Mussels in Trouble: Nature's Water Filters in Massive Die-Off

Oct 25, 2019

The mighty Pheasantshell mussel, the workhorse water filter of rivers (center) is experiencing mass die-offs in VA and other parts of the world
Credit Kurt Holtz

The Fresh water mussel is nature’s river cleaner. But every autumn, for three years running, there’s been a mass die off of one of the most important species. Biologists say, if this continues, what’s at stake is nothing less than our global river ecosystems.

The Clinch River system in southern Appalachia is world class, rich with wildlife from pretty much the entire food chain, top to bottom.  But it’s the bottom that has scientists extremely concerned. A keystone species of freshwater mussel that filters a large portion of these waters is dying by the thousands and biologists are desperate to find out why.

Tony Goldberg is an infectious disease epidemiologist and a veterinarian from the university of Wisconsin, Madison Veterinary School.  "We're at ‘ground zero.’ This, the Clinch River is the best studied example of this. But throughout the world there are muscle populations that are experiencing what we're calling mass mortality events where you'll walk out onto the river and you'll see unusually large numbers of fresh dead mussels."

Goldberg is part of a team of people who are investigating, what we now know is a global phenomenon of mussel mortality events.   “This is happening here in the southeast, it’s happening in the Pacific Northwest, it’s even happening in Europe. We have collaborators in Spain who have been sending us samples and asking for our help.

Jordan Richard is a biologist with the U. S. Fish and Wildlife Service who studies freshwater mussels in this watershed. He says a die-off like this can happen when pollutants make their way into the water, overwhelming even the mussels’ ability to clean it up. But here, where the Powell and Clinch rivers run undammed down the mountains, the water quality is actually good. He says, “Just based on water quality, it doesn't make sense that, that would be the real culprit.”

Richard is holding three different mussel species, still wet from the river, shells glistening in the sun, their tissue still white, looks fresh, but a trained biologist knows they are newly dead.

“And so, it's not abnormal at a shoal, where you have hundreds of thousands of mussels living together on any given day, to find one or two that have died recently. But when you see one of these and one of these and 50 of those, in like a 10-minute walk through the site, that gives you a clue that something is happening to these.”

Richard is referring to the prized pheasant shell mussel, Actinonaias pectorosa, the workhorses of the waters. Larger than most mussel species, they sit almost fully buried in the river bottom, only the tips of their shells sticking up, quickly opening and closing, as though gulping the water as it flows. They’re responsible for the majority of water filtration in any river they inhabit.

Richard says, that why “we call them the backbone of the ecosystem. They, really useful at sort of making their own habitat better for themselves, but also better for everything else, because they take (murky water) so when you see cloudy water instead of clear water, what they're doing is they're taking the stuff that makes the water cloudy and they're feeding on it, but they're also pulling it out of suspension and clarifying the water." 

Richard and Goldberg are now able to time the die off season, to the autumn of the year, after 3years of tracking the phenomenon. And each year, like clockwork, as – whatever this is-- returns to these waters, it kills more and more pheasant shell mussels without harming any of the other 46 species of mussels here. One theory is that it’s a new virus, or some imbalance in the microbiome that affects only this species.

Goldberg say, “We all know about the microbiome and how that's so important for just about everything. It could be that there are invisible changes that have altered the microbial community and the river or the mussels. And if that's the case, wouldn't it be nice if we could get ahold of some healthy mussel microbiomes and give them to captive reared mussels or re-instill them into wild ones to give them a little bit of an immune boost.” 

And no list of possibilities is complete without this one.  “Climate change is the 800 pound gorilla in the room.” Goldberg says.  “And this could be related to climate change, but we don't have the luxury of saying, well, if we want to save the mussels, we have to fix climate change. We don't have that much time. We just don't have the luxury to wait.” 

Goldberg and Richard think this year’s die-off season has peaked; the shells of dead pheasant mussels littering the rivers’ edge. For now, the waters still look pristine on a late fall day.  And while declines in abundant species like this one have been seen before, they’re normally linked to extremely high spawning events, with too many offspring for all to survive.  Unless and until these scientists confirm what’s causing this, they can only say, this is an event unlike any they’ve yet seen.