A Breakthrough In A Mollusk Mystery

IRA FLATOW: This is Science Friday. I’m Ira Flatow. And now it’s time to dive in to a mollusk mystery.

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IRA FLATOW: Freshwater mussels in the US are having a really bad time. It’s estimated that 70% of freshwater mussel species in North America are extinct or imperiled. That is a shocking number.

There’s a good chance you haven’t heard about this. Mussels aren’t the most engaging of creatures, right? They don’t pull at the heartstrings like a fuzzy little mammal might. And these are not these saltwater mussels that arrive on your seafood plate.

But mussels play an important role in aquatic ecosystems. So that’s why scientists are doing their best to figure out what the heck is going on here and why the declines are so drastic. And now scientists have a lead to this great mystery.

Joining me today to talk about these mussels in trouble are my guests Jordan Richard, a Fish and Wildlife biologist for the US Fish and Wildlife Service in Madison, Wisconsin– he’s also a PhD student at the University of Wisconsin– and Eric Leis, a parasitologist and fish biologist at the La Crosse Fish Health Center in La Crosse, Wisconsin. Welcome both of you to Science Friday.

ERIC LEIS: Hi.

JORDAN RICHARD: Hey, glad to be here.

IRA FLATOW: Jordan, when did it become apparent that freshwater mussels were having a bad time?

JORDAN RICHARD: Well, it’s been apparent for well over the last 100 years. They’ve really had a whole series of issues. It started with pollution in the Industrial Revolution, and then river impoundment. Then they faced pearl culture harvest, where people would just harvest them by the thousands just to see if they had a pearl in them. And then before plastics became big, mussels were actually the number one source for shirt and clothing buttons, so people would harvest their shells just to punch out little circles for using buttons.

So they already had their huge share of issues, but then more recently we’ve started to notice what we call these mass mortality events or die offs. And it’s really been kind of a mystery. More recently, in the Clinch River in Virginia and Tennessee, we saw this seasonal mass mortality that started back in 2016.

And the Clinch is kind of the crown jewel of aquatic biodiversity in the Southeast. There’s 133 species of fish and 46 extant species of mussels in the Clinch River and its tributaries alone. So it’s really worrying to see mass mortality in one of these biodiversity hotspots.

IRA FLATOW: Are we talking about a specific kind of mussel that’s dying off in that river, or are they overall not doing well?

JORDAN RICHARD: The ones we’re seeing die in the big numbers are actually one of the most common species. So it’s not one of the 20 federally endangered species that lives here, but it’s the common one that we’ve kind of counted as our bedrock of the mussel community. They’re always around. They’re always abundant. And they’re always providing this critical ecosystem services that mussels give us. So it’s really worrying to see a big proportion of the total number of mussels dying when you lose a species like this.

IRA FLATOW: Yeah, to have a die off in the most common species, you’re going to be losing the greatest number of mussels.

JORDAN RICHARD: Exactly. They’re also one of the largest. Mussels come in all shapes and sizes. When you lose 80% or more of the Pheasantshell biomass– that’s the specific mussel we’re talking about– you’re actually losing a quarter or a half or even more of the total mussel biomass in a given stretch of the river.

IRA FLATOW: You know I mentioned at the top that these are not the mussels– the saltwater mussels– that people eat. Give us a crash course in why these freshwater mussels are important for the environment.

JORDAN RICHARD: So I always start with the analogy of a healthy functioning ecosystem is like a house. And in that analogy, mussels are the foundation. So they play these critical roles as ecosystem engineers where– they’re filter feeders, and so they live down in the bottom of the river where a lot of the biomass of the river is concentrated.

So you picture, classically, fish are swimming around in the water. But a lot of the things that live in the river– snails, crayfish, mussels, and macroinvertebrates and all the things that fuel the food chain– they all live down in the bottom between the rocks. And so what mussels do is they filter everything that’s flowing down the river and the overlying water column.

And they take it in and they filter it. The water gets decontaminated. They have really great properties for detoxifying a lot of compounds for people and other animals. So they’re cleaning the water, but they’re also taking in nutrients and sources organic carbon, processing them, and then depositing them down in the sediment where all these other creatures live.

So they’re simultaneously cleaning the water and then fueling the bottom of the food chain in the river. And then when they die, their shells persist for decades. And they make really great habitats. They add a lot of complexity. If you go out on a shoal and you start picking up shells, you very quickly find a lot of little fish and crayfish and snails and small mussels. All sorts of things depend on those nooks and crannies that you get when you have a healthy mussel ecosystem on the shoals.

IRA FLATOW: Let me go to the mystery of why freshwater mussels are declining. And you have a lead, right? Possibly a virus, or viruses– is that right?

JORDAN RICHARD: We do. After a lot of hard work by a lot of people, we really clamped down and put on our epidemiology hats to investigate this thing because it’s really tricky to figure out what’s going on. Do you need to look at all the species? Is it just this one that seems disproportionately affected?

