There’s no getting around it — parasites are icky. But they’re also complex organisms that help control host populations and keep food webs balanced in terrestrial and aquatic ecosystems. They’re also disappearing at an alarming rate, according to researchers at the University of Washington. After studying fish specimens dating as far back as 1880, the researchers found a steep decline in marine parasites in Puget Sound. The decline was strongly correlated with rising water temperature, suggesting that climate change could pose a significant threat to parasite species.
Chelsea Wood is a professor of aquatic and fishery science at the University of Washington and lead author of the study. She joins us to explain the role parasites play and what their loss could mean for different ecosystems threatened by climate change.
Note: The following transcript was created by a computer and edited by a volunteer.
Dave Miller: This is Think Out Loud on OPB. I’m Dave Miller. There’s no getting around it, parasites can be scary. They are also fascinating, complex organisms that play important roles in entire ecosystems and some of them are disappearing at an alarming rate, according to researchers at the University of Washington, who found a steep decline in some marine parasites in Puget Sound over the last 100 plus years. Chelsea Wood is an associate professor of aquatic and fishery sciences at the University of Washington and the lead author of a new study about this. She joins us now. It’s good to have you on Think Out Loud.
Chelsea Wood: Thanks so much for inviting me.
Miller: Let’s start with the basics. What is the definition of a parasite?
Wood: Parasites are organisms that live in or on other organisms and that inflict a fitness cost on those hosts, basically that make them sick.
Miller: So this is not a symbiotic relationship. They somehow hurt the host.
Wood: That’s right. There are other kinds of symbionts, like mutualists, and we define those as organisms that live in or on a host, that give a fitness benefit to that host. The thing that distinguishes mutualists from parasites is that the parasites are bad for the hosts.
Miller: Which is why it’s a little bit counterintuitive that a decline in parasites would somehow have negative impacts on an entire ecosystem and we’ll get to that as we go. What are the various life cycles of different parasites?
Wood: That’s the cool thing about parasites and that’s the first thing that attracted me to parasitology. Their life cycles are incredibly diverse and when you first learn about them, they seem just improbable and alien. There are simple life cycles among the parasites. Some use just one host species and they move from one individual of that species to the next. Think of the common cold, COVID-19. These are things that only require one host species to persist.
But there are other kinds of parasites that have more complex life cycles where they absolutely require hosts of more than one species. Think about the malaria parasite. People can’t transmit malaria to other people. It’s impossible, no matter how hard you try, you cannot give another human malaria.
Miller: Even if I have, say, like a cut on my finger, and somebody else has a cut on their finger and we touch fingers, that’s not going to do it?
Wood: Absolutely. What needs to happen is there has to be a mosquito host intervening between two humans in order for transmission to be successful. And that is a parasite that has a two host life cycle. That’s relatively easy to wrap your mind around. We’ve all thought about malaria at least once or twice. But there are parasites that require even more than two host species to complete their life cycles. They might need 3, 4, even 5 host species to complete their life cycles. And that’s where things get really weird and alien.
Miller: Why did you want to study the prevalence of parasites over time in Puget Sound?
Wood: Well, I kind of have the impression that we’re all operating under the assumption that infectious diseases are on the rise and we certainly can’t be blamed for having this impression. Think about what we’ve gone through in the past couple of years. Not just COVID-19, but we’ve had a number of other really disturbing disease outbreaks that seem to be intensifying in frequency. There seems to be more of them than there used to be, and so I was interested in this impression that we have that there’s more parasitism happening now than happened in the past. It turns out that that’s a really hard assumption to test because there just isn’t a lot of data on how parasitism has changed over the past couple of decades, at least for wildlife parasites. No one’s been keeping track. And so we don’t have data sets to go to, to understand whether there truly is an increasing burden of parasitism, in the contemporary world compared to historical times. And that’s why I was excited to get these new data to weigh in on that question.
Miller: You say new data, but if I understand correctly this, it’s new data based on old specimens, which seems like it’s a key repository for you and your team. Can you describe how you were able to actually track this history?
