Freshwater mussels have no eyes or brains. Here’s how they fool fish.

Freshwater mussels have no eyes or brains. Here’s how they fool fish. - Vox.com

November 14, 2022

Shimmering on the rocky bed of the White River in Missouri is something that appears like a small fish. It has fins and eyes, and it wiggles as if it's swimming.

To bass that hunt in these waters, the creature looks like food. But when the predators go in for a bite, they don't get a meal. Instead, they're blasted in the face with a stream of baby freshwater mussels, which latch onto their gills.

Fooled. The small fish lookalike is not a fish at all. It's actually part of a mussel — a gooey creature encased in a shell.

A female broken-rays mussel, and its appendage resembling a small prey fish.

Once they latch onto the bass's gills, the baby mussels, or larvae, become parasites; they live inside the gills, taking nutrients from the fish, and getting a free ride to a different part of the river. After a few days or weeks, they'll disembark and start a life on their own.

The fish disguise — formed by part of the mussel's fleshy body — evolved to lure in predatory fish. And the clip below shows just how convincing some of these lures can be. They're especially impressive considering that mussels have no eyes or brains or any idea of what fish look like.

There are 300 or so freshwater mussel species in North America, and many have their own ingenious tactics to lure in hosts. But that's not the only feature that makes these animals so remarkable. They also clean pollution from water, help prevent erosion, and provide habitat for other animals.

The bad news is that freshwater mussels are imperiled. Massively. Roughly 70 percent of the North American species are threatened and more than two dozen of them have already gone extinct. That makes them one of the most endangered animal groups on the planet.

More funding for conservation would help, experts told Vox, but that can be a problem for species that lack charisma. Mussels tend to have a reputation of being glorified rocks. In reality, they're rockstars — unique, talented, and they can put on a spectacular show.

The extremely strange sex lives of mussels

Mussels are bivalves, like clams and oysters, meaning they live inside a hinged shell with two parts, or valves. Perhaps you know mussels because you've eaten them before, but those are typically saltwater mussels — which, I'm sorry to say, are far less exciting than their freshwater counterparts.

The US, and especially the Southeast, is the epicenter of freshwater mussel diversity. "When people think of biodiversity, they usually think of coral reefs and rainforests, but the southeastern United States is the global hot spot for freshwater mussels," Chris Eads, a mussel expert at North Carolina State University, told Vox. "We have something that nobody else in the world has."

Mussels collected from the Verdigris River, Kansas.

Many freshwater mussels are beautiful on the outside; their shells are colored with greens, yellows, and browns, and striped with black. Their insides, however, are spectacular — especially those lures.

Unlike marine mussels, most freshwater mussels exploit fish (or in some cases, salamanders) to spread their offspring, and they use lures to draw them in.

The larvae of freshwater mussels on the gills of a small fish.

Remarkably, some mussels depend on a specific host fish, and they'll tailor their lures to them. The primary host of the rainbow mussel, for example, is the rock bass, so it mimics one of its favorite foods: a crayfish, gangly legs and all. Just check out the video below.

"It can move its body back and forth and make it look like this is a crayfish swimming backwards," Eads said of the mussel and its lure. "This mussel has no idea what a crayfish is. It's really amazing."

Mussels have gills, too, which is where they house their larvae, Eads said. When a fish strikes the lure — like in the example above — it can rip those gills open, he said, causing the mussel to "spray the fish with a face full of larvae."

Incredibly, some mussels can also produce lures that float outside of their shelled body. For example, the orange-nacre mucket mussel, shown below, emits two masses of babies encased in mucous tubes that float side by side. Connected to the mussel's shell with a string of mucus, the lure looks just like a minnow and even waves in the water like a real fish.

The lure of an orange-nacre mucket mussel.

When a predator swims by, it could think the mucous tube is a minnow, and attack, rupturing the packet of baby mussels. In the process, the fish would take in water containing the larvae, which would pass over its gills (where the larvae latch on).

Equally impressive, some mussels create out-of-body lures disguised as aquatic insects to attract smaller fish. Those worm-like structures in the image below are actually packets of mussel larvae, from a species called Ptychobranchus occidentalis, that look like bug larvae. They even have fake eyes.

