: just show that that's exactly what's happening.
Speaker 3: And so there were some categories of behavior you were
Speaker 3: looking for, scratching, darting, et cetera. How did you come
Speaker 3: up with these categories, is there a possibility that there's
Speaker 3: some kind of category of behavior that the fish are
Speaker 3: doing different between the two populations that you didn't think
Speaker 3: to observe.
Speaker 2: Yeah, that's so.
Speaker 1: First of all, in in Lafferty and Morris's paper, they
Speaker 1: watched the fish for a long time. They came up
Speaker 1: with like any category of weird behavior that they could
Speaker 1: think of. That's called like an ethogram, where you watch
Speaker 1: for a long time and you you know, take notes
Speaker 1: on any behaviors that you think are relevant.
Speaker 3: But it feels a little subjective.
Speaker 2: Right, yes, behavior exactly, Yes, it is sort of subjective.
Speaker 1: I'll actually be very interested in what AI does to
Speaker 1: this field, because if you can have computers analyzing everything
Speaker 1: about behavior, it's some you know, is a computer able
Speaker 1: to analyze behavior in a way that's different and maybe
Speaker 1: less subjective than how human eyes do. And there's some
Speaker 1: programs now that analyze behaviors that weren't available when I
Speaker 1: was doing it.
Speaker 3: No, it's still going to be biased, just in a
Speaker 3: way you don't understand as well.
Speaker 2: Yeah, no, you might be right.
Speaker 1: So anyway, so I looked at their ethogram, what what
Speaker 1: behaviors they thought were important and then I watched also
Speaker 1: and looked to see if I was seeing something different.
Speaker 1: And I've also watched fish from a couple of different estuaries,
Speaker 1: so this is not the only study I've done with
Speaker 1: these fish. And so I've spent a lot of time staring.
Speaker 3: At fish, the glamorous work of science.
Speaker 2: That's right, that's right. Maybe I haven't captured all the
Speaker 2: important behaviors, but I feel like I.
Speaker 3: Have all right. And so you saw this darting effect,
Speaker 3: so compare contrast that with the original effect in the earlier.
Speaker 2: Paper less strong.
Speaker 1: So before they didn't see that darting was the behavior
Speaker 1: most strongly associated with this brain infecting parasite, and they
Speaker 1: saw that behaviors were like four times more pronounced in
Speaker 1: their infected fish. So we're seeing a different behavior most
Speaker 1: strongly impacted by the parasite and a lower magnitude. And
Speaker 1: we don't know if that's because it was a different
Speaker 1: population that we.
Speaker 2: Were looking at.
Speaker 1: It could be because the wildfish in the other study
Speaker 1: had a whole community of parasites, and a lot of
Speaker 1: those community of parasites also go to predatory birds. Maybe
Speaker 1: each of those parasites are like manipulating a different part
Speaker 1: of the behavior, or like they're all sharing the cost
Speaker 1: of manipulating. There's a lot of different reasons, but at
Speaker 1: the end of the day, what I found was that
Speaker 1: the effect is a little bit more complicated and a
Speaker 1: little bit smaller than what had been found before.
Speaker 2: So no one cites my work exactly.
Speaker 3: But this was the exact goal of this studies to
Speaker 3: tease this part and try to understand more specifically what
Speaker 3: is the effect of this parasite. Yes, and so the
Speaker 3: answer is that on its own, this parasite is not
Speaker 3: doing everything that the original guy saw.
Speaker 2: That's right, that's right.
Speaker 1: But so let's dive in a little bit to what
Speaker 1: we know about how the parasite might be doing this. Yeah,
Speaker 1: the first thing you usually wonder when you discover that
Speaker 1: a infected organism is getting eaten by another organism more
Speaker 1: often is like, well, is the parasite just debilitating it
Speaker 1: in some way? Like is it just making it slower,
Speaker 1: easier to catch or something? And that doesn't seem to
Speaker 1: be the case. So, first of all, you know, just
Speaker 1: like observations, if you watch these fish, they've got eight
Speaker 1: thousand parasites on the brain. Man, it's like a carpet.
Speaker 1: It's so crazy to see all these parasites. But they're
Speaker 1: not like swimming on their sides, they're not having trouble
Speaker 1: catching their food. They school normally, like they look normal.
Speaker 1: And my friend Laura Nadler, who worked on this system
Speaker 1: as a postdoc, she stuck them in these little metabolic
Speaker 1: chambers and she measured their acute metabolism, so she like,
Speaker 1: you know, got them to swim real fast.
