[water running] - [Erin] I just think their strategy is really cool.

I think that things that don't get a lot of attention are kind of interesting and cool.

Yeah, I think freshwater mussels are cryptic and they're really neat.

- [Narrator] There's a hidden world beneath the surface of rivers and streams and the freshwater mussels that live there are vital to the health of those waterways.

- Here's one right here.

- Is that alive?

- It's a live mussel.

- All right.

So how was on, it looks like what's on it on its side there.

So how does it live in the stream?

How does it exist here?

- Yeah, so if my hand is the bottom of the stream, they kind of bury themselves and kind of nestle in just a little bit like that.

And then they'll open a little bit on the top.

And here there two siphons are where they're breathing the water in and out.

So they bury themselves a little bit, but not completely.

- So the current's coming this way, so it's gonna be open enough so the water just flows through it.

- That's right.

- It filters.

- Yes.

The water will flow and it will breathe it in and release the water out in a perfect way.

- How much water can a mussel filter in a day?

Any sense?

- Depends on the mussel, but I think a lot of mussels can filter like 30 gallons of water in an hour.

- An hour?

- It' amazing.

Freshwater mussels breathe the water in terms of they pull the water in, they sort through it, figure out what's food and what isn't food, and then they release water that is cleaner on the other side.

So it eliminates bacteria and viruses and pull sediment out of the water into the bottom of the river, making our rivers cleaner.

- The mussels transfer that stuff out of suspension into the bottom of the stream where it's a critical food resource for aquatic insects, bacteria, crayfish.

And all those creatures in the bottom of the stream are sort of the foundation of the food web in a stream.

They're important to game fish and virtually everything else in Australia.

- [Narrator] That filtering helps the environment.

It also helps people.

- Which means that when you turn on your tap, you're gonna have clean drinking water and it's not gonna cost too much because these mussels are out here.

Some people say that they're the lungs of the river, other people say they're the liver of the river, and they're really gonna help keep our water clean as a part of that healthy ecosystem.

- [Narrator] Trouble is freshwater mussels are dying at unprecedented rates.

- 152.3.

- The one silo they grew, the other one was pretty full of sand.

- [Narrator] Scientists are scrambling to figure out not only what is wiping out freshwater mussels, but also what can be done to save them.

- There are a lot of ideas about why mussels are disappearing and many of those have good support in one specific area.

Others are more just speculation at this point.

But there's never been a study to try to look at what factors might be associated with either healthy mussel populations or declining mussel populations across a large area.

- It does look beautiful here and everything does seemingly look okay, but still they're not doing well.

So I think that's the interesting aspect of it is like, "Okay, so yeah, this technically on the outside looks really good, but it's still not."

- [Narrator] Researchers from federal and state agencies and universities are in the midst of a three year study.

They are monitoring 100 rivers and streams across 13 states.

10 of those waterways are in North Carolina.

This is the Catava River.

- Streams are this different little universe that has a lot of unseen factors going on as far as water quality, sedimentation, what this stream looked like a hundred years ago, and what it looks like now.

It's normal to us what this stream may look like but what it is supposed to look like is a factor that people alive today probably haven't seen.

And mussels are filter feeders so we're not seeing that with the naked eye.

That's all things that are going on that require a closer inspection.

- [Narrator] Researchers collect water samples, sediment samples, and cholerae samples of the river bottom for analysis.

Oxygen levels in the water are checked.

- You can imagine like when you eat food that doesn't have a lot of the nutrients you need, you can't grow as big as you could if you had all the nitrogen and phosphorous and carbon you needed to grow big and reproduce.

- [Narrator] And in a unique experiment, scientists place 20 mussels into four silos that water flows through.

The weighted silos are then located in various points in the stream bed, fast and slow moving currents, as well as calm eddies.

Three months later, the mussels in the silos are collected to see how well they grew.

- So they're growing.

- In a stream like this, I mean, it could be that this is normal.

This stream is cool.

There's obviously not much in the water.

- So maybe not as much.

- Not as much food, exactly.

So that's one of the things we're trying to figure out is what kind of growth should you expect to see.

- [Narrator] All of the mussels grew, but the growth rate varied.

- We take the mussels that we've had in our silos, the enclosures that our baby mussels have been growing in for three months.

We'll take those out at the truck, put them in vial, and freeze them in liquid nitrogen so that those samples will be used later to look at a a host of different things including parasitology, toxicology, virology.

We're just trying to test as many things as possible, anything that people have thrown out as potential causes for mussel decline or any issues that could be going out.

And we're really trying to just capture all of that so that we can look at it and see if we see any trends across the large area.

- So we're looking at that biological part.

We're also looking at sediment.

How are the sediments in the streams?

And might that be having an impact on the mussel populations.

We're also looking at freshwater mussel food and other kind of water quality indicators, as well.

So we're looking at a lot of data to investigate what's happening.

- [Narrator] The study's initial findings hint that freshwater mussels may be facing their own pandemic, but that's not confirmed.

What is known is that some species have completely disappeared from rivers.

More species could be lost soon.

- Ecosystems are really complex and I think we often don't understand all of the relationships that are happening until maybe we lose something.

So for the average person, maybe they've never seen a freshwater mussel and maybe they never will.

That's okay.

But every single person needs clean water and that's the cornerstone of a thriving community.

So we want to make sure that we are protecting biodiversity before we lose it.