Wood: Sure. I guess they’re better called ‘new old data’ because as you point out, they are based on some specimens that are up to 140 years old. We figured out that you could get data on the parasite burden of fish from fish floating in jars held in natural history museums. When you go to a natural history museum, you’re going to see a couple of fish floating in jars, maybe some other vertebrates as well, maybe even some invertebrates. But what you see in the public space of the museum is a teeny tiny fraction of what the museum actually holds. And in the basement, in the spaces that are off limits to the public, are millions of fish floating in fluid preservatives. And those fish contained inside of them all the parasites that were there at the time of their death. The collections can stretch back in North America, to the late 1800s. European collections stretch back even further. And these fish and other vertebrates are just parasite time capsules, waiting to give us information on what the parasite burden was at their particular time and place.
Miller: The work that you and your team have just been doing, cutting up these fish and looking for tiny worms or whatever, is this what the folks who put the fish in the jars 140 years ago had in mind? I guess I’m just wondering why these jars, millions of them, are in basements of huge museums all around the world?
Wood: These specimens are what we call vouchers, they’re little biodiversity capsules that tell us a ton about the ecosystems in which that organism existed. They certainly were not originally collected so that I could go into them and pull their parasites out. I’m confident that the original curators and collection managers who accessioned those specimens were never dreaming that they would be used for parasitological analysis. But they also never dreamt of all the other uses for these specimens, including genetics, other kinds of infectious disease, understanding taxonomy and systematics of these fish and all the new things that we’ve yet to dream up that can be used to understand stuff about the ecosystems that these animals came from.
They’re in museums because they’re part of our biodiversity heritage. And in many cases they’re our only opportunity to learn things about the past. And at least speaking for my own research, without these specimens, we would have no way of knowing what the parasite burden of a fish in Puget Sound might have been in 1880. But thankfully with these specimens, we have the capacity to learn that.
Miller: Can you give us a sense for the diversity of the parasites that you found in these samples?
Wood: Puget Sound is pretty typical when it comes to marine ecosystems. The fish hosts a very broad diversity of parasitic forms. We have a number of fila including some common parasites from the phylum platyhelminthes. Those are the flatworms, they include cestodes or tapeworms and trematodes or flukes. We also find a lot of animals in the phylum nematoda. Not all the nematodes are parasitic, but obviously the ones that we find in these fish are and they are quite a number of them. And then, aside from the worms, we also commonly find crustaceans, parasitic copepods that cling to the gills or the skin of these fish and a number of other clades as well. So we’re spanning a huge chunk of the tree of life when we look just at the parasites within a fish.
Miller: You described one of these, in the press release from the University of Washington, as ‘an unbelievably gorgeous tapeworm.’ I don’t think I’ve ever seen those words used to describe tapeworms. Can you make us fall in love with this worm? What’s special about it?
Wood: Yeah, I stand by that quote. [Laughing]
This tapeworm is one of my favorites. It comes from the order Trypanorhyncha, which is a huge order of tapeworms that all infest sharks as their final host. And there are some unique challenges of living in the gut of a shark. One is that the gut is slippery. You’re trying to stay in the intestine, but of course the muscular contractions of the intestine are pushing material through. So you’ve got to kind of swim against the stream of poop in order to stay in your position in the intestine and the Trypanorhynchs have a really lovely and clever solution to that problem. They have heads, no eyes, no mouth and no gut tract, but they do have a head and that head is armed with four tentacles which are typically retracted into the head. The animal can actually evert those tentacles, one by one, at will, and when it does, it shoots them out really quickly. Each tentacle is armed with thousands of backward facing spikes. And this is part of what makes these animals really beautiful. Those backward facing spines reflect and refract light so that you get kind of a rainbow effect on the tentacles. The tentacles themselves coil and the way that they move is just really spinuous, almost like a ballet dancer. They are meant to lay these tentacles on the intestinal lining of the shark. They dig those recurved spines in and then they retract the tentacles to anchor themselves in place. So not only do they look just beautiful, under a microscope, but they’re also perfectly adapted to the environment where they’re supposed to be.
Miller: I don’t know if I’ve fallen in love with them, but I have a great respect for them. It seems like mission impossible inside a shark’s poop filled insides, and I did not think I would have such respect for these tapeworms.
Let’s turn to the decline. Can you put it in perspective? I mean, how big a decline did you find in some of these populations?
