If crabs ever made a zombie movie, it would probably start something like this.

The scene: A pleasant day in the waters of Chesapeake Bay. Our protagonist, a freshly molted mud crab, is looking forward to his future — thinking about having some mud crab babies one day. . . .

And then, dramatic music swells as a dastardly barnacle swims up behind him. It’s the dreaded Loxo! While the opening credits roll, we watch the Loxo larva burrow into the crab and take over his body. When the transformation is complete, our crab looks like he’s undergone a sex change, so he’s better equipped to carry and protect sacs of millions of parasites. Which is what he’ll do, again and again, until he dies.

Unlike “The Walking Dead,” this zombie situation is very real and affects up to 80 percent of both male and female mud crabs in the region.

“It feels like a pretty bad deal for the poor crab,” says Carolyn Tepolt, a postdoctoral fellow researching the issue at the Smithsonian Environmental Research Center in Edgewater, Md. But it makes for a pretty interesting segment of the Smithsonian’s new podcast, Sidedoor. (It’s part of Episode 4, called “Master of Disguise,” which will be available Nov. 23.)

To be clear, these crabs aren’t the kind you steam and smack with mallets. They’re wee things about the size of a thumbnail. But they do play a key role in the local ecosystem because they’re a favorite snack for other creatures.

The focus of Tepolt’s research, however, is whether there’s a “co-evolutionary arms race” raging between the crabs and the Loxo barnacles. Crabs in the Chesapeake are relatively “naive,” she explains, because they’ve been battling the parasite only since it arrived there 50 years ago. In Florida, where the Loxo is native, far fewer crabs end up affected: just 1 percent.

She says this difference may be a sign of evolution in action. Over time, perhaps local crabs will become better at fending off the parasites, too.

And it’s always good to know more ways to fight zombies.