These worm-like structures are packets of mussel larvae that can trick fish into thinking they're food.

"It's just an amazing example of evolution," said Wendell Haag, a fisheries biologist at the US Forest Service, of these kinds of lures. "It's mind-boggling to think about."

Here's a closer view of those packets of mussel larvae that impersonate aquatic insects. If you look closely, you can actually see tiny, bead-like mussels inside of them.

A fragment of a mussel's gill (like fish, mussels have gills) that contains lures that look like aquatic insects.

Yet another kind of mussel, known as a fluted kidneyshell, creates lures that mimic the pupa of a blackfly, which small fish like to eat. Here's what those lures look like.

Packets of mussel larvae in the shape of a blackfly pupa.

For comparison, here's a real blackfly pupa.

Other mussels take a slightly different approach: They'll open their shells, and when host fish poke their heads inside, the mussels will clamp down, almost like a venus flytrap.

With the fish's head partway inside, the mussel will release a stream of babies. After a few minutes, once the fish stops struggling to escape, the shell lets go. Take a look.

So yes, mussels are undoubtedly metal. And these are just a handful of examples of the incredible mimicry they display. (For more, check out this gallery with images and video by Chris Barnhart, a biologist at Missouri State University.)

We really need mussels — and not just as food

Mussels are essentially little Brita filters. They feed by filtering microscopic organisms and debris out of the water, cleaning streams in the process.

You can see this in the time-lapse images below, showing two tanks of dirty water: one with and one without mussels.

"They have huge gills and they just filter water all day long and all night," said Caryn Vaughn, a freshwater ecologist at the University of Oklahoma who's writing a book about mussels.

A single mussel can clean between 10 and 20 gallons of water each day, removing harmful algae, bacteria, and even metals. Together, their impact is enormous. In one 2011 study, researchers estimated that mussels in a 480-kilometer (298-mile) stretch of the Mississippi River filtered about 14 billion gallons a day. A large wastewater treatment plant filters a small fraction of that.

In some cases, mussel filtration might even work too well. In the 1980s, the invasive zebra mussel arrived in Lake Erie; now there are thousands of them per square meter in some parts of the lake. A paper from 2012 suggests that zebra mussels can filter the entire volume of the lake in less than a month and have made its water 600 percent clearer. This isn't good for Erie: The invasive mussels filter out so much plankton that there isn't enough for other animals to survive.

To be clear, however, native freshwater mussels bring mostly benefits. Beyond filtration, they can suck up nitrogen in their tissues and shells, Vaughn said, which is especially important in rivers that bisect farmland where fertilizer is abundant. Their shells also create structures in the riverbed for insects to hide and fish to nest, she said. (If you want to learn more about the benefits of mussels, check out this overview paper that Vaughn wrote in 2018.)

The "mysterious" mussel decline

Last fall, the Fish and Wildlife Service declared 23 species extinct in the US. Most news reports at the time (including ours) focused on the more charismatic species on the list, like the ivory-billed woodpecker and Bachman's warbler. But eight of the species, or roughly a third, were mussels.

Animals that live in fresh water tend to be in bad shape as it is, and mussels are likely the most threatened of all freshwater organisms, according to Andrew Rypel, a freshwater ecologist at the University of California Davis.

Why? Scientists aren't exactly sure.

Historically, a big problem was the damming of rivers. Throughout much of the 20th century, large sections of rivers were turned into reservoirs. Without flowing water, river-adapted fish disappeared, as did the mussels that used them as hosts. "You have these situations where the host fish are declining, and then the mussels are declining after them," Rypel said. "It's like a double impact."

Yet even as the dam-building frenzy mellowed, mussel populations have continued to fall precipitously, even in rivers that appear otherwise healthy, Haag said.

Water pollution likely plays a role; even though mussels filter out pollution, they can only handle so much before they start breaking down, Rypel said. Plus, mussels are, for the most part, stuck in place, so they can't simply swim away from a source of pollution like fish can.

But the decline still remains something of a mystery, Haag said. It could be due to disease, he said, or the spread of invasive clams called Corbicula fluminea, or both. Ultimately, "we just don't know," he told Vox. "It's really mysterious."

A female broken-rays mussel in Beaver Creek, Missouri.

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