Wood: What we discovered was that the parasites that used one host species in their life cycle were fine. The parasites that used two host species in their life cycle, also fine. But parasites that use three or more host species plummeted in abundance and the degree of decline was pretty staggering. It was 11% per decade. And of course, this is just a study of Puget Sound. But if you look at the scientific literature on conservation biology, you’ll find that people are deeply alarmed by 8, 9, 10% declines in things like mammals or birds or even insects. So the decline is severe enough that if people actually cared about parasites, it would trigger conservation action.
Miller: There are so many questions to follow from what you’ve just said. But first, I’m wondering if you have a theory for the data that you just explained? Why is it that the more host species that a parasite needs for its life cycle, the larger drop you saw in its population over time?
Wood: I like to explain this by looking at something completely different. Rube Goldberg machines are familiar to many people. If you don’t know what they are, Google it and find some videos. They’re machines that accomplish a very simple task through an overly complicated array of mechanisms and each mechanism depends on the one that comes before it. Rube Goldberg machines are coolest when they’re very complicated, when there are many, many mechanisms that all must work in order to achieve the end goal of the machine, in order to kind of complete the cycle of mechanisms. And the thing that makes them cool and impressive is that the more mechanisms you have, the likelier it is that one of those mechanisms is going to break.
Now, I want you to imagine that we take our Rube Goldberg machine and we’ve built it, it works perfectly indoors and now we take it outdoors into a rainstorm. Of course, at some point, some mechanism is going to break. You’ve changed the conditions and it’s inevitable that you’re going to have a breakdown of the machine at some point, when one of those mechanisms breaks down. Complex life cycle parasites are much like these Rube Goldberg machines, they have many, many host species in their life cycle and each of those host species needs to be there and be there at the right time in order for the parasite to complete its life cycle. And when it’s operating under the conditions in which it evolved, it usually works. That’s why it exists.
But as soon as we introduce a change in the environment, like climate change, that’s when you start to get the risk of one of those mechanisms breaking down and if you lose just one mechanism, the whole machine shuts down. That’s why we think these very complex life cycle parasites are the ones that are most susceptible to climate change.
Miller: So this takes us into the biggest overarching question, which is why this matters. What role do parasites play in ecosystems?
Wood: Well, we’re just starting to discover those roles and to explain this, I like to go back to predator ecology, where we’re actually a couple of decades ahead of parasite ecology. Back in the 60s and 70s, we thought of predators like wolves and bears as vermin. There were government bounties on their heads. We persistently persecuted them because they scared us and they took our livestock and we didn’t really understand what their point was in an ecosystem.
Then predator ecologists started to understand, started to do the experiments and the studies needed to quantify the role that predators play in ecosystems. And a great example comes from gray wolves in Yellowstone. They were eradicated in the twenties and the entire food web fell out of whack. Grazers started becoming over abundant and they denuded all the vegetation that other species needed to thrive, like beavers and bison. When wolves were reintroduced in the 90s, it was like a wave of green washed over Yellowstone, balance was restored, other species that depended on the resources that the grazers overgrazed, came back and we saw that predators play this really key role in ecosystems.
We believe that parasites have some of the same roles in ecosystems and we’re just learning what those are. We know that like predators, parasites keep a cap on the abundance of some of their host species, preventing things from becoming over abundant, like the elk in Yellowstone. We also know that parasites do some really unique things. They’re often transmitted from prey to predator and they can just hang out and wait for that to happen. Or they can evolve ways to manipulate the prey to be likelier to be eaten by the predator host. They can do that by making the prey clumsy or slow or reckless. And in that process, parasites essentially feed predators. They push energy up the food web and they’re doing this all kind of in secret right now. They’re doing these services for us that we don’t recognize, but we will miss those services when parasites are gone and they stop providing them.
Miller: Chelsea Wood, thanks very much for joining us. I really appreciate it.
Wood: Thank you so much for having me.
Miller: And I should say if you ever also want to teach science communication to your fellow scientists, I think many of our listeners would be very appreciative. Chelsea Wood is an associate professor of aquatic and fishery sciences at the University of Washington. She joined us to talk about the new paper that she and her team put out, showing a huge decline in certain parasites in Puget Sound over the last 140 years.
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