The strange and fascinating life of a freshwater mussel
Oh, you think mussels are basically rocks? I’m afraid you are very wrong.
They use realistic fish or crab lures to deposit their larvae
Shining on the rocky bed of the White River in Missouri is something that looks like a little fish. It has fins and eyes, and it moves like it’s swimming. For the bass that hunt in these waters, the creature looks like food. But when predators bite, they don’t get food. Instead, they are shot in the face by a stream of freshwater mussels, which cling to their gills.
The bass has been tricked. The little fish is not a fish at all. Actually, it’s part of a mussel, a slimy creature trapped in a shell.
Once attached to the gills of the sea bass, the mussel larvae become parasites; they live inside the gills, taking nutrients from the fish and traveling for free to a different part of the river. After a few days or weeks, they will disembark and start a life on their own.
The fish costume, made up of part of the mussel's fleshy body, evolved to attract predatory fish. And the video clip below shows just how convincing some of these lures can be. They're especially impressive considering that mussels have no eyes or brains and no idea what fish look like.
There are about 300 species of freshwater mussels in North America, and many have their own clever tactics for attracting hosts. But that’s not the only feature that makes these animals so remarkable. They also clean up water pollution, help prevent erosion and provide a habitat for other animals.
The bad news is that freshwater mussels are in danger. Strongly. Approximately 70 percent of North American species are threatened and more than two dozen of them have already become extinct [PDF]. That makes them one of the most threatened animal groups on the planet.
More funding for conservation would help, experts say, but that can be a problem for species that lack charisma. Mussels tend to have the reputation of being superb rocks. In reality, they are rock stars: unique, talented and can put on a spectacular show.
The extremely strange sex life of mussels
A female mussel with broken rays. its appendix resembles a small prey fish. Courtesy of Chris Barnhart
Mussels are bivalve, like clams and oysters, meaning they live inside an articulated shell with two parts or leaflets. You may know mussels because you’ve eaten them before, but those are typically saltwater mussels that, I’m sorry to say, are much less exciting than their freshwater counterparts.
The United States, 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 world’s hot spot for freshwater mussels", said Chris Eads, a mussel expert at North Carolina State University. "We have something no one else has in the world".
Many freshwater mussels are beautiful on the outside; their shells are colored green, yellow and brown, and striped black. Its interior, however, is spectacular, especially those lures.
Unlike marine mussels, most freshwater mussels take advantage of fish (or, in some cases, salamanders) to scatter their young and use lures to attract them.
Surprisingly, some mussels depend on a specific host fish and will adapt their lures to it. The main host of the rainbow mussel, for example, is the sea bass, so it mimics one of its favorite foods: a crayfish, gangly legs and all. Watch the video below.
"It can move its body from side to side and make it look like 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 also have gills, which is where they harbor their larvae, Eads said. When a fish hits the lure, like in the example above, it can open those gills, he said, causing the mussel to "spray hundreds of larvae on the fish's face."
Incredibly, some mussels can also produce lures that float outside their shellless bodies. For example, the orange-pearl mussel, shown at right, emits two masses of larvae encased in mucous tubes that float side by side. Connected to the mussel shell with a string of mucus, the lure looks like a minnow and even waves in the water like a real fish.
When a predator swims by, it might think the mucous tube is a minnow and attack, rupturing the mussel larval pack. In the process, the fish would take in the water containing the larvae, which would pass through its gills (where the larvae latch on).
Equally impressive, some mussels create lures outside the body disguised as aquatic insects to attract smaller fish. Those worm-like structures in the image below are actually packets of mussel larvae, of a species called Ptychobranchus occidentalis, that look like insect larvae. They even have false eyes.
"It's just an amazing example of evolution," Wendell Haag, a fisheries biologist with the US Forest Service, said of such lures. "It's mind-boggling to think about it."
Below is a closer view of those packages of mussel larvae posing as aquatic insects. If you look closely, you can see inside them little beaded mussels..
Another type of mussel, known as ribbed kidney shell, creates lures that mimic the pupa of a black fly, which small fish like to eat. Here’s what those lures look like:
For comparison, here is a real blackfly pupa.
