This Mussel Does an Incredible Impression of a Fish

This is what a mussel in the throes of an identity crisis looks like. Or, more likely, it looks like a fish so that other predatory fish nip at it, releasing the mussel's parasitic larvae. These clamp onto the fish's gills and absorb nutrients.

Just be yourself, the saying goes. Cheesy, yeah, and fine enough for humans, unless you’re an identity thief or whatever. But in the animal kingdom, pretending to be something you’re not is a smart move. Some geckos, for instance, look exactly like leaves, while some cuttlefish cross-dress to fool their rivals.

But no deception is more complex than that of a genus of freshwater mussels called lampsilis, which call North America home. The mantle flesh that spills out of the females’ shells is not only shaped like a fish, but moves like one, every so often twitching with a flip of its “tail.” When a predatory fish like a bass attacks the lure, the mussel fires its larvae in the dupe’s gills. Here the parasitic young attach and drain nutrition from their host before ejecting and settling on the riverbed. (The Aristocrats!)

Typically lampsilis species mimic minnows, those sad little punching bags of the river world. The lure has two halves—the left and right side of the fish—that have characteristic stripes, and even sport eyespots for extra trickery. Varieties also differ in how vigorously they’ll flop their lures around, with some committing several hours straight to twitching to reel a predator in.

A lampsilis mussel doing its best impression of a fish. Why? Because you’re not the boss of it, that’s why.

And when the dupe falls for it, everything else is automatic. The predator strikes, and the gills between the two halves of the lure rupture, releasing a cloud of larvae from special chambers. “So the mussel isn’t saying, ‘Hey, there’s a fish close by, I’m going to release my larvae,’” says Bernard Sietman, a mussel biologist at the Minnesota Department of Natural Resources. “It’s actually the act of the attack that ruptures the gills.”

This is bad news for the dupe, on account of the way predatory fish like bass hunt: They’re like vacuums with fins and eyeballs. When the hunter finds something it fancies, it approaches and rapidly opens its maw, and the resulting suction drags the prey in. Unfortunately for the dupe, though, this is a fantastic way to get gills full of mussel larvae.

So the larvae—which look like tiny versions of their parent, only opened up wide and lacking the fake-fish bits—move through the mouth and into the gills, snapping shut on the filaments. Here they’ll hold tight, absorbing nutrients from the host, though Sietman notes the mechanism involved here is still a mystery. Depending on the water temperature, the larvae will stay as few as 10 days, maybe 20 if the water is colder. Some species will even ride out a winter if the water is particularly cold, exiting their host in the warmth of spring.

Now, just like our bodies mount defenses against invading nasties like viruses, the fish doesn’t take kindly to the mussel larvae’s assault. “When [the larvae are] attached to the gill,” Sietman says, “the response of the fish tissue is to grow epithelial cells around that spot”—epithelial cells being cells that collectively act as a shield, like your skin, for instance.

The young mussel ends up encapsulated, but this doesn’t faze it. Well, at least if it’s found the right host—that is, a particular species of fish. If the wrong kind of fish happened to attack its mother and inhale the larva, the host’s immune response will overpower it. But otherwise, the species the larva is meant to attack can’t conquer it (some varieties of lampsilis, though, get a bit carried away with things and parasitize multiple fish species).

Lampsilis larvae looking all coy. We all know what you’re after, lampsilis larvae.

And this would seem to present a problem for the mussel’s future life outside a fish, yet somehow—and Sietman says exactly how isn’t yet known—the parasite drops out of the capsule and sinks to the riverbed. The fish goes about with whatever it chooses to do with its life, though a serious infestation of mussel larvae can kill it.

At this point the mussel doesn’t look that different from when it started. “Most species don’t even grow in size, but the internal organs change completely,” Sietman says. “And so it’s sort of the caterpillar to the butterfly scenario, it’s a complete change of the internal structures of the larva.”

So why go through all the trouble? Why doesn’t the mussel just release its young into the water column, like any number of other freshwater mussels (whose larvae also find fish to parasitize, only by floating around willy-nilly)? After all, such complex parasitism is serious evolutionary trickery.

Well, in addition to getting the nutrition it needs to develop, the larva gets a secure home relative to the perils of sticking it out on the riverbed, where plenty of predators lurk. Also, I’m not sure if you’ve ever met a mussel, but they aren’t exactly celebrated for their speed, so by hitching a ride with a fish, a mussel’s offspring end up better dispersed.

Those white bits are the mussel’s gills. Puncture those and the larvae fly free. Or leave the mussel alone like a decent human being.

Then we might ask how on Earth this could evolve in the first place, how the mussel could not only wonderfully impersonate a fish, but figure out how to invade its hosts. It turns out that no deliberateness is at work here, just evolution’s beautiful, millennia-long ballet.

A lampsilis mussel didn’t show up one day doing an great impression of a fish. Individual organisms of course vary, like you and your siblings, and those with beneficial variations have a better chance of reproducing and passing down those variations. So mussels that looked a bit like fish attracted predators, which helped ferry their young—boosting the mussels’ chances of reproductive success. As the generations wore on, small changes that made the mussels look more and more like fish accumulated, resulting in the fantastical lures.

It’s a shame, then, that lampsilis and other freshwater mussels in North America are in trouble. “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,” Sietman says. You name it and for the past 100 years it’s been hitting mussels hard: habitat destruction, agricultural runoff, dams—which in particular hurt them, because dams interfere with the movements of their fish hosts.

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