And so what we’ve found now is a novel virus that’s tentatively named Clinch densovirus 1. And it is strongly associated with the mortality, which means that in our sampling over time, we’ve collected sort of a case control design where we have healthy mussels, and then mussels that are dying from these sites and other sites scattered around the watershed. And we do all sorts of next generation sequencing and build out virus genomes of novel viruses that have never been seen before. And then use our statistical tools to see which ones are the most associated.

And so we described 17 new viruses in freshwater mussels from the sampling, but one of them was 11 times more likely to be found in sick mussels compared to healthy mussels. And orders of magnitude higher viral loads were found in the sick mussels. So it’s a really strong association at this point.

IRA FLATOW: Eric, do you have any idea why the viruses are attacking now? What is happening in the environment?

ERIC LEIS: It’s difficult to say what’s happening now. From the sick mussels, we observed less bacterial diversity. And we saw species that were commonly secondary pathogens. So they’re known to infect other animals, including fish, later after there’s already a primary infection established.

And we also found a bacteria that was associated with contaminated soils, that had the ability to degrade hydrocarbons. So we’re not sure if there is some sort of contaminant persisting there and this bacterial species is some sort of biological indicator. But it would be possible that there are changing conditions, or possibly things in the environment that would allow for this virus to kind of gain a hold and cause disease. But that’s something that we’ll continue investigating.

IRA FLATOW: Tell me about how you would do this sleuthing on this. Which part of the mussel are you actually looking at when you try to figure out if it has a virus? Is it blood? Does a mussel have blood? Where do you look for it?

ERIC LEIS: So the mussel does have blood, and it’s called hemolymph. So they have an open circulatory system, so it’s not a closed circulatory system like you see in humans. And we basically take the blood, or the hemolymph, from the adductor muscle.

So we crack the shell open slightly, and we use a needle and a syringe to slowly draw the blood out. And it looks sort of watery, but you can see little particles in it. And the hemocytes are basically their blood cells. You can kind of see them in the hemolymph solution.

IRA FLATOW: Why has it taken so long to crack through this mystery of why freshwater mussels are doing poorly?

ERIC LEIS: Well, part of the reason is that we’ve kind of needed a technology like next generation sequencing. And so next generation sequencing has been used in human medicine. And we kind of needed that to trickle down to applications and fish and wildlife health where we could have a tool where we could see all the viruses that could be present in a sample. And so that’s really been the foundation of our analysis.

Another part of it has been that there hasn’t been a cell line established from freshwater mussels that we can use to try to isolate these viruses. So from a fish population– if we think a viruses infecting fish– we can take tissue from those fish, and homogenize it and dilute it, and put it on the fish cells that we have growing in the lab in plastic flasks.

And so if the virus was in that population of fish, it would hopefully infect those cells, and we can observe that. And it’s called cytopathic effect, where the virus is basically infecting the cells and lysing them. And so once we have that, we know that we have a highly concentrated, more purified form of the virus that we can use for downstream applications like molecular analysis, electron microscopy, or even in vivo trials where we take that virus and expose otherwise healthy fish to the virus to see if it is indeed pathogenic.

And that’s something that we haven’t had with freshwater mussels. And so we’ve been– it’s really more of a guessing game for us. We’ve been trying to isolate the virus through using the hemolymph samples that we have where we know the densovirus is present. And we put that sample onto either fish cells or mosquito cells in an effort to get that virus to replicate.

IRA FLATOW: So you’ve really had no “mussel” memory to work from on this?

ERIC LEIS: That’s for sure.

IRA FLATOW: Jordan, we’re talking so much about viruses these days. Do you think this is a relatively good time for this news about mussels to come out?

JORDAN RICHARD: Yeah, I think it’s really made communicating what we’ve been finding and working on a whole lot easier because everyone’s sort of had this forced primer on virology 101, and not just the basics of viruses, but sort of how a viral infection unfolds. I was sort of thinking about it earlier when you were asking about, why are we just seeing this now and why might this be happening? It’s part science and part philosophical question. You know, I’m like why is COVID happening right now? It’s a whole series of factors.

But it’s been so much easier to talk about what we’re finding and the intricacies of it because everyone knows a lot more about virology than they did 12 months ago, especially things like comorbidities and the fact that in a population the mortality isn’t going to be 100%. There are very few viruses like Ebola where the mortality rate is close to 100%. It’s going to be viruses, but also bacteria and underlying health.

And that all ties back into the river ecosystem. Is the river healthy already? Might healthy mussels in a clean river will be able to fight off viral infection better than those in a polluted river? Those kind of details can be really hard to talk about, especially since there’s– no one’s really talked about freshwater mussel viruses before. But everyone has this nice parallel that the entire world’s staring at as we talk about it.

IRA FLATOW: I want to pick up on that last point that you just made because it’s interesting. There are some creatures that go through massive die offs that people have really strong emotional reactions to, like polar bears or orangutans, for example. And mussels aren’t exactly the cuddly, easily anthropomorphized kinds of animals. How do you get people to care, Jordan, about mussels?