Speaker 2: And I don't really know how you do this kind
Speaker 2: of stuff and fish.
Speaker 3: This is like what they do to Olympic athletes.
Speaker 2: Right, treadmills And yeah, I don't know how you treadmill.
Speaker 3: Is I'm imagining a fish on a treadmill right now?
Speaker 2: Yeah, yeah, yeah.
Speaker 1: She did a bunch of different things to like get
Speaker 1: their like maximum metabolic rate and they're resting metabolic rate
Speaker 1: and blah blah blah.
Speaker 3: Okay, she fed them red bull and stuff like this.
Speaker 2: I'm sure she did. Yeah, I'm sure she got a
Speaker 2: protocol and a permit and all.
Speaker 1: That for that. And what she found, like using our
Speaker 1: fish from our lab experiments where we had like hatched
Speaker 1: them in the lab, infected, some didn't infect others that
Speaker 1: the infected fish, and the uninfected fish had like no
Speaker 1: difference in metabolism, So it doesn't look like having a
Speaker 1: bunch of fish on your brain is that energetically expensive. Like,
Speaker 1: they're pretty efficient at resource extraction when they're on the brain.
Speaker 1: They're like, you know, semi dormant after they finish growing,
Speaker 1: So it doesn't seem like it's that. And their body
Speaker 1: condition is good if you compare them to uninfected fish.
Speaker 1: They have, like their gonads are about the same size
Speaker 1: because ecologists do grow stuff like way gonads, and like
Speaker 1: they seem just as good at having babies. They have
Speaker 1: the same amount of fat reserves. Like, so it doesn't
Speaker 1: seem like they're making them debilitated in any way that
Speaker 1: we can measure.
Speaker 3: Well, what about coming at it from the other direction
Speaker 3: and asking, like, how is the parasite benefiting, because that
Speaker 3: might be the other side of the equation. If the
Speaker 3: parasite is like extracting something specific from the brain or
Speaker 3: from the fish, then maybe you could understand the impact
Speaker 3: of losing that on the fish.
Speaker 1: Uh so, Okay, So the idea would be that, like
Speaker 1: the parasite is extracting like a neurotransmitter or something.
Speaker 3: I don't know what is the benefit to the parasite.
Speaker 2: What do you mean, what is the benefit of the parasite.
Speaker 3: To being on the brain? Like, what is this doing
Speaker 3: there that's helping it?
Speaker 1: Yeah, So usually what the parasites do at this stage
Speaker 1: is they grow a little and then they kind of
Speaker 1: like they reach you know, asymptotic growth. They grow, and
Speaker 1: then they stop growing. And I think they mostly stop
Speaker 1: growing because while they're growing, they're forming a cyst around them,
Speaker 1: and this cyst protects them from the immune system. And
Speaker 1: when that cyst finishes growing, they've sort of run out
Speaker 1: of room to grow, and so that's as big as
Speaker 1: they're going to get. But presumably they can extract some
Speaker 1: resources across the cyst wall and maybe even secrete some
Speaker 1: stuff across the cyst wall. Presumably somebody has looked to
Speaker 1: see what they are sucking up on the brain, but
Speaker 1: I don't think we've looked to see if they're like specifically,
Speaker 1: like I think they're sucking up like nutrients and stuff,
Speaker 1: but I don't know if they're specifically sucking up like
Speaker 1: serotonin to try to mess with brain stuff.
Speaker 3: Because for example, like a tapeworm lives in my gut,
Speaker 3: eat some of my food, I get less food. So
Speaker 3: it's easy to understand from the tapeworms benefit what the
Speaker 3: impact is on me. I don't know. I'm not a parasitologist,
Speaker 3: but it seems like maybe an avenue. So what do
Speaker 3: we know about how these parasites are influencing these fish
Speaker 3: spoiler alert.
Speaker 1: We don't know at the end of the day, but
Speaker 1: we think that what might be happening is that the
Speaker 1: parasite is making it so that the fish doesn't respond
Speaker 1: with as much of a stress response as it should.
Speaker 1: So essentially the parasite is like dampening the stress response.
Speaker 1: And so here's what we think is happening. So there's
Speaker 1: a part of the brain in fish that is usually
Speaker 1: associated with the stress response, and when you look at
Speaker 1: that part of the brain, we see that the typical
Speaker 1: response to the chemicals in the brain in fish when
Speaker 1: they're stressed is much lower when the fish has a
Speaker 1: bunch of parasites in that part of the brain. And
Speaker 1: so that's got us wondering, like if a predatory bird
Speaker 1: is in the area, for example, is maybe what's happening
Speaker 1: is that like uninfected fish would be like.