Other mussels take a slightly different approach: they open their shells, and when the host fish poke their heads in, the mussels close, almost like a fly-catching venus.
With the fish’s head partially in, the mussel will release a stream of babies. After a few minutes, once the fish stops fighting to escape, the shell is released. Take a look:
So yes, mussels are definitely amazing. And these are just a few examples of the incredible mimicry they show. (For more information, see this gallery with images and videos by Chris Barnhart, a biologist at Missouri State University).
We really need mussels, and not just as food
Mussels are essentially small Brita filters. They feed by filtering microscopic organisms and water waste, cleaning streams in the process.
You can see this in the time lapse images below, which show two tanks of dirty water: one with and one without mussels.
"They have huge gills and they just filter the water all day and all night," said Caryn Vaughn, a freshwater ecologist at the University of Oklahoma who is writing a book about mussels.
A single mussel can clean 10 to 20 gallons of water per day, eliminating harmful algae, bacteria and even metals. Together, their impact is enormous. In a 2011 study, researchers estimated that mussels on a 480-kilometer stretch of the Mississippi River filtered about 14 billion gallons per 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 invading zebra mussel reached Lake Erie; there are now thousands of them per square meter in some parts of the lake. A 2012 article [PDF] suggests that zebra mussels can filter the entire lake volume in less than a month and have made the water 600% clearer. This is not good for the Erie: the invasive mussels filter so much plankton that there is not enough for other animals to survive.
However, to be clear, native freshwater mussels provide mainly benefits. Beyond filtration, they can absorb nitrogen into their tissues and shells, Vaughn said, which is especially important in rivers that divide farmland where fertilizers abound. Their shells also create structures on the riverbed for insects to hide and fish to nest, he said. (For more information on the benefits of mussels, see this general document that Vaughn wrote in 2018).
The "mysterious" decline of the mussel
Last fall, the Fish and Wildlife Service declared 23 species extinct in the U.S. . Most news reports at the time focused on the most charismatic species on the list, such as the ivory-billed woodpecker and the Bachman warbler. But eight of the species, or about a third, were mussels.
Animals living in freshwater tend to be in bad shape, and mussels are probably 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 major problem was the construction of dams in rivers. Throughout much of the 20th century, large sections of rivers became reservoirs. Without running water, the fish adapted to the river disappeared, as did the mussels that used them as hosts. " You have these situations where the host fish are decreasing and then the mussels are decreasing after them," Rypel said. "It’s like a double impact".
However, even as the frenzy of dam construction eased, mussel populations continued to fall precipitously, even into otherwise healthy rivers, Haag said.
Water pollution probably plays a role; although mussels filter pollution, they can only handle a limited amount before they start to decompose, Rypel said. In addition, mussels, for the most part, are trapped in place, so they cannot simply swim away from a source of pollution like fish do.
But the decline remains a mystery, Haag said. It could be due to a disease, he said, or the spread of invasive clams called Corbicula fluminea, or both. Ultimately, "we just don’t know," he said. "It’s really mysterious".
Whatever the cause, some scientists and environmental advocates are working to recover these molluscs. Chris Eads of NC State, for example, is raising mussels in captivity to finally release them into the wild. Meanwhile, a nonprofit in Delaware is working on a multi-million dollar project to restore native mussels to clean water in the Delaware River basin.
Funding for mussel conservation remains a challenge, at least statewide. " We tend to put our conservation dollars into charismatic megafauna, "Vaughn said. "It’s a problem".
However, a new bipartisan bill, known as the US Wildlife Recovery Act (RAWA), could help, Eads said. The bill provides states and indigenous tribes with money to conserve wildlife, regardless of how popular a species is. It passed the House in June and is scheduled to pass in the Senate as early as this fall.
Overall, it would also help if people cared more about freshwater mussels, Eads said. They need a new brand, and they clearly deserve it. Remember that they have no brains or eyes, but they can fool a variety of animals in strange costumes. And they provide one of the most essential services, water filtration, for humans. That is, mussels are not boring, but magnificent. We need them, and there are many reasons to love them, even if you don’t eat shellfish.
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