This Parasitic Mussel Pretends to Be a Fish

Chris Barnhart, Missouri State University
Chris Barnhart, Missouri State University / Chris Barnhart, Missouri State University

When you think about shellfish, “devious” is probably not the first word that comes to mind. That is, unless you’ve met the parasitic mussel Lampsilis, the trickiest thing on two shells. To attract their hosts, Lampsilis wiggles a fish-shaped lure. When a would-be predator strikes, a pouch inside the lure bursts, releasing a swarm of parasitic larvae into the big fish’s face.

The genus Lampsilis is home to more than 20 species of freshwater mussels, all with a proclivity for fraud. Their delightful common names (Waccamaw fatmucket, lined pocketbook, Alabama lamp naiad) don’t really capture the sophistication of the mussels’ snare.

The scheme starts, as so many do, with sex. During mating season, male mussels spew their sperm into the water column. Because mussels are filter feeders, the sperm-water is sucked in by nearby females. Once their eggs have been fertilized, the females transfer them into a special brood pouch. Soon, the eggs will morph into a special type of parasitic larvae called glochidia.

Little teeny mussels!

Those itty-bitty shells are not just for show. The larvae are going to need them for what comes next.

Once her larvae have reached the glochidia stage, a female Lampsilis mussel goes into total Decepticon mode. She pushes two flaps of her mantle together to form an extremely convincing fish decoy. Inside that decoy, like a terrifying crunchy center, is the brood pouch full of glochidia.

As soon a potential host approaches, the female mussel starts wiggling her faux fish. And it’s not just any old fish; each Lampsilis lure is customized to imitate the prey of nearby host fish. Unlike real prey, the decoy doesn’t swim away, which makes it especially irresistible. It’s just sitting there!

So the big fish bites, and then things get kind of gross. The bite punctures the brood pouch and releases a cloud of glochidia.

The little larvae make their way into the fish’s gills, where they clamp on with their tiny shells. Then they dig in and start growing.

The host fish’s body is understandably not okay with this. Its immune system fights back, enclosing each larva in a cyst. But like tiny honey badgers, the larvae don’t care.

They’ll ride the big fish train until they morph into the next growth phase. Sometimes that’s 10 days, and sometimes it takes months. When the time is right, the mini shellfish pop out of their flesh prisons and sink onto the riverbed, where they’ll live out the rest of their lives. In most cases, the host fish simply swims away, suspiciously lighter than it was a few minutes ago.

Why do this? Two reasons: defense and dispersal. Once it gets past its host’s immune system, a baby mussel buried in the gills of a bass is pretty safe. And unlike their parents, Lampsilis larvae are cruising. The bass bus will take them to places their parents have only dreamed of.

For simple animals, Lampsilis mussels rely on a pretty complicated scheme. Fortunately for them, it works; unfortunately, fish fraud may not be enough to save them. “Freshwater mussels are the most, or among the most, imperiled group of freshwater organisms in North America, and some cases you could argue in the world,” Lampsilis expert Bernard Sietman told WIRED.

To preserve the mussels’ majestic weirdness, fish and wildlife agencies have begun rearing mussel larvae in laboratories and releasing them into their river habitats. That’s right: Even humans have started to do the mussels’ bidding.