JORDAN RICHARD: It starts with just making sure that people know mussels exist. They’re such cryptic little creatures. Most of the time when you’re seeing mussels in a river, what you’re actually seeing are the shells of mussels that have already died and have remained in the ecosystem and serve as nice nooks and crannies for other things. But all the live mussels dig themselves down the bottom of the river. And so we have mussels the size of salad plates where they’re buried so deeply in the river that you might only see a quarter of an inch of them sticking up out of the bottom of the river.

And so it starts with just explaining that they exist, and then getting into all that really cool ecosystem services they provide. I mean, they truly make the world a better place for people and all other animals around them by cleaning water, processing nutrients, stabilizing riverbeds. They’re just so important. It’s kind of like the foundation of your house. You’ve got to have it.

IRA FLATOW: Yeah, that’s a very interesting point. Let me remind our listeners that I’m Ira Flatow and this is Science Friday from WNYC Studios.

In case you’re just joining us, we’re talking about a mussel mystery, the die off of freshwater mussels. We think it might be a virus of some kind. Talking with Jordan Richard, Fish and Wildlife Biologist for the US Fish and Wildlife Service, and Eric Leis, Parasitologist and Fish Biologist at the La Crosse Fish Health Center in La Crosse, Wisconsin. Jordan Richard is in Madison.

If we eat mussels that come from saltwater, marine mussels, why are we not eating mussels– these freshwater mussels– if they’re the size of a dinner plate?

JORDAN RICHARD: Well, for one thing a lot of these freshwater mussels are very long-lived, some of them up to 100 years. And so for the same reason that they’re useful to us, we also don’t want to eat them. They clean that water by bioaccumulating the toxins and things that are passing by in the river water. And so if you’re eating a large, old freshwater mussel, one, it’s going to be really rubbery, tough, and not taste very good, and two, you’re going to pick up every contaminant that mussel’s ever drank in for the last 80 years.

IRA FLATOW: Aren’t these saltwater mussels eating the same sort of toxics and cleaning the saltwater environments?

JORDAN RICHARD: They are, to a certain extent. But the freshwater mussels are more directly exposed to all of the runoff that we have. There’s a famous quote. I hope I’ll get it right. But it’s– they say that, “rivers are the gutters down which run the ruins of continents.” And so, another way of putting it is that rivers are like the veins of the entire life on a continent, and mussels are frequently known as the livers of the rivers. So just like sometimes we avoid eating liver because it’s a toxin processing center, you want to avoid those mussels.

IRA FLATOW: I love it, the livers of the rivers. You know, that’s how you can tell people about how much trouble the mussels are in by coming up with a phrase like that.

JORDAN RICHARD: Yeah, to be fair I did not come up with that one, but I really love to use it. It’s probably my snappiest one-liner for why everyone should love mussels.

IRA FLATOW: And it’s a good one. Eric, do you have a hope that we can reverse the course of how poorly mussels are faring right now? Maybe we should use that phrase a little more often?

ERIC LEIS: Definitely. We’re hoping to develop diagnostic assays– we’re working on it right now– that will be used to find healthy populations of mussels that we can then target for use in repopulation and restoration efforts.

IRA FLATOW: And what do we still not know about mussels, Jordan, that would really help crack this mystery about the disease that’s killing them?

JORDAN RICHARD: So many things, but to really get to the heart of the disease matters, we really need some measures of basic freshwater mussel health. It’s something that you take for granted in human health. And we even have other things like fish health, where we can take a blood sample, and we can run it through analyzers, and we know what the parameters should look like. In people, everyone knows that 98.6 is the right temperature, and if you’re above or below that, there’s something wrong.

We don’t have any sort of numbers like that for mussels. We can’t easily take their temperature, so to speak. And so another piece of what we’re working on is these metabolic assays to figure out how can we quickly and effectively measure whether a given mussel or population is doing well, is struggling, is facing some sort of health issue. It’s a real black box that we’re all untangling at once as we dig into these viral mysteries.

IRA FLATOW: Eric, same question to you. What would you like to know that would really help to crack open this mystery?

ERIC LEIS: I think the distribution of the virus, and just the other viruses that could be present in the environment as well. Once we find out the viruses and other diseases that are important in mussel health, then we can work to not only limit their spread, but again select healthy populations like they use to restore the impacted populations.

IRA FLATOW: That’s quite interesting. That’s about all the time we have for today. I think we all learned a lot about freshwater mussels, the liver of the river. Jordan Richard, a fish and wildlife biologist for the US Fish and Wildlife Service in Madison. He’s also a PhD student at the University of Wisconsin. Eric Leis, a parasitologist and fish biologist at the La Crosse Fish Health Center in La Crosse, Wisconsin. Thank you both for taking time to be with us today.

ERIC LEIS: Thank you, Ira.

JORDAN RICHARD: Thank you.

IRA FLATOW: You’re welcome.

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