Speaker 2: Ah predatory bird, I gotta run away.
Speaker 1: But in a fish that has these parasites, are the
Speaker 1: parasites making the fish be like, eh, no, big deal,
Speaker 1: probably not gonna choose me. In fact, actually I'm gonna
Speaker 1: shoot forward really quick.
Speaker 2: Oh look at how cool I am.
Speaker 1: And I should say that we have measured these differences
Speaker 1: in infected and uninfected fish brains, but we have not
Speaker 1: tied these differences to behavior or tied these differences to like,
Speaker 1: you know, fish getting eaten by birds more often. So
Speaker 1: this is just like preliminary observations and how these brains
Speaker 1: look different.
Speaker 3: So all that makes sense, except for the part where
Speaker 3: being less stressed makes them dart more. Because I thought
Speaker 3: darting was a response to like, oh no, I think
Speaker 3: I'm gonna get eaten.
Speaker 1: Yeah, but so so it is a response to oh no,
Speaker 1: I think I'm gonna get eaten, but usually it ends
Speaker 1: in and so I'm going to hide, right, and so,
Speaker 1: but here it's like, oh no, I'm gonna get oh wait,
Speaker 1: oh wait, what was I running from? And there's another
Speaker 1: part of the brain that's associated with locomotion, and you
Speaker 1: tend to find a high density of the parasites there.
Speaker 1: And so I mentioned that you know, the parasites are
Speaker 1: sort of like a carpet on the brain, but they're
Speaker 1: like a carpet, but they also tend to like be
Speaker 1: a little bit more dense in two parts of the brain,
Speaker 1: and one of those parts deals with stress, and one
Speaker 1: of those parts deals with locomotion. And so you know,
Speaker 1: again these are just observations. We haven't linked it to
Speaker 1: behavior yet, but it could be that, like, you know,
Speaker 1: there's some stimulation of the locomotion part of the brain
Speaker 1: to get those fish moving, to get the darts going.
Speaker 1: And then the part where usually they're like and now
Speaker 1: go run and hide is being turned off. So they're
Speaker 1: moving around, but they're not hiding.
Speaker 3: Maybe so these fish are just like more chill, they
Speaker 3: sound like of anything. They might be happier they could be.
Speaker 1: And so actually part of how we get funding for
Speaker 1: this system is we say, you know, look, maybe what
Speaker 1: the parasite is doing is it's secreting some chemical that
Speaker 1: is suppressed saying stress. Maybe there's like a treatment for
Speaker 1: anxiety or something that we could find by something that's
Speaker 1: being secreted by this parasite. There could be a novel
Speaker 1: compound that we could discover with this parasite.
Speaker 3: Wasn't ozembic discovered when they were studying like lizard venom
Speaker 3: or something crazy.
Speaker 2: You never know, I think.
Speaker 1: HeLa HeLa monster or venom or something like that. Yeah,
Speaker 1: you never know where the next treatment's going to be found.
Speaker 3: Your science could change, Hollywood.
Speaker 1: That's right, that's right. One of the cool things about
Speaker 1: studying fish is that if you stick them in a
Speaker 1: beaker of water, the steroid hormones, which are things like cortisol,
Speaker 1: which is associated with your stress response, and things like
Speaker 1: testosterone and estrogen and stuff like that, they leak across
Speaker 1: the fish's gills and into the water and they'll reach
Speaker 1: an equilibrium, so the concentration in the water is equal
Speaker 1: to the concentration in the blood. And you have to
Speaker 1: do some validating work, but you can repeatedly make measurements
Speaker 1: of hormones in fish without needing to draw any blood
Speaker 1: by just putting them in beakers of water. And so
Speaker 1: I collected cortisol levels from fish repeatedly, and these fish
Speaker 1: had different levels of parasites on their brain, and we
Speaker 1: found that the density of parasites on their brain does
Speaker 1: impact how much cordisol they release, But it wasn't in
Speaker 1: a like nice, predictable way. And this is when I
Speaker 1: decided I didn't want to work on hormones and neurotransmitters.
Speaker 2: Anymore because I had a prediction.
Speaker 1: And I talked to the experts and I was like,
Speaker 1: there was supposed to be a straight line up, but
Speaker 1: instead it made au and they were like, yeah, this
Speaker 1: stuff never works out the way we think it's gonna
Speaker 1: and I was like, I'm done.
Speaker 2: I'm done. I'm not doing this anymore.
Speaker 3: I've had enough snail gonads. I'm moving on right.
Speaker 2: I don't even like pipettes, and so I was that
Speaker 2: was it for me.
Speaker 3: But that is one of my fears about biology is
Speaker 3: that you're doomed to working in complex systems. You can
Speaker 3: never really use reductionism and simplify things because everything you're
Speaker 3: doing is most interesting in a complex system, which means
Speaker 3: it might be forever before you actually untangle stuff. It's
Speaker 3: incredible to me that we've made any progress in biology
Speaker 3: at all.
Speaker 1: I have to admit a lot of my friends are
Speaker 1: working on these kinds of problems, and I am so
Speaker 1: glad they are, because this is how we get to
Speaker 1: the bottom of things, like, you know, why are parasites
Speaker 1: becoming resistant to our drugs, and so like super important questions.
Speaker 1: I found it really frustrating, and I was like, I can't.
Speaker 1: I don't think I can keep doing this.
Speaker 3: I think something that's not widely enough appreciated about how
Speaker 3: people end up in the science field. To end up
Speaker 3: in is that you have to be excited about the
Speaker 3: big questions of the field, but also you have to
Speaker 3: find the day to day work fun. Yeah, and so
Speaker 3: many fields are fascinating the questions they ask, but then
Speaker 3: the day to day work is very different. You know,
Speaker 3: it's like working in a lab pipetting or you know,
Speaker 3: making a laser operate well or whatever. And you have
Speaker 3: to be interested in both sides of it if you're
Speaker 3: going to spend your life doing it, because it's not
Speaker 3: every day that you're answering the big questions. Mostly it's
Speaker 3: the day to day work, and so you've got to
Speaker 3: find that niche where the craft is also fun.
Speaker 1: I can't tell you how many days I wanted to
Speaker 1: hurl that pipette across the lab.
Speaker 2: I do not enjoy pipetting. I have friends who get
Speaker 2: in the flow, and I do not. I have no flow.
Speaker 3: Well, the wonderful thing about humanity is that we're all
Speaker 3: into different stuff. And so yeah, some of us trapes
Speaker 3: through rainforests and get their socks wet while studying spiders,
Speaker 3: and other people look up at the sky and wonder
Speaker 3: about all of that, and some people like to pipet.
Speaker 3: So because of that, we have a glorious diversity in
Speaker 3: all of the science stories that we extract from the universe.
Speaker 3: We do.
Speaker 1: And if I may so, you know, I hope that
Speaker 1: work continues in the system, because this could be the
Speaker 1: routes to another treatment for anxiety.
Speaker 2: Maybe this will be helping me a decade down the road,
Speaker 2: we can hope.
Speaker 1: But also there are other killy fish species and a
Speaker 1: bunch of estuaries in North America, And there are other
Speaker 1: Uplorcus species and a bunch of estuaries in North America.
Speaker 1: So it might be that similar interactions like this are
Speaker 1: playing out and a bunch of our super productive ecosystems,
Speaker 1: so this parasite might be impacting, you know, the flow
Speaker 1: of energy from aquatic systems to terrestrial systems all throughout
Speaker 1: North America. And we don't understand this very well. And
Speaker 1: a lot of people are interested in migratory birds. I
Speaker 1: think fish are cooler than birds. I'm going to die
Speaker 1: on that hill. But you know, bird people, you guys
Speaker 1: are cool too, and so this kind of stuff matters.
Speaker 3: And that's some of the excitement of science that you
Speaker 3: never know around which corner or under which snail gonad
Speaker 3: is going to be some amazing discovery that really changes
Speaker 3: the world. And it takes somebody like pushing on a
Speaker 3: question that seems maybe minor and in a corner, but
Speaker 3: that reveals a thread that you can use to unravel
Speaker 3: our understanding of something much broader. And when you're doing science,
Speaker 3: you never know, like, is today the day I'm going
Speaker 3: to learn something mind blowing? Because there have been days
Speaker 3: like that in the history of science. We just all
Speaker 3: hope that one of them is going to happen to us.
Speaker 2: That's right, Well, thanks for listening to me ramble. Hell everybody,
Speaker 2: I love this parasite and I.
Speaker 3: Thought that was interesting, even without any parasites controlling my
Speaker 3: brain and telling me what defined